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UBC Theses and Dissertations

The interactions of sockeye and kokanee salmon (Oncorhynchus nerka), mysids (Mysis relicta), and macrozooplankton… Wright, R. Howie 2006

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THE INTERACTIONS OF SOCKEYE AND KOKANEE SALMON {ONCORHYNCHUS MYSIDS (MYSIS RELICTA), AND MACROZOOPLANKTON I N ^ K A H A AND OSOYOOS LAKES, BRITISH COLUMBIA  NERKA),  By R. HOWIE WRIGHT B.Sc, The University of British Columbia, 1995  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Resource Management and Environmental Studies)  THE UNIVERSITY OF BRITISH COLUMBIA  © R. Howie Wright, November 2005  Abstract  Skaha and Osoyoos lakes are within in the British Columbia portion of the Okanagan Basin. Each lake has an indigenous population of Oncorhynchus nerka and the lakes have been invaded by the opossum shrimp Mysis relicta through downstream migration from Okanagan L . where they were introduced in 1966.  O. nerka and M. relicta compete for zooplankton food resources. Generally, in lakes where M. relicta have been introduced, the O. nerka population declines. Impacts of M. relicta include changes in seasonal abundance and species composition of zooplankton, leading to effects on phytoplankton, fish, trophic levels, lake nutrient processes, pollution, and parasitism. However, M. relicta is also a potential food source for O. nerka and may be a benefit. The interactions between these two species and zooplankton are variable and may be lake specific.  Historical and current information related to O. nerka, and M. relicta and zooplankton population dynamics was collected for Skaha and Osoyoos lakes. This information was compared to the mysidinduced effects observed in other studies. In addition, the diel vertical migration of O. nerka and M. relicta was monitored and their diets analysed to evaluate the interactions between these species, including the potential of M. relicta as a food source.  Within a decade of the introduction of M. relicta into Okanagan Lake, the'kokanee population in Skaha Lake underwent a five-fold decline; however, M. relicta is not directly responsible for this decline. The numbers of sockeye in Osoyoos Lake are highly variable, with many factors affecting their survival and M. relicta may become a future contributing factor.  Since M. relicta entered Skaha and Osoyoos lakes there has been no significant change in the abundance of cladocerans, species diversity of cladocerans has declined, and the annual pulse in cladoceran densities has been delayed in Skaha L . M. relicta are a food source to O. nerka as early as  ii  their year of emergence. The interactions between M.  relicta  and O. nerka in each lake vary  seasonally, and are influenced by zooplankton production and abiotic factors. Bioenergetic investigations are recommended to better quantify the competitive interactions between O. nerka, M. relicta  and zooplankton in these lakes.  iii  Table of Contents Abstract Table of Contents List of Tables List of Figures Acknowledgements Introduction : 2. Methods 2.1 Study Area 2.2 Mysis relicta invasion and Oncorhynchus nerka 2.3 Zooplankton 2.4 O. nerkalM. relicta interactions 2.4.1 When and where O. nerka/M. relicta interact 2.4.2 O. nerkalM. relicta interactions 3. Results 3.1 Mysis relicta Invasion and Oncorhynchus nerka 3.1.1 Mysis relicta Invasion 3.1.2 Oncorhynchus nerka 3.2 Zooplankton 3.2.1 Historical changes in zooplankton 3.2.2 Species changes in percent composition 3.2.3 Seasonal changes 3.3 When and Where Mysids and Nerkids Interact 3.3.1 Nerkid recruitment to Skaha and Osoyoos lakes 3.3.2 Mysis relicta abundance and size , 3.3.3 NerkidVmysid diel vertical migration monitoring 3.3.4 Nerkid/mysid zone of tolerance 3.4 Predator-prey interaction between nerkids and mysids 4. Discussion 4.1 Recent changes 4.2 Potential future effects 5. Conclusions and Recommendations on the outlook for Skaha and Osoyoos lakes 5.1 Conclusions/Recommendations 5.2 Outlook on Skaha and Osoyoos lakes Bibliography .  ii iv v vi vii 1 6 6 14 15 18 18 23 25 25 25 26 30 30 32 37 43 43 44  •  63 69 72 77 78 84 88 88 91 93  iv  List of Tables Table 1. Fish species composition for Skaha and Osoyoos lakes Table 2. Mysis relicta invasion and relative abundance in Skaha and Osoyoos lakes Table 3. Juvenile O. nerka mean abundance - autumn acoustic estimates Table 4. Beach seine summary for littoral presence of sockeye fry in 2002 for Osoyoos Lake.. Table 5. Summary of M. relicta densities in Skaha and Osoyoos lakes Table 6. Main and secondary food items used by nerkids in the two study lakes Table 7. Summary of variables of interest for Skaha and Osoyoos lakes  11 26 28 43 44 76 79  v  List of Figures Figure 1. Okanagan B a s i n  2  Figure 2. Location o f Skaha L a k e study sites  8  Figure 3. Location o f Osoyoos L a k e study sites  9  Figure 4. Kokanee escapement into Penticton Channel from Skaha L a k e from 1971-2004  27  Figure 5. Osoyoos Lake sockeye escapement to Okanagan R i v e r 27 Figure 6. M e a n length-at-age comparison of autumn nerkids samples between study lakes and other B . C . interior lakes 29 Figure 7a and b. Cladoceran density from selected years between 1969 to 2002 for Skaha and Osoyoos lakes 31 Figure 8a and b. Copepod density from selected years between 1969 to 2002 for Skaha and Osoyoos lakes 31 Figure 9. Percent composition for cladoceran species in Skaha L a k e (1969, 1971, 2003) 33 Figure 10. Percent composition for copepod species in Skaha L a k e (1969, 1971, 2003) 34 Figure 11. Percent composition for cladoceran species i n Osoyoos Lake (1969, 1971, 2003)... 35 Figure 12. Percent composition for copepod species in Osoyoos Lake (1969, 1971, 2003)  36  Figure 13a-d. Daphnia 2001 and 2002 seasonal trend for Skaha and Osoyoos lakes  38  Figure 14a-d. B o s m i n a 2001 and 2002 seasonal trend for Skaha and Osoyoos lakes 41 Figure 15a-i. Density distributions for Mysis relicta in Skaha and Osoyoos lakes from D F O sampling in 2001 and 2002 45 Figure 16 a-h. Skaha L a k e 2001 size frequency histograms 48 Figure 17. a-p Skaha L a k e 2002 size frequency histograms 50 Figure 18 a-h. Osoyoos L a k e 2001 size frequency histograms 54 Figure 19 a-p. Osoyoos L a k e 2002 size frequency histograms 56  M. relicta densities for 2001 and 2002 60 21. Skaha L a k e released juvenile M. relicta densities for 2001 and 2002 60 22. Osoyoos L a k e gravid M. relicta densities for 2001 and 2002 61 23. Osoyoos L a k e released juvenile M. relicta densities for 2001 and 2002 61 24. Mysis relicta weighted mean lengths (April-November 2002) in Skaha L a k e and north  Figure 20. Skaha L a k e gravid Figure Figure  Figure Figure basin o f Osoyoos L a k e 62 Figure 25a-g. D i e l vertical migration monitoring o f mysids and nerkids for Skaha and Osoyoos lakes 2002 65 Figure 26. Zone o f tolerance for nerkids and mysids for Skaha L a k e 2002 71 Figure 27. Zone of tolerance for nerkids and mysids for Osoyoos Lake 2002 71 Figure 28. Percent frequency o f M. relicta i n O. nerka stomachs (0 ) from August to February for Skaha and Osoyoos lakes 73 +  Figure 29. O. nerka (0+) lengths for Osoyoos and Skaha lakes 2002 74 Figure 30. Percent frequency o f M. relicta in O. nerka stomach samples (1+ and older) from July to November 2002 for Skaha and Osoyoos lakes 75  vi  Acknowledgements  I would like to thank Kim Hyatt and Tom Northcote and the rest of my committee, Les Lavkulich and Ken Hall. I would like to thank the Okanagan Nation Alliance for permitting a Gitksan to work on their territory. I would like to thank the Okanagan Nation Alliance Fisheries Department (ONAFD) staff Deana Machin, Fabian Alexis, Shayla Lawrence, Kari Long, Herb Alex, and Alfred Snow for helping to collect the field data. Thanks to Paul Rankin and Barry Hanslit of Fisheries and Oceans Canada at the Pacific Biological Station and Steve Matthews of the Ministry of Water, Land and Air Protection (MoWLAP) Penticton Fisheries Branch, for their support and access to reports and historical data. Thanks to Brent Phillips of Summit Environmental Consultants Ltd for final edits of the thesis. Lastly thanks to Dawn, Keegan, and Brenden for their patience.  vii  Introduction  The Okanagan Basin is an important tributary watershed to the Columbia River. It is a major valley of the southern interior of B.C. and runs north to south through a series of Okanagan lakes (Okanagan, Skaha, Vaseux, and Osoyoos) and its river to the confluence with the Columbia River at Brewster, Washington (Nasmith 1962; Pinsent et al. 1974b). In addition, northeast of the main valley (Figure 1) is a parallel series of lakes that flow north through Ellison, Wood and Kalamalka lakes and then south-west into the Vernon Arm of Okanagan Lake (Nasmith 1962).  Human populations in the Okanagan have grown continuously since the 1900's, establishing major city centres in Vernon, Kelowna and Penticton (Northcote 1996; Stockner & Northcote 1974). Concerns about water quality and quantity combined with rapid human population growth in the Okanagan Basin led to the Canada B C Okanagan Basin Study (Canada-British-ColumbiaConsultative-Board 1974), which included determining the value of the fisheries resource in the basin (Stockner & Northcote 1974). One outcome of the Okanagan Basin Study was the establishment of a water board to reduce nutrients in the lake system. More recently, the Okanagan Lake Action Plan (Andrusak et al. 2004) has been documenting the status of the fisheries resource in Okanagan Lake and the results of efforts to bolster kokanee populations.  Noting its importance for the sport fishery, the Okanagan Basin study identified rainbow trout,  Oncorhynchus nerka (kokanee and sockeye, or either referred to as nerkids) as important fish species and recommended works in all lakes, particularly Okanagan, Skaha and Osoyoos lakes (Pinsent et al. 1974a). However, other than Okanagan Lake, little work has been conducted on Skaha and Osoyoos lakes since then.  1  Cautery  Lo^ke c*H\ei <W lOKm *0  T»0  MO  5t>  ATTN  2  Dams, flood control projects involving channelization and dyking, and the impacts from Grand Coulee Dam and the Grand Coulee Fish Maintenance Project (GCFMP) have decimated salmon populations in the Okanagan basin (Ernst 1999; Fish & Hanavan 1948; Fryer 1995). Only sockeye are present in appreciable numbers today. Steelhead and chinook are.seen periodically in the Okanagan River and it is unknown i f these are strays from other systems or remnant populations. In addition to habitat loss, altered stream and river discharge have led to deterioration of fish habitat in the Okanagan Basin. As early as the 1930's it was documented that water management for irrigation resulted in some streams becoming seasonally dry near their confluence with Okanagan Lake (Clemens et al. 1939). Fish population reductions and losses have impacted a major First Nations fishery once located at Okanagan Falls, in addition to other upriver fishing stations for chinook, sockeye, steelhead and coho (Ernst 2000; Kennedy & Bouchard 1975). Today there is a limited Okanagan Nation fishery for Okanagan sockeye in the Okanagan River mainly at the base of Mclntyre dam.  The Okanagan sockeye is one of the last two remaining viable sockeye populations within the Columbia River (Wenatchee population is the other) and these fish rear in Osoyoos Lake (Hyatt & Rankin 1999). Sockeye, the anadromous form of nerkids, generally spend one year in their freshwater nursery lake feeding on plankton prior to migrating to the ocean. The best opportunity to rebuild the Okanagan sockeye population may be to reintroduce this population to its historic range and an approach that has begun by planting hatchery-reared Okanagan sockeye fry into Skaha Lake as part of a multi-year experimental reintroduction program (Smith 2003). Part of the evaluation of this program is to determine the effects of the reintroduced sockeye population on resident kokanee, the non-anadromous form of nerkids. The kokanee population of Skaha Lake is below historical abundances.  Mysis relicta  (hereafter referred to as mysids) are a late-glacial colonizer from separate eastern and  western reftigial stocks (Vainola et al. 1994). They are an omnivorous species that was proposed as a  3  food source for rainbow trout in their transition from planktivory to piscivory. Following their introduction into Kootenay Lake in 1949 there were initially positive results, although the benefit was not to rainbow trout, but to the kokanee population (Northcote 1973). Unfortunately, these preliminary results brought about introductions of mysids into numerous lakes in British Columbia (Lasenby et al. 1986; Martin & Northcote 1991), and it was not until the mysids had established dense populations, up to decades after introduction, that negative impacts were noted (Nesler 1991). Mysids and nerkids both feed on plankton in the pelagic areas of a lake system during specific times of their life history and both undergo diel vertical migrations (Johannsson et al. 2001; Levy 1991; Northcote 1991), and thus are often in competition for food. It is this competition for food between mysids and nerkids that is thought to have contributed to the decline in numerous kokanee populations in B.C., including the population in Okanagan Lake, where mysids were introduced in 1966 (Ashley et al. 1998, MS; Northcote 1991). There had been a previous proposal to introduce mysids into Okanagan Lake as a food source for lake whitefish (Coregonus clupeaformis) in 1939 (Clemens et al. 1939; Northcote 1991) that was not implemented. Skaha and Osoyoos lakes now have mysid populations subsequent to the introduction into Okanagan Lake. Introductions of mysids do not always coincide with nerkid population declines, some populations may increase in abundance (Lasenby et al. 1986). This is contingent upon whether the benefit of introduction (i.e. a food source) outweighs the cost of competition for zooplankton and the balance between these factors is lake specific, and has not been established for Skaha and Osoyoos lakes. The identification of impacts of mysid introductions are often confounded by concurrent natural and anthropogenic influences during the establishment of mysids in a lake system. In addition, suitably detailed information on pre-mysid conditions in lakes is usually lacking. Since controlled introductions of cultured sockeye fry have recently begun in Skaha Lake and mysids are beginning to establish a population in Osoyoos Lake, these present two opportunities to improve our understanding  4  of the interactions among planktivorous fish populations, mysids and zooplankton communities. The purpose of this investigation was to review historic and new information on the state of biotic and abiotic conditions in Skaha and Osoyoos lakes to: •  Establish a reliable set of baseline observations to serve as a chronological data set on the state of the zooplankton, mysid and planktivorous fish populations in advance of potential changes that may occur as a consequence of future sockeye fry introduction into Skaha Lake and/or mysid population increases in Osoyoos Lake.  •  Obtain evidence of effects of historic changes in the abundance of planktivorous fish, mysids, limnetic zooplankton communities, and their interactions in these lakes; and,  •  Identify current factors that may be important in controlling the potential strength of current and future interactions among zooplankton, mysids and planktivorous fish in these lakes.  The results of the investigation will assist in projecting the outcome of re-introducing sockeye into Skaha and, possibly, Okanagan lakes, as well as in evaluating the potential impact of mysids on sockeye in Osoyoos Lake.  5  2. Methods  The chain of lakes located in the lower Okanagan valley has been the subject of several intensive investigations over the past 50 years. Results from these studies suggest many similarities among valley bottom lakes, but also some differences with respect to historic and current physical, chemical and biological features that may be of special interest here. Descriptions of some of the most relevant features are provided below, including a brief narrative of major historical changes (e.g. water regulation, water quality and species composition of lake biota), because of their potentially important influence on processes in the limnetic communities that are the focus of the current thesis.  Northcote (1991) summarizes some of the problems with mysid introductions and their impacts on fish populations such as kokanee. Impacts of M.  relicta  include predation on zooplankton, direct and  indirect effects on phytoplankton, effects on fish, with other implications on upper trophic levels, eutrophication, pollution, and parasitism. The methods below describe tests of specific hypotheses identified in previous studies as to the potential effects of mysids to supplement the three general objectives.  2.1 Study Area  Skaha Lake (Figure 2) is a 2,010 hectare lake with a mean depth of 26 m, a maximum depth of 57 m, and a theoretical water residence time of approximately 1.2 years (Pinsent et al. 1974b). The littoral habitat, defined as the lake area in the 0-6m contour, is approximately 16% of surface lake area (Pinsent et al. 1974b). The lake can be divided into two basins, with the large north basin being the main one of interest. Okanagan Falls Dam, at the outlet, regulates water levels in Skaha Lake. Immediately upstream of Skaha Lake is the city of Penticton where the Okanagan River (herein referred to as the Penticton Channel) flows through a reach that was channelized in the 1950's. Upstream from this channelized inlet to Skaha Lake is Shingle Creek (entering on its west side) and  6  Ellis creek entering on its east side. The Penticton Channel then continues to the outlet of Okanagan Lake where a low head dam regulates lake level and flows in Okanagan River.  Osoyoos Lake (Figure 3) can be divided into three basins; north, central, and south. Zosel Dam, at the outlet of the south basin, regulates Osoyoos Lake levels. The south basin, bisected by the Canada/US border, and the central basin undergo anoxic hypolimnetic conditions and elevated epilimnetic temperatures such that habitat is unsuitable for pelagic rearing salmon during the summer and fall. Therefore, the emphasis of this study is on the north basin of Osoyoos Lake, which is 1,500 hectares in area, with a mean depth of 20.7 m, a maximum depth of 63 m, and a water residence time of approximately 0.7 years. The littoral habitat is approximately 23% of lake surface area (Pinsent et al. 1974b).  Over the past century water has become progressively more regulated with the use of outlet dams on headwater and mainstem lakes for water consumption and flood control (Symonds 2000). Major works have included alteration of the Okanagan Lake outlet to regulate lake levels for flood control and downstream flow. B y the 1950's, the Okanagan basin mainstem lakes all had outlet dams and the Okanagan River was channelized to its current state. In addition, human population growth in the Okanagan significantly influenced productivity on phytoplankton, zooplankton and fish population in mainstem lakes with increasing untreated sewage output, especially prior to the 1970's. Studies conducted during the early 1970's lead to an effective nutrient reduction program (Jensen & Epp 2001).  7  P»g\M-6 3,. 4  svcdKa Lake (A<A«p-teMro*v PWT.T <i.a\. \qm  8  Staged ar&a. O-fcm  i  i  Lake  i  1  1  PW 5c<v\e  ; ojoce'i • SkftWv Uk e(od^eA -from Pmsft^- d. «J. w? kb)  9  Fish species composition in Skaha Lake has also undergone drastic changes due to the invasion or introduction of several exotic aquatic species since the early 1900's (Table 1). Studies show that Skaha Lake currently contains 19 species of fish of which 13 are native and 6 are exotics (Alexis et al. 2003; McPhail & Carveth 1994; Pinsent et al. 1974a). Sockeye (O. tshawytscha),  coho (O.  kisutch),  and steelhead (O.  mykiss)  nerka),  chinook  (O.  are all thought to have historically returned  to Skaha Lake (Ernst 2000); however, only resident forms of two anadromous salmon remain, rainbow trout and kokanee. In addition to the exotic lake whitefish (Coregonus  clupeaformis),  rainbow trout and kokanee are routinely found in the offshore limnetic waters, while the other fish species can generally be classified as littoral or benthic residents. Kokanee is the numerically dominant species in the limnetic area of Skaha Lake and consequently is one of the focal points for the current study. Skaha Lake kokanee spawn in the Penticton Channel, mainly upstream of the Shingle and Ellis creeks and also in these two tributaries.  10  Table 1. Fish species composition for Skaha and Osoyoos lakes Skaha Lake Littoral  Limnetic Indigenous  0. nerka  Exotic  Coregonus clupeaformis  O. mykiss  Indigenous  Catostomus  Exotic  Micropterus dolomieui  macrocheilus C. catostomus  Perca flavescens  Ptychocheilus  Ictalurus melas  oregonensis Mylocheilus caurinus -  Lepomis gibbosus  Acrocheilus alutaceus Cyprinus carpio Couesius plumbeus Richardsonius balteautus Cottus asper Prosopium williamsoni Prosopium coulteri Lota lota  11  Table 1 (continued). Fish species composition for Skaha and Osoyoos lakes Osoyoos Lake Limnetic Indigenous O. nerka  O. mykiss  Littoral Exotic  Exotic  Indigenous  Coregonus  Catostomus  Micropterus  clupeaformis  macrocheilus  dolomieui  C. catostomus  Perca  Ptychocheilus  Ictalurus melas  flavescens  oregonensis Mylocheilus  caurinus  Lepomis  gibbosus  Acrocheilus  alutaceus  Cyprinus  carpio  Couesius  plumbeus  Pomoxis nigromaculatus  Richardsonius  balteautus  Lepomis macrochirus  Cottus asper  Tinea tinea  Catostomus  Micropterus  columbianus  salmoides  Prosopium  williamsoni  Lota lota Prosopium  coulteri  12  Osoyoos Lake currently contains 24 species of fish, of which 14 are native and 10 are exotic that have invaded since the 1900's (Alexis et al. 2003; McPhail & Carveth 1994; Pinsent et al. 1974a). Sockeye is the main salmon species that returns to Osoyoos Lake with chinook and steelhead seen periodically. Coho are most likely extirpated. As with Skaha Lake, the majority of species are littoral dwellers with only three species regularly found in the limnetic area of Osoyoos Lake. Sockeye is the dominant species in the limnetic zone of Osoyoos Lake and is a focal species of this study. Anadromous fish can spawn in areas as far upstream of Osoyoos Lake as Mclntyre Dam (Figure 3). In the 1950's, the river was channelized for approximately 17 kilometres above the mouth at Osoyoos Lake. The remaining six kilometres, ending at Mclntyre Dam, were either not channelized or the dykes were set back from the channel for more natural river processes to occur.  13  2.2 Mysis relicta invasion and Oncorhynchus nerka  It is hypothesized that the invasion of mysids has negatively affected the Skaha kokanee population and has resulted in a decrease in their overall population, such as is documented in other lakes (Morgan et al. 1981; Morgan et al. 1978; Rieman & Falter 1981). In order to understand some of the interactions between mysid and nerkid populations in Skaha and Osoyoos lakes it is important to know the abundance of each population in the two lakes. Historic data on mysid stocking and seasonal abundance in the Okanagan basin were reviewed and summarized. Also, records of kokanee and sockeye adult abundance were reviewed to establish historic population variability and document changes in the population of kokanee in Skaha Lake since the invasion of mysids. Adult Skaha Lake kokanee have been enumerated for the Okanagan Basin Study (1971), during the Summerland Hatchery egg take (1972-1974), and in more recent years, by the Ministry of Environment (1989 to 2002). Fisheries and Oceans Canada (DFO) has compiled, reviewed, and developed an escapement data set for Okanagan sockeye (Stockwell & Hyatt 2003). High densities of nerkids can also influence macrozooplankton, and specifically cladocerans (Burgner 1991). Therefore, the seasonal distribution and abundance of juvenile sockeye and kokanee in Osoyoos and Skaha lakes, respectively, have been documented through acoustic and trawl surveys conducted by Fisheries and Oceans Canada. Surveys have been conducted for juvenile sockeye in Osoyoos Lake since May 1997 and for juvenile kokanee in Skaha Lake since May 1999, resulting in standardized population estimates (Hyatt & Rankin 1999). Juvenile density estimates have been compared to the numbers of spawners for each population. hi addition to documenting nerkids population sizes, the length and weight of nerkids in the fall have been compared between Okanagan, Arrow and Kootenay lakes as an indication of whether nerkids in the study lakes are currently in a food-limited environment (see section 2.4.2 for study lake sampling  14  methodology). Length and weight information were obtained from Pieters et al. (1998; 2000; 1999) for Arrow Lake, Ashley et al. (1999) for Kootenay Lake, and Andrusak et al. (2004) for Okanagan Lake. For Arrow Lake the mean lengths at age were from two (1997 and 1998) pre-fertilization and one (1999) fertilization year. The highest and lowest yearly mean lengths at age were used for Kootenay (1983-1996) and Okanagan lakes (1985-2003) as a size range of the age structure. 2.3 Zooplankton In addition to interacting with each other, both M. relicta and O. nerka influence macrozooplankton communities, cladocerans in particular (Burgner 1991). For both study lakes, the macrozooplankton communities in 2001 and 2002 were dominated by cyclopoids and diaptomids, with the dominant cladoceran being daphnids (Wright 2002; Wright & Lawrence 2003). It has been demonstrated in other lakes that after the invasion of mysids, there can be a decline in cladocerans, or a delay in their annual density increase, the "cladoceran pulse" (Cooper & Goldman 1980; Goldman et al. 1979; Langeland 1988; Morgan et al. 1981; Spencer et al. 1999; Vogel & L i 2000). While Osoyoos Lake is only beginning to show the presence of mysids, they are thought to have been present in Skaha Lake since the early 1970s (Pinsent et al. 1974b; Truscott & Kelso 1979). Mysids may remove enough metalimnetic zooplankton production to become a competitor with planktivorous fish (Johannsson et al. 1993; Johannsson et al. 1994). In laboratory studies, mysids preferred cladocerans to copepods and the consumption of cladocerans was sufficient to delay the annual density pulse (Rieman & Falter 1981; Spencer et al. 1999). It is hypothesized that the zooplankton abundance and/or composition in Skaha Lake has changed following the invasion of mysids and that there has been little change in the zooplankton abundance and/or composition in Osoyoos Lake over this same period because mysids have only recently become established. In order to test the hypotheses that densities of copepods and cladocerans have decreased following the appearance of mysids in Skaha Lake and that densities have not changed in Osoyoos Lake  15  between 1969 and 2002, the densities of copepod and cladoceran taxa were compared as were the ratios of total copepod to cladoceran numbers. The comparisons were made using late-summer/fall samples that had been collected from geographically similar lake sample sites. Late-summer/fall samples were used because the largest data set was available. For both lakes, data were used for three time periods (1) from 1969 and 1971 (Pinsent et al. 1974b), (2) from 1977 and 1978 (Truscott & Kelso 1979), and (3) 2001 (Wright 2002) and 2002 (Wright & Lawrence 2003). Unfortunately, no zooplankton samples were available prior to the appearance of mysids in Skaha Lake. However, since previous studies have shown that it can take a decade for mysids to reach high enough densities to appear in predator stomachs (Lasenby et al. 1986), we can assume that mysids in Skaha Lake were present in low numbers during the zooplankton sampling in 1969 and 1971 for Skaha Lake and can thus represent the pre-mysid condition. All samples that were used were from deep water sites. When more that one sample was availablefromthe late-summer/fall the average of all samples within that period was used. One confounding factor is that in 1977 and 1978 it is not clearly stated that nauplii were included in copepod counts; however it is assumed that they were since two species of nauplii were listed as dominant in Skaha Lake and high numbers of unidentified nauplii listed as dominant in Osoyoos Lake (Truscott & Kelso 1979). Another data problem is that within the Truscott and Kelso (1979) report, the 1978 cladoceran and copepod abundances listed were identical to the 1977 abundances and different from that listed in the subsequent graphical presentation. Because of this, the abundances for 1978 were based on visual estimates from the graphical presentation instead of the table. Finally, there was a change in sampling gear starting in 2001. In the earlier samples a 75 um mesh Wisconsin plankton net with a 25 cm diameter opening was used, while a 100 um mesh net with a 45 cm opening was used for 2001 and 2002.  To test for differences in zooplankton numbers between years within each lake a nonparametric Kruskal-Wallis Analysis of Variance (ANOVA) was used. An all-pairwise comparison test  16  (Conover-Inman) was used to identify significant differences in density within selected years. A Mann-Whitney U - test was used to test for differences in density between lakes for 2001 and 2002.  To test the hypothesis that mysids cause a shift in zooplankton species composition, comparisons were made within each lake using zooplankton samples from late-August 2003 to compare to samples from 1969 and 1971. Percent composition by species for cladocerans and copepods were used for the comparison.  To test the hypothesis that mysids have delayed the annual pulse of cladocerans, as previously reported elsewhere (Rieman & Falter 1981), seasonal trends in key zooplankton taxa in Skaha Lake (high mysid abundance) and Osoyoos Lake (relatively low mysid abundance) were compared. The zooplankton taxa of interest were Daphnia and Bosmina, both of the cladoceran genu. In 2001, monthly sampling was conducted from April to November. Sampling was increased in 2002 to biweekly sampling for the same period. As for mysid sampling, two sites were sampled in each lake (Figures 2 and 3).  Mysid sampling in 2001 and 2002 was conducted at night using a plankton net (45 cm diameter, terminal mesh size of 105 urn). The net was lowered to a vertical depth of 25 m and hauled back up at approximately 1 m/s. A Rigosha® flow meter was used to measure the flow rate through the net. Samples were preserved in 4% formalin in 250 mL glass jars and were shipped for analysis to the Pacific Biological Station (PBS) in Nanaimo, B C . Samples were processed as described in the mysid methodology. Both a regular count and a scan for rare species were conducted.  The samples were analysed using an I B M computer-based caliper measuring system and dissecting scope. Macrozooplankton were classified to the genus level, except for the samples used to compare species composition. The wet weight of each individual was estimated using a length/weight regression equation. The software program Zebra2® was used to manage and summarize the data (Rankin et al. 2000).  17  2.4 O. nerkalM. relicta interactions Mysids can play a dominant role in food web interactions, with their role heavily influenced by their rate of consumption by fish, including nerkids (Chipps & Bennett 2000). The consumption of mysids by nerkids depends on several physical factors such as lake depth, epilimnetic lake water temperatures, dissolved oxygen levels in the hypolimnion, and also their level of entrainment in lake outflow basins (Martin & Northcote 1991). Since there is an ongoing program to reintroduce sockeye to Skaha Lake and efforts to increase the population in Osoyoos Lake, it is important to understand the factors that control the availability of mysids as food for nerkids. Improved understanding involves documenting when, where, how and to what effect do sockeye, kokanee, mysids and zooplankton interact in these lakes. 2.4.1 When and where O. nerkalM. relicta interact O. nerka that have recently emerged may either immediately recruit to the limnetic portion of a lake or remain in the littoral area of the lake for some period prior to recruiting to the limnetic area (Burgner 1991). To determine if nerkid fry use the littoral area in Osoyoos and Skaha lakes, a 3 meter deep by 30 meter long net (10 meter long stretched-in bunt with 3 mm mesh and 10 meter wings tapered from 10 mm stretched mesh to 25 mm stretched mesh) was used to sample recently emerged nerkids from the 2000 brood year in the littoral zone of Osoyoos Lake near the mouth of Okanagan River (Figure 3). Previous beach seining attempts in Skaha Lake had been unsuccessful because the coarse substrates permitted the fry to escape (Long 2001). Sampling occurred once emergence of sockeye was evident (Lawrence 2003) and continued every three weeks until no sockeye were caught. Catch per unit effort (CPUE) was calculated for each survey date to follow early season increases in fry abundance as they recruit to the littoral zone environments and then later as they move offshore.  18  It is important to document mysid abundance, life history and seasonal distribution in order to understand their interactions with nerkids and zooplankton. Sampling for mysids in the two study lakes has been conducted between 1998 and the present by DFO and the Okanagan Nation Fisheries Department (ONAFD) (Rankin et al. 1998; Rankin et al. 1999; Rankin et al. 2001, 2003; Wright 2002; Wright & Lawrence 2003). The location of sample stations is given in Figures 2 and 3. Mysid sampling was conducted monthly between April and November 2001 and biweekly during this period in 2002 at two stations per lake. Additional mysid sampling was conducted by Fisheries and Oceans Canada (DFO) at ten sites per lake during seasonal acoustic and trawl surveys (ATS) (Rankin et al. 2003) in these years. The DFO results were used to evaluate the variability in mysid densities within the study lakes and to calibrate mysid densities obtained using different sampling methods. The sampling methodology used by DFO and the ONAFD was similar and is described below. All sampling was conducted at night using a "mysid net" (0.75 m diameter opening, with a terminal mesh size of 300 um). Vertical haul sampling was conducted by lowering the net down to 25 m or 35 m (depending on lake depth at the sample site) then hauling the net in at a speed of approximately 1 m/s. A Rigosha® flow meter was used to determine the flow rate through the net. Samples were preserved in 4% formalin in 250 mL glass jars and were shipped for analysis to the Pacific Biological Station in Nanaimo, BC. Further details can be found in Rankin et al. (2000). Sprules et al. (1981) provide a summary of the methodology commonly used to analyze zooplankton and mysid samples. The samples were stained with methylene blue to improve visibility when measuring then poured into a Folsom splitter, and sub-sampled to a concentration suitable for analysis, which used similar methods to those described for zooplankton. Mysids were classified into adults and subadults. The adults were further categorized into adult females, gravid females (carrying eggs), released females (egg pouch with no eggs), and adult males. The sub-adults were categorized into juveniles (sex not distinguishable), immature females, and  19  immature males. The wet weight of each individual was estimated using a length/weight regression equation. The software Zebra2® was used to manage and summarize the data.  Seasonal densities for all mysids, gravid females, and juveniles, as well as seasonal size-frequency results (DFO unpublished data) were summarized for the study lakes to determine the life history of mysids in each lake. Mean lengths for each life history stage of mysids were summarized for 2001 and 2002 to examine their potential as a food source for nerkids in Osoyoos Lake.  Hydroacoustic surveys were conducted in both lakes to monitor diel vertical migrations of mysids and nerkids throughout the growing season (May to November) in order to understand their potential to directly interact. Acoustic technology has previously been used to study the diel vertical migrations of both nerkids and mysids (Beamish 1966; Beeton & Bowers 1982; Levy 1987, 1989, 1991; Narver 1970; Northcote 1964; Rieman & Bowler 1980). Previous work on sockeye in Osoyoos Lake has shown that most juvenile sockeye rear in the north basin of the lake because of limnological constraints in the central and south basins (Hyatt 2003, personal communication; Hyatt & Rankin 1999), so all surveys for this project were conducted in the north basin of the lake.  Hydroacoustic surveys were conducted along a single transect in each lake that included the deepest part of the lake. The transect in each lake was selected from those used by DFO for ongoing monitoring. Each monthly survey was conducted over a 24-hour period, from April to November, using a Biosonics© sounder and a model 115 Portable Chart Recorder provided by DFO. The transect in Skaha Lake is referred to as "Gillies Point" and the one in Osoyoos Lake as "Monashee Coop" and are located close to mysid and zooplankton sample sites of the same name (see Figures 2, 3). Acoustic instruments were set at a frequency of 420 kHz, a threshold of 0.06 Volts, a range of 80 m, gain 12, a pulse width of 0.4 m, and a paper speed of 3.  Surveys were conducted near the end of each month, corresponding in 2002 with DFO's April, August, October, and November acoustic and trawl surveys. Within each lake at least five passes  20  were conducted of the transect during a 24 hour period. During each survey there were two daytime passes, one at dusk, and between two and four at night. Conducting surveys at the same time as D F O was important since DFO uses a 70kHz Simrad sounder ( E Y M 20069) that does not record mysids, so the DFO results were used as a check for distinguishing between mysids and juvenile fish. Unfortunately, there was difficulty with the Biosonics sounder for October and November of 2002 so only some of the passes were completed. In April 2002 a stationary survey was completed at the deepest part of the transects, just before dusk, to monitor mysid migration.  Qualitative analysis of monthly acoustic results consisted of visually identifying the upper limit of mysid migration towards the surface of the lake and identifying concentrations of nerkids, where possible. This involved a comparison of characteristic fish and mysid signatures on echograms obtained with the 420 kHz Biosonics sounder relative to the 70 kHz Simrad sounder. Also, beginning in May 2002, at each zooplankton station along the transect in each lake, a daytime and night time vertical haul was completed using the standard plankton net to confirm the depth at which mysids were present. The net was incrementally lowered, between 1 m and 5 m, in each successive haul at a sampling site until mysids were observed in the sample. In addition, to distinguish between nerkids and mysids in echograms during each pass of the transect, the boat was stopped for several minutes with the echosounder still operating. Fish were distinguishable because they were repeatedly marked with the echosounder in the echograms while mysids weren't.  Lake depth also may influence interactions between nerkids and mysids. In Okanagan Lake (immediately upstream of Skaha Lake), mysids undergo diel vertical migration, remaining at depths of 90-150 m during the day and migrating up to the thermocline area during the night (Levy 1991). Kokanee were also found to undergo a similar diel vertical migration, with juvenile nerkids remaining at depths of 50-80 m during the day and migrating up to the thermocline area or shallow waters during the night (Levy 1989, 1991). In contrast, older age class kokanee remained in shallow waters or in the thermocline area during the day, dispersing down to depths of 50 m at night. Levy (1991)  21  found similar results in Okanagan Lake, with O. nerka and M. relicta were segregated during much of their diel vertical migration cycle. However, in spite of having opposite migration patterns, Levy (Levy 1989, 1991) reported that in Okanagan Lake, kokanee larger than 300 mm fed heavily on mysids.  Since Skaha Lake is shallower than Okanagan Lake, it may be hypothesized that there is a greater potential for mysids to be consumed by nerkids of all ages in Skaha Lake. Okanagan Lake has a mean depth (maximums in parenthesis) of 76 m (242 m) while Skaha and Osoyoos (north basin only) lakes are shallower, 26 m (57 m) and 2 1 m (63 m) respectively (Pinsent et al. 1974b). In addition, one could hypothesize that mysids would play a greater role as a prey species in Skaha Lake than in Osoyoos Lake, due to the higher density of mysids in the former.  Another hypothesis is that thermal barriers restrict mysid vertical and horizontal migration and reduce access to the zooplankton in the warm epilimnion waters (Northcote 1991). The literature suggests that the 'zone of tolerance' for mysids is > 1.5 mg/L for dissolved oxygen and <15 °C for temperature (Bowles et al. 1991; McEachern 1999, M S ; Sherman et al. 1987). Mysids were found to be physiologically constrained when in epilimnetic waters above 15 °C since their gut residency time and feeding rates decrease such that it is bioenergetically costly living at these temperatures. Thus high epilimnetic temperatures create a refuge for macrozooplankton during thermal stratification (Chipps 1998; Goebel et al. 1995), although smaller mysids are more temperature tolerant than larger ones (Rudstam et al. 1999). While, mysids are found in the warm epilimnion in Okanagan Lake they may be limited to short periods of time in this area (Whall & Lasenby 1999 MS).  It was hypothesized that after Skaha Lake undergoes thermal stratification and temperatures exceed 15 °C, a thermal refuge is created that reduces the influence of mysid predation on macrozooplankton.  To answer these questions, water temperature data for Skaha and Osoyoos lakes were compiled for the period between April and November 2002. In addition, in 2002 temperature ( C) and dissolved  22  oxygen profiles (mg/L) were taken at least biweekly for both lakes from April to August and weekly between September and early October.  At each sample site, vertical profiles of temperature ( C) and dissolved oxygen (mg/L) were taken at 2 m intervals using a calibrated YSI model 52 dissolved oxygen meter that is sensitive to the nearest 0.1 mg/L.  The night distribution of mysids and nerkids within the higher temperature epilimnion was used to test the hypotheses that the upper limit of temperature tolerance for mysids is 15 °C and 17 °C for nerkids (Hyatt 2001, personal communication). The daytime distribution of these organisms was used to test the hypotheses that the lower limit of dissolved oxygen tolerance for mysids is 1.5 mg/L and for nerkids is 4 mg/L (Hyatt 2001, personal communication).  2.4.2 O. nerka/M. relicta interactions  Trawl and gill net captures served as a source to evaluate mysid influences on growth rate and the potential of mysids as a food source for nerkids in Skaha and Osoyoos lakes. Trawls were conducted with a 2x2 m net at varying depths depending on acoustic results, water temperatures, and dissolved oxygen conditions (Rankin et al. 2003). The sinking gill nets in Skaha Lake were set either at locations 4 m off the bottom or where 10-12 degrees Celsius water occurred, as identified by temperature/dissolved oxygen readings immediately north and south of the west end of the Skaha Lake transect (Figure 2). Five or six gill nets were used for each gang ranging in diagonally stretched mesh size from 2.5 cm to 14 cm. In Osoyoos Lake, gill nets were set either near the Okanagan River inlet to Osoyoos Lake or near the west end of the Osoyoos Lake transect (Figure 3). Gill nets were set prior to dusk and lifted the next morning.  Nerkids caught in the trawl samples were measured for length and weight then sent to Zootec Services for analysis of stomach contents. The stomach contents were examined for  23  presence/absence o f mysid shrimp, major food items were identified, and samples were stored i n formalin for further analysis i f required.  Nerkids caught i n gill nets were treated the same as those caught i n trawl samples except that the fish were also aged, their sex was determined, and only presence/absence o f mysids was noted from the stomach contents. W h i l e detailed stomach contents analysis was not conducted general observations on the main food items present by sample date were recorded. Length-at-age was compared between kokanee captured i n the two study lakes, as were growth rates i n sub-yearling kokanee (Skaha Lake) and sockeye (Osoyoos Lake). T o examine the importance o f mysids as a food source for nerkids, plots were made for each lake and sample date o f the percent o f sub-yearling and older nerkids with mysids i n their stomach. Lastly, to determine the extent o f dietary overlap between nerkids and mysids, the major food items noted from the nerkids stomachs were compared to reported diets o f mysids.  24  3. Results  3.1 Mysis relicta Invasion and Oncorhynchus nerka 3.1.1  Mysis relicta  Invasion  Documentation of approximately when mysids invaded the study lakes was compiled as a baseline of when possible kokanee/sockeye/zooplankton responses might have occurred (Table 2). Mysids were introduced into Okanagan Lake in 1966 with low abundances recorded in the early 1970's (Pinsent et al. 1974b). However, it is not known if they were sampled during the daytime or evening. If they were included in daytime zooplankton sampling, mysids are known to occupy the deeper waters, not accessible to zooplankton sampling methods used then. Generally, it takes 10 years for mysids to become evident in stomachs of predators when introduced into a new lake system (Lasenby et al. 1986). Nevertheless, it is known that by the 1980's with subsequent Okanagan basin implementation evaluations, high densities of mysids were observed in not only Okanagan, but also in Skaha Lake (Truscott & Kelso 1979). Mysid surveys conducted in 1989 by provincial ministry staff using vertical trawls identified a high abundance of mysids in Okanagan Lake but none were caught during numerous attempts in Osoyoos Lake. However, during evening zooplankton sampling in 1997 (Rankin et al. 1998), mysids were taken in low abundance in Osoyoos Lake, with moderate levels the next year.  25  Table 2. Mysis relicta invasion and relative abundance in Skaha and Osoyoos lakes  Lake  Mysid absent  Low abundance  Moderate abundance  High abundance  Okanagan  Before 1966  1969 to 1971  By mid 1970's  After 1980  Skaha  Before 1970 probably  1969 to 1971  By mid 1970's  1976 to 1978  Osoyoos  Before 1989  1997  2002  Not yet reached  3.1.2 Oncorhynchus nerka Spawner escapement data for Penticton channel, the main kokanee spawning area for Skaha Lake, were examined for the period following the invasion of mysids to determine whether there was a correlation between increasing mysid abundance and decreasing nerkid abundance. While, the earliest escapement data for Penticton channel is from 1971, the mysids were still in low abundance so were unlikely to have significantly influenced nerkid abundance yet. Between 1971 and 1975 escapement estimates for Penticton Channel were about 50,000 adults, dropping to 7,560 (SD 4,600) between 1989 and 2001. More recently (2002-2004) the average escapement has increased to 82,060 (SD 12,760), an order of magnitude greater than the previous 13 years. These estimates do not include the Shingle or Ellis creeks escapement numbers, which were likely about a thousand and a few hundred, respectively. Osoyoos sockeye escapement between 1953 and 2002 were highly variable, averaging 24,400 (SD 23,060) and ranging between 113,323 in 1967 to less than 2000 (escapement in parentheses) in years 1961 (1,382), 1963 (1,402), 1978 (1,932), and 1998 (2,048) (Figure 5).  26  100 in •a  c ra in 3 O  rc  o E o a.  ra o 10  90 80 70 60 50 40 -\ 30  LU  ~ <  20 10  <SK  Qs.  7  <9>  <9o  <&>  <§p <2g  <2p  <P  0  <9  Q  'iPr,  ' Q  0  "To. 'Or,  ^JL  Year  Figure 4. Kokanee escapement into Penticton Channel from Skaha L a k e from 1971-2004  cU oO cl ol lcl oO cI oO cI oO cI o. O c oI O c oI fc fol cN o c o c o O Ol S !D U Ol -vl CD ->• CO Ol  c-ov ci -ovci oo co oo oc oo occoocooo cc oo cc oo ccooccooc co o S I D - ' U C n S I D - ' U U l S l t l  1  N5 hO O O O O CO  Year  Figure 5. Osoyoos L a k e sockeye escapement to Okanagan R i v e r .  1971 escapement from (Pinsent et al. 1974a); 1972-1974 escapement estimate from hatchery broodstock collectors (Summerland Hatchery M E L P Files), and 1989-2004 from Steve Mathews fax 07-07-2003 and M E L P File 34560-20/Kokanee Spawning - Okanagan Region Kokanee Spawning Summary for 2004 Stockwell and Hyatt (2003) 2  27  For the six years when comparable data are available, the mean fall juvenile nerkid abundance has been considerably greater in Osoyoos Lake than in Skaha Lake (Table 3). Even though there was an increase by an order of magnitude in escapement in the Penticton Channel in 2002 and 2003, the subsequent autumn in-lake abundance didn't increase. In Osoyoos Lake,, when escapements of sockeye were abundant, there was a subsequent increase in the autumn in-lake abundance.  Table 3. Juvenile O.  nerka  mean abundance - autumn acoustic estimates  3  Skaha  Osoyoos  Number/Hectare  Number/Hectare  1997  -  1,000  1998  -  4,040  1999  200  300  2000  300  1,200  2001  200  3,100  2002  300  2,200  2003  400  900  2004  300  700  Lake  4  As an indication of mysid-induced effects on the nerkid population, the mean length-at-age of nerkids from the two study lakes and three other mysid-bearing lakes in British Columbia (Arrow, Kootenay, and Okanagan) were compiled (Figure 6). The 2+ aged kokanee from Skaha Lake were among the largest of this age class sampled from the five lakes, as were aged 0+ and 2+ nerkids from Osoyoos Lake, so for the study lakes, there is little indication of a mysid-induced effect on nerkid size.  From Rankin et al. (2003) and Hyatt unpublished data Includes sockeye that were reintroduced into Skaha Lake (sk/kok ratio results of 0 fish not received to date) 3  4  +  28  o Kootenay (min) • Kootenay (max) X Upper Arrow (1999) S  200  X Lower Arrow (1999) OUpper Arrow(1997)  ®  + Lower Arrow (1997) -Upper Arrow(1998) - L o w e r Arrow (1998) O S k a h a (2002) E3 O s o y o o s (2002) A O k a n a g a n (min) O O k a n a g a n (max)  2 Age  Figure 6. Mean length-at-age comparison of autumn nerkids samples between study lakes and other B.C. interior lakes.  29  3.2 Zooplankton  3.2.1 Historical changes in zooplankton There were no significant differences in the densities of cladocerans in Skaha Lake after the invasion of mysids, which does not support the hypothesis that the abundance of cladocerans would decline after the invasion of mysids (Figure 7a). A Kruskal-Wallis test showed there were no significant differences among cladoceran densities in Skaha Lake over the years with available data (P = 0.5332). In Osoyoos Lake there has been a recent decline in cladoceran density (Kruskal-Wallis test P = 0.023), even though mysid densities, their supposed predators, are lower in Osoyoos Lake than in Skaha Lake (Figure 7b). Pair-wise comparisons found significant differences between 1977 and 1978 with 2001 and 2002, and between 1969 with 2002. A comparison was also made between the densities of cladocerans in the two study lakes, with significantly more cladocerans in Skaha Lake than in Osoyoos Lake in 2002 (P = 0.0148) but not in 2001 (P = 0.1143). A recent decline was evident in copepod density of the two lakes (Figures 8a and b). A KruskalWallis test found significant differences in copepod numbers in both Skaha and Osoyoos lakes (P = 0.0111 and P = 0.0103 respectively) when samples froml969,1971,1977 and 1978 were paired with samples from 2001 and 2002. There were no statistical significant differences between Skaha and Osoyoos lakes in 2001 or 2002 (P = 0.3429 and P = 0.9591 respectively).  30  Osoyoos Lake  Skaha Lake 25  I  25  20 -  |  20  15 -  01  10 5  8.2  10.7  1  m 11.3 LP 5.6  5.5  5.5  |  10  J  5  I  0 1969  1971  1977  1978  1969  D  CM  1971  1977  Is/c  f| 175.5  200 -  XI  150 100 -  «s I)  150 116.0  >.  m 53.5  50  45.9  0 1969  1971  1977  1978  2001  2001  i  7  2002  * 153.3  •>  100 ^  1978  m  250 -j  (Indi  E qj 190.7  m2.7  Figure 7b. Cladoceran density from available years between 1969 to 2002.  m 248.4  200  m 5.9  Year  Figure 7a. Cladoceran density from available years between 1969 to 2002.  250 -,  m 12.5 m 8.3  0  o  2002  2001  • Year  m 16.3  15  "Hi  c CD Q  86.6 * 87.5  70.1  50 -  : 35.4  I 42.5  0 -  2002  1969  1971  1977  1978  2001  2002  Year  Year Figure 8a. Copepod density from available years between 1969 to 2002.  (note: error bars one SD)  Figure 8b. Copepod density from available years between 1969 to 2002.  3.2.2 Species changes in percent composition  To see if there have been species changes in the zooplankton communities for cladocerans and copepods in the two study lakes, a comparison was made between their percent composition in zooplankton samples from late-August 2003 and late-summer 1969 and 1971. Separate comparisons were made for each lake.  There have been changes in the species composition of cladocerans in Skaha Lake, with Daphnia longerimus and D. pulex only observed in the 1969 and 1971 samples (Figure 9). The cladoceran community is now (2003) dominated by three of the smaller species, D. thorata, Bosmina longirostris and Diaphanosoma leuchetenbergianum. The samples indicate that there was a greater diversity of cladocerans in 1969 and 1971 (seven species present) than in 2003 (three species present). Changes in the copepod community are less dramatic, with no species losses, only relative shifts in abundance. The samples indicate that between 1969/1971 and 2003 the relative abundance of Diaptomus ashlandi has decreased compared to Cyclops bicuspidatus thomasi (Figure 10).  In Osoyoos Lake, the cladocerans community appears to have lost Daphnia longerimus and D. pulex since the 1969 and 1971 samples since none were present in the 2003 samples (Figure 11). However, in all three sample years the dominant zooplankton species was the cladoceran Diaphanosoma leuchetenbergianum one of the smaller species. There were no obvious differences in the copepod community among the years with adequate data (Figure 12).  32  8  01969 111971 o 55 C/3  O  • 2003  5  o U  cw o  OH '  1 0  %  X  4  %  4  % Zooplankton  Figure 9. Percent composition for cladoceran species in Skaha Lake (1969,1971, 2003)  UJ  45  I  I  40  B1969  35  •  1971  • 2003  3 0  25 o ° 20 o 15  i I  10  t  5 0  i  fef; JS2  ~lft.-"i.". JJi)  A: c  4  % 6y  V  v  v  X  4  Zooplankton Figure 10. Percent composition for copepod species i n Skaha Lake (1969, 1971, 2003). -1^  8  c o  01969  St o 5 o a, o o i-  B 1971  • 2003  -*-»  S3  o J !-i  CU  1  0 A  •to.  %  4  o.  %  zooplankton Figure 11. Percent composition for cladoceran species in Osoyoos Lake (1969,1971,  2003)  45 40 a o £ 30  H 1969  |  25  • 1971  o ° 20  • 2003  3  5  -4—»  C  0)  i  2  1 5  a  10 5  <u  0  I  4,  4-  <0  J  V X  2>o  4:  X  zooplankton Figure 12. Percent composition for copepod species i n Osoyoos Lake (1969, 1.971,2003).  Co  3.2.3 Seasonal changes The timing of the annual cladoceran pulse was compared between Skaha and Osoyoos lakes in 2001 and 2002, with the pulse appearing about four weeks later in Skaha Lake than in Osoyoos Lake in 2001 and about six weeks later in 2002. The abundance of Daphnia species in Skaha Lake did not increase noticeably until late-June in both 2001 and 2002 (Figures 13a,b). The peak abundance of  Daphnia  in Skaha Lake occurred by the  middle of August in both years, with numbers falling to very low levels by early-October. However, the amount of within season variability was high so the Kruskal-Wallis ANOVA test found no significant differences in Daphnia abundance between seasons in either 2001 (P = 0.66) or 2002 (P = 0.62). The peak abundance of  Daphnia  in Osoyoos Lake occurred between late-May (2001) and late-July  (2002) after which they declined to relatively low levels for the remainder of the year (Figures 13c, d). A Kruskal-Wallis ANOVA test found no significant differences in Daphnia abundance between seasons in 2001 (P = 0.074). However, a significant difference was found between seasonal samples in 2002 (P = 0.016). Pairwise comparisons showed significant differences between the July 9 and July 23 samples with all other samples. Thus, as previously mentioned, the seasonal Daphnia pulse in Skaha Lake (August; although not significant) occurred about a month later on average than in Osoyoos Lake (late July). In addition, the peak density of Daphnia in Skaha Lake was greater than in Osoyoos Lake for 2001, but densities were similar in the two lakes in 2002.  37  Skaha Lake 120000 100000 E  80000 -J  J2 E  60000  Z  40000  J3<  /  A Q—  20000 0  -B—  43^-  -B"O  Date Figure 13a. 2001 Daphnia Seasonal Trend. 120000 100000 80000 E 3  60000 40000 20000  ^ r ^ ^ . e-  1 \  X  Date Figure 13b. 2002 Daphnia Seasonal Trend.  Note: error bars one S D  \  \  O s o y o o s Lake  -BDate Figure 13c. 2001 Daphnia Seasonal Trend. 120000 100000 80000 J§ 60000 40000 20000 •  t=|-  X Date Figure 13d. 2002 Daphnia Seasonal Trend.  Note: error bars one S D  -B-  Bosmina  densities were not significantly different between the two lakes in either 2001 or 2002.  Bosmina  in Skaha Lake showed two peaks in abundance during the 2001 growing season and  multiple peaks in 2002 (Figure 14 a, b), although the peaks are not significant Kruskal-Wallis ANOVA tests of P = 0.35 and P = 0.80 respectively). In 2001, the peak densities in Skaha Lake were on July 19 and November 29, but, in 2002, peak densities were observed on June 18, August 8, and slightly on October 17, with peaks decreasing in magnitude as the year progressed. In Osoyoos Lake, the Bosmina peak densities in 2001 occurred on June 24 and October 17 (Figure 14c, Kruskal-Wallis ANOVA test of P = 0.24) but, in 2002, a major peak for Bosmina occurred on June 18, followed by secondary peaks October 3, and November 14 (Figure 14d, Kruskal-Wallis ANOVA test P = 0.054). In summary, in 2001 and 2002, the initial peak in Bosmina density was approximately four weeks later in Skaha Lake than in Osoyoos Lake, which supports the hypothesis that mysids delay the bosminid pulse in that lake. However, the same phenomenon was not observed in 2002 when initial peak densities in both lakes occurred in mid-June.  40  S k a h a Lake 80000 60000 40000 20000 0  —03-EH  %  IS/  Date Figure 14a. 2001 Bosmina Seasonal Trend.  80000 60000 £ 40000 E 2  20000  1  V ""J  Date  \  w  w  U  %  \  ~<*  Figure 14b. 2002 Bosmina Seasonal Trend.  Note: error bars one S D  C>  S  O s o y o o s Lake 80000 60000 E  40000 20000 0  vs. Date Figure 14c. 2001 Bosmina Seasonal Trend.  80000 i 60000  20000 -\  \ %  <fr  V  V  %  \o  Tb  Date Figure 14d. 2002 Bosmina Seasonal Trend.  4^  to Note: error bars one S D  ^  3? \S>  %  YD  >  3.3 When and Where Mysids and Nerkids Interact  3.3.1 Nerkid recruitment to Skaha and Osoyoos lakes The 25%, 50%, 75% and 100% sockeye emergence dates for Okanagan sockeye in 2002 were estimated to have occurred on April 8th,April 11th, April 15th and May 20th respectively (Lawrence 2003). This corresponds well with the timing of sockeye fry that were seined in the littoral zone of Osoyoos Lake (Table 4). Table 4: Beach seine summary for littoral presence of sockeye fry in 2002 for Osoyoos Lake.  Date  Temp (°C)*  Date  Site  Number per 100 m  April 10  5.7  April 8  7  2.3  April 10  5.7  April 8  8  8.3  May 8  10.8  May 3  7  1.0  May 8  10.8  May 3  8  8.3  May 22  13.4  May 17  7  7.0  June 6  13.1  June 3  8  5.7  June 10  16.3  June 14  7,8  0  2  ^surface temperature After peak emergence sockeye fry were found in the littoral zone of Osoyoos Lake. However, as temperatures increased from 13.1 and 16.3 °C between May 22, 2002 and June 14, 2002, juvenile sockeye moved offshore into the pelagic area of the lake.  43  3.3.2 Mysis relicta abundance and size  In Skaha Lake, between 1999 and 2002, mysid abundance ranged from a high of 151.8 /m in 2000 to 2  a low of 43.5 /m in 2001 (Table 5). Between 1998 and 2002 mysid abundances in Osoyoos Lake 2  have varied from a low of 9.6 /m in 1999 to a high of 35.5 /m in 2001 (Table 5). 2  2  Table 5. Summary of M. relicta densities in Skaha and Osoyoos lakes . 5  Lake  Skaha . Number /m  Osoyoos 2  Number/m  1997  -  present  1998  -  22.0  1999  89.5  9.6  151.8  32.0  2001  43.5  35.5  2002  140.7  10.5  2000  r  2  Mysid densities by DFO sampling at 10 sites were highly variable (Figure 15 a to i) and suggest the O N A F D collected samples from two sites may not adequately reflect mysid population abundance in the entire lakes.  Size frequency histograms for mysids in Skaha (Figures 16 and 17) and Osoyoos (Figure 18 and 19) lakes in 2001 and 2002 indicate that mysids in these lakes have a one-year life cycle. This is further supported by the high seasonal (autumn) abundance of gravid females in both lakes (Figure 20 and 22), which suggests that they release their young in spring and early summer and then die. The density of juvenile mysids peaked during spring/early summer, with mysids reaching maturity by late- summer (Figures 21 and 23).  5  Rankin et ai. (2003) and Hyatt unpublished data  44  Skaha Lake (June 15, 2001)  Stn. 1  Figure 15a  Figure 15b Skaha Lake (November 29, 2001) 300  -I  250 200 in  §5 100  Q  Site Figure 15c  Figure 15a-i. Density distributions for Mysis  relicta  in Skaha  and Osoyoos lakes from D F O sampling in 2001 and 2002.  Figure 15e Osoyoos Lake (November 27, 2001) 300 250 CM  E  200  %  150  Qcu  100 -  in c  50 0  Stn. 1  5  10  6  Site  Figure 15f Figure 15a-i. Density distributions for Mysis  relicta  in Skaha and  Osoyoos lakes from D F O sampling in 2001 and 2002.  Osoyoos Lake (August 27, 2002) 300 250  10  Figure 15g Osoyoos Lake (October 30, 2002) 300  N  250  °*E 5.  200  £ . 150  I Q  100 50 0  Stn. 1  3  4  5  6  7  8  9  10  8  9  10  Site Figure 15h Osoyoos Lake (November 25, 2002) 300  N  250 "E 200 i * in  § Q  1  5  0  100 50 0 r —  Stn. 1  3  4  5  6  7  Site Figure 15i  Figure 15a-i. Density distributions for Mysis relicta in Skaha and Osoyoos lakes from D F O sampling in 2001 and 2002.  47  800 600 600 400  400  A  200  200  £ CS  &  &  &  •$>  **  y  &  \*" A  *  <£>  J  ^  N  >  *°  %  >y  KC>  V  A  K  CD  A  ,<f  -«—>  cu  Figure 16a. May  Figure 16b. June  S-l  cu &, S-i CU  600  H  600  400  H  400  (Z3 CU  200  200  Q  0  i>  k.""P  ifl *  *  -$>  ifl •CV  • A  K<0 «  « N*>  <$> <a  <p  0  «  ^ <<*  Figure 16c. July  Size (mm) Figure 16a-h. Skaha Lake 2001 Mysis relicta size frequency histograms.  00  or  <V  xp  v  0  <b  t>  «p  <o  <y  %  >  C)  N  &  .V  Figure 16d. August  Size (mm)  -t>> &  -e>  ,<S  T3 <D  fe  600  600 ^  400  400  200  200 0  0 /?>  ^  <V«  N >  **  „ *  *  *  *  *  ^  n?  ^  .<£ /  r>  ^  **  ~  \*"  %  * *  y  O?  y  ^  >  S-H  (U  •t->  Figure 16f. October  Figure 16e. September S-H  t-i  600  600 CO  a Q  400  400  H  200  200 0 ^  ,>  ^  ~*  •0'  N  6 ,  v ^  N  •?>  ^  n?  ^  %" <r  * *  <V  *  J°  I?  %  ^  A  J  .>  J  N  Figure 16g. November  S i z e (mm) Figure 16a-h. Skaha Lake 2001 Mysis relicta size frequency histograms.  Figure 16h. seasonal average  S i z e (mm)  ^>  ^  <b  J>  ^  J?  300  300  200  200  100  100  CD  %  «S>  CO  fe  „«P  fe  „«P  =t> ^ fe^ ^ fe^ fe« „•  <b „"P  <a  k"P  J  <V ^ ^ ^ .<»  >  fe  Figure 17a. April 11, 2002  Figure 17b. April 24, 2002  fe OH  fe  a CO  C  CD  Q  300  300  200  200  100  100 0  0 <V  «p fe  fe  «p V  fe  «p fe  <b >$>& «p  »p io  y  \»  *p  \«  ,p  ^  >p  ^  «p  Figure 17c. May 9, 2002  Size (mm) Figure 17a-p. Skaha Lake 2002 Mysis relicta size frequency histograms. o  ^  >  •O'  N  v  ^  •  #fe*fe*„* * * * * ^ N  Figure 17d. May 22, 2002  Size (mm)  /  300  300  200 •  200  100 -  100  T3 <L>  fe  cr  1'''»"'"I J  Co*P  *P V  *P &•  co l-l  fi CD Q  *  <l>  N  »>  >  ,<f  ^  >  •O' N  •  V  <V*  **  „*•  - •  *y  - •  \ k  * •  - '  < iy  \ ^  0?  * '  »<y  Figure 17f. June 18, 2002  Figure 17e. June 6, 2002  <D CL)  co  '  0  300  300  200  200  100  100 0  0 N  <b  fc Co  *S>  <b >°  9>  <V ^ °  A  s!> „«P  ^> ,A°  »t>  T-  ^> ,v°  ^  ^ °  Figure 17g. July 9, 2002  Size (mm) Figure 17a-p. Skaha Lake 2002 Mysis relicta size frequency histograms.  0*  fc  <b  fc*  %  Q  <W  ' >$>  „*• .„* <b <S <V fc N  N  N  Figure 17h. July 22, 2002  Size (mm)  fe  N  «?>  ^  <&  4  ^  S  T3 CD  % cy  300  300  200  200  100 -]  100 0 4  0 <V fe*  ^  fc  fe fc*  % fe*  <V yfe N  J <V  fc  *fe  N  N  fc  fe  N  N  fe  ^  N  <$ «"  fe  fe  <V  <}.  >  fe*  fc <v  fc*  fe fe*  <V  ^5 ^  *  °  ^  fe  N  fc  »fe  N  „<s>  N  v  „ > °  fc  fe  N  N  fe  N  ,£>  -j.  A  nfe  CU  Figure 17j. August 20,2002  Figure 17i. August 8, 2002 OH i-i  tu  s CO  C  cu  300  300  200  200  100 -|  100  P  (ZZt  0 .«p  „>pfc  fe  ,<>fe  *  fe  - •  K  fe  V  ^  N  ^  N*  f e  A  f  A  ^  .of  Figure 17k. September 5, 2002  Size (mm) Figure 17a-p. Skaha Lake 2002 Mysis relicta size frequency histograms.  <V fe*  fc fe fe fe <W <V* fc* fe* „ * ^ N  N  *  fc  N  fe  *  N  fe  <,  Figure 171. September 20, 2002  Size (mm)  nfe  J>  f , <e  300  300  200  200  fe t-c  100  100  0  0  *p fe  >p V  >p fc  *p fe  "p >  „"p ^  *p <i*  ^"p N  fc  *p V  >>p \*~  *p  <i> ^ fe* ^ <f  * fe^ ^  N  Figure 17n. October 17, 2002  Figure 17m. October 3, 2002 i-l  OH  fe fi in fi D  Q  300 ^  300  200  200  100  100 0  0 <V fc fe fe ,» «p «p *p «p <5 V fc^ fe a.* ' * ^ v  5  N  A ^ <n W  0  f>°  g > ° sfe  •o*> °  .4<?*  Figure 17o. November 14, 2002  Size (mm) LU LO  Figure 17a-p. Skaha Lake 2002 Mysis relicta sizefrequencyhistograms.  /y  fe*  r>  0*  J> f>  fc*  N"  •*.„<»  ^  f  > °  Figure 17p. Yearly Average  Size (mm)  > °  N  S-H  200 160 120 80 40  200 160 120 80 40 0  a  &  *p  »p  S-i  ,o  .o  <b  .*  & o>° rA° b>N > ° < b Kcs* Q> K> N  ^ o>°  G > °  ^5  jy  ^  <§  J*  J°  n>°  J° ^°  >$> o .o  N  ^\vo\vO  >$> & N  V  ^  <P e  ^  &  tu CU  Figure 18a. May  CU  Figure 18b. June  OH  <u fi g C/3  <u Q  200 160 120 80 40 0  200 160 120 80 40 0 nP  y  >  x p  p xP Jb < Jb „ &p » ^p * p <b ^ ^ > ^  $  *0  #  K  •& ^ < f r  <  ^°  y  nf  j*  V  Figure 18c. July  Size (mm) Figure 18a-h. Osoyoos Lake 2001 Mysis  J°  J°  a?  >$> & >> & >$> ^ JV _<P KO .O , C - .o' ^ <b ^ »5V »> .«  Figure 18d. August  Size (mm) relicta  size frequency histograms.  are  200 160 120 80 40 0  200 160 120 80 40 0  r*  *P  N*  &  ^  &  1-1  a  ^  <?  >  f  j>  _«>  f  Figure 18f. October  Figure 18e. September  1-1  ^  ** „* ^  (-1  S >,  •«->  De  a  200 160 120 80 40 0  200 160 120 80 40 0  T>  *  <b  <l  Q  „>P .«P «P  N  r  >  <V  rJ"? K£ ^  N  *  fc  N  %  N  ^>  K.^ K> NS>  ^  N.^  &  Figure 18h. seasonal average  Figure 18g. November  Size (mm) Figure 18a-h. Osoyoos Lake 2001 Mysis  <V  Size (mm) relicta  sizefrequencyhistograms.  f  f  T3  CD  300 -  300  200 -  200  100 -  100 0  0 --  c3 <J  \"n * \"s.^\  V  <0P  ? N  (-1  O  <V  0  *fc  *b  N  %  0  N  f> f> <o ° ^  T>°  <£  .0  CD  fy  \° <S? ^ r'P a." <®  K  \  &  %  ^ ° «> ^ 0  >P ^  < /  s  "CD  a  Figure. 19b. April 24, 2002  Figure 19a. April 11, 2002  CD  d, S-i  CD  v  aCP  — '  >>  • ,—4  De  CI  300 -  300  200  200  100  100  0 ^  _j  ^  N  *  «* >* v* •*  „*  \  0  j—  0  ^  ^  *<y ^  K<£  Figure 19c. May 9, 2002  Size (mm) Figure 19a-p. Osoyoos Lake 2002 Mysis relicta size frequency histograms.  /V  *-°  fc  *0  S>  \V  »* . * v v v > v y \fc  Figure 19d. May 22, 2002  Size (mm)  C$>  ^  300  300  200 -\  200  100  100  0  0  T3 »-< Kt  .'V  CT tn  1  -  %  NV  N* 1  \*  \  %  f  J\  ^  <t>°  •O' \^  J? J°  y  \N  ^o ^o ^o N  N  F E  r>  ^  o  N N  93  ^  T?  ^  tU  Figure 19f. June 18, 2002  Figure 19e. J u n e 6, 2002 tU CM S-I  tu  e  C  <U  Q  300 -\  300 ^  200  200  100  100  0  0 >  Figure 19g. July 9, 2002  Size (mm) Figure 19a-p. Osoyoos Lake 2002 Mysis relicta size frequency histograms.  _<b  _9>  ,$>  ^  Figure 19h. July  Size (mm)  N  fc  22, 2002  K%  ^  T3 CD Si  300  300  200  200  100  100 j_  0  ^  Si  CD CD  lo* <?  J fc*  6  WO*'  ^  **  ^  ^  ^  ^  *  ^  ^  ^  ^  KO^  /  ^  ^  <V* fc* *  Figure 19i. August 8, 2002  Si  KO*  ^  „* ^  ^  ^  ^  ^  ^  ^  A  *  *  Figure 19j. August 20, 2002  CD  OH SH  CD  a 300 CO  C  CD  Q  300  H  200  200  100  100  0  0  4-  J  6  <V*  fc*  J>  <b*  N °  <y *  & r>  N *  o  &  ^o"  J'  ^  Figure 19k. September 5, 2002  Size (mm) Figure 19a-p. Osoyoos Lake 2002 Mysis relicta size frequency histograms.  <V fc <b v? ><p <o* O V fc <b  ^  <y \*  \  fe  <v* fc* N  Figure 191. September 20, 2002  Size (mm)  \% <$> ^ </  300  300  200  200  100  100  0 !-l  v?'  cr  V  o  KO  N  V  0 wo*  1? xp* J> J* N° ^ ^ ^ # ^ x? X? xp x? & fc ' <bxO ^KO <l> „fc <b <t> c f N  (V.^ fe  v  v  N  N  v  J>  xO*  $  &  *  N  $  %  &  ^  N  co  Figure 19m. October 3, 2002  cu  Figure 19n. October 17, 2002  a S-i  CU  a S-i  cu  a  CO  d cu  Q  300  300 200 100 0  200 100  J>  J?  «P  *P v  *P  0  xP*  N°  HP  o <  ^ xp  ^  ^  xO  b p s ? < V  xO N  xO  $ xO  JA  ^  f>  ^ ° <v* ^°  J>  ^  *P  N> x?  N  \ * xO  N  xO  ^  xO  N  n?  xp*  f c ^ > ^ . < ^ r0>  Figure 19o. November 14, 2002  Size (mm) Figure 19a-p. Osoyoos Lake 2002 Afysz's relicta size frequency histograms.  Figure 19p. April - November Yearly Average  Size (mm)  x<*  f  f= 30  1  25  CD  «  2 0  >• 15 "5 CD 10  Nov  CM" CN  2002  Sept  CM  CM O O CM  IS? CN  Is."  June  July  CN  June 7, 2002  o"  CN  2002  2001  o  May  2001  o>  2002  2001  CM  April  2001 |s"  May  = illll  >, —3  CN O O CN  ai .  <  Date  CN O o CN CN CM O)  <  Figure 20. Skaha Lake gravid M. relicta densities for 2001 and 2002.  error bars one SD no error bar means one measurement  60  CM  14  E 12 |  10  ^ "5  6  5> 4  i  2 m  co  re eg  3  O"  I--"  CM  | C M  CM  CO  z  CM CM  CN  c  o  Q. CN <  g  ~,  re  CM  co" CN CD O C 3  O CM  m"  CN CN  £ CM > . § 3 CM  O) o  lO CN  m  0) fN  2  CO  Date  s  a.  fc ™  o  CD CN CO  Figure 22. Osoyoos Lake gravid M. relicta densities for 2001 and 2002.  CN  200  E  a 160  I » 120  Z  80  40  in  oo" >.  CM  o  c  ™ CM  CO  z  g o  D. CN <  g  C CM N ~. " ! CM  CO J M CD O C 3  O CM  CN  g  T-  co §  >-  E CN  3  g •8*8 o  CN  O)  3"  o  lO CN  in  CM CM  ° Q. O CD CN  O CN  CO  Date  Figure 23. Osoyoos Lake released juvenile M. relicta densities for 2001 and 2002.  error bars one SD no error bar means one measurement  61  In terms of weighted mean lengths (Figure 24), in Skaha Lake mysids were significantly longer each 6  month than in the previous month, except for August, based on a two-sample t-test assuming equal variances (P values sequentially from May to November were 0.01,0.001, 0.001, 0.10, 0.05,0.01, and 0.03). The mysids in Skaha Lake grew from 6.95 mm in April to 14.21 mm in July, and then only added another 1.6 mm, to 15.81 mm, in the subsequent four months. In contrast, mysids in Osoyoos Lake grew more slowly, increasing from 4.17 mm in May to 12.49 mm in August, before decreasing slightly to 11.94 mm in mean length for September, and then increasing to 14.82 mm in mean length for November.  4 -  2 0 Apr  May  June  July  Aug  Sept  Oct  Nov  Dec  Month  Figure 24. Mysis  relicta  weighted mean lengths (April-November 2002) in Skaha Lake and north  basin of Osoyoos Lake.  Weighted mean is the adjusted mean based on proportion of the mysid life history type (juvenile, male and female juvenile, adult female, gravid female, released female, and adult male). E.g. the arithmetic mean of juveniles at 6mm and adult males at 20 mm is 13 mm. But there were 4 juveniles and only 1 adult male: Thus the weighted mean is (6+6+6+6+20)/5 = 8.8 mm  62  3.3.3 Nerkid/mysid diel vertical migration monitoring  In order to better understand the potential interactions between mysids and nerkids, hydroacoustic surveys were conducted in both lakes to monitor diel vertical migrations of mysids and nerkids throughout the growing season (May to November). Hydroacoustic survey results indicate that mysids underwent vertical diel migrations (Figure 25). They remained on or near the bottom during daylight hours, migrating up into the upper parts of the lake near sunset, and returning towards the bottom prior to dawn. During the April survey, it took about 30 minutes for mysids to migrate from the lake bottom to about 10 m below the lake surface. The echosounder was used in a stationary position from prior to dusk until after dark to observe mysid migration.  There were seasonal and between lake differences in the vertical diel migration of mysids. As the season progressed, the daytime uppermost limit of their scattering layer was observed higher up in the water column (i.e. off the bottom) in Osoyoos Lake September 27. For the daytime surveys from April to August 19 in Osoyoos Lake, the uppermost limit of the mysid scattering layer was consistently about 10 m off the lake bottom. During the August 27 and September 25 surveys the uppermost scattering layer was about 15 m off the bottom. This is in contrast to that of Skaha Lake where the mysid layer was consistently on or near the lake bottom, except for the September 25 survey when little diel migration was observed (Figure 25).  Concentrations of nerkids in Skaha Lake could be identified on five of the seven survey dates (May 29, June 27, July 17, August 21, and September 27; as shown in Figure 25. On these five survey dates, 16 out of the 32 sampling intervals had concentrations of nerkids, of which 12 overlapped with the mysid layer. A l l of the observations for concentrations of nerkids overlapping with mysids were made during the evening surveys. The four times that concentrations of nerkids were observed but did not overlap with mysids were from daytime observations.  63  Concentrations of nerkids in Osoyoos Lake could be identified during all survey dates (Figures 25). During 35 of the 47 surveys where concentrations of nerkids were observed, 28 of them (80%) overlapped with the mysid layer.  In summary, nerkids and mysids occur near each other in both lakes, but more commonly so in Osoyoos than in Skaha Lake.  64  Osoyoos Lake  Skaha Lake April 23  April 25  Time (24hrs)  Time (24hrs) K  0 5 10-] 15 20 25 30 H 35 40 45 50  N  v  <P  #  <b-  A-  <v-  of*  ^  °>-  N  «P ^ * rjv  I •  t  I  J_  Figure 25a  K  v  A-  A  -  May 29  June 4  Time (24hrs)  Time (24hrs)  «3-  V  Figure 25b Figure 25. Diel vertical migration of mysids and nerkid concentrations in Skaha and Osoyoos lakes  0\  1 diamonds are upper mysid scattering layer at survey time, rectangles areas are nerkid concentrations, and dashed lines are sunset/sunrise times  v  ^  *  <y  *  ,.vf  <o-  *  ..<y  V  99  Upper level of mysid scattering layer and nerkid concentrations in meters  g  co' CD  CO CD  CD N> O  L9  Upper level of mysid scattering layer and nerkid concentrations in meters  CD OI  CD  3-  3  i £ 1  01  a  CD  CD  CD  aCL  CL  CQ  89  Upper level of mysid scattering layer and nerkid concentrations in mete TI CO c  to'  M OI  lo OI CO  s  O  TI c  s  CD  3  <Q CO  o' 3  3 o 3 O  Tt  a  3. 3  CO  o  3 •< (fl  d  ci  3  w  CD  "S  CO 3  Q.  #  3  w  CD  aci  5  i o  w  j» o V) 7? CO 3*  co CO 3  Q.  O  C O  o o o  tn  67  ?r CD (/> N> O O ho  (  O  (A O ><  o o  3.3.4 Nerkid/mysid zone of tolerance The literature suggests that the upper limit of temperature tolerance for mysids and nerkids is 15 °C and 17 °C respectively, and the lower limit of dissolved oxygen tolerance is 1.5 mg/L and 4 mg/L respectively. These limits were compared to observed locations of these organisms in relation to temperature and dissolved oxygen levels to determine whether the suggested "zone of tolerance" was supported. Mysids observed in Skaha Lake during the April and May diel surveys did not appear to be influenced by temperature, as they were not observed in the upper 12-15 m even though temperatures were suitable (Figure 26). During the June 27, July 17, and August 21, 30 surveys surface waters were warmer than 15 °C and mysids were not observed in these warm water; however during the September 27 survey mysids were observed as shallow as 5 m, even though water was above 15 °C to a depth of 18 m. All nerkids distinguishable during the surveys in Skaha Lake were in waters cooler than 17 °C except during the August 21 survey, when nerkids were observed as shallow as 6 m even though water temperatures were above 17 °C to a depth of 14 m. Dissolved oxygen levels were above the identified threshold levels for both organisms during all surveys in Skaha Lake. Mysids observed in Osoyoos Lake were generally observed in water that was less than 15 °C, except for during the August 30 survey when mysids were concentrated at a depth of 12 m even though water above 15 °C extended to 16 m (Figure 27). Dissolved oxygen levels throughout the lake were above 1.5 mg/L throughout the survey period, not dropping below this level until mid-October. However, even though dissolved oxygen level were above 1.5 mg/L at the bottom of the lake mysids were concentrated above the bottom during the day suggesting that the preferred dissolved concentration is greater than 1.5 mg/L. Nerkids in Osoyoos Lake were generally found in water cooler than 17 °C except during the August 27 survey when fish were observed at a depth of about 9 m even though water temperatures were above 17 °C to a depth of 14 m. Nerkids in Osoyoos Lake  69  were generally concentrated in water with more than 4 mg/L of dissolved oxygen, except for during the September survey when nerkids were concentrated in water with less than 4 mg/L of oxygen. In September the bottom 20 m of the lake had less dissolved oxygen than the identified threshold.  70  Temp. (C)/Oxygen (mg/L)  0 4 8 12 16 20 E . 24 .c Q. 28 <B D 32 -  —A  Temp (17 0C)-O. nerka  —*<  DO (4 mg/L)-0.nerka  36 • -c- - -Temp (150C)-mysid  40 44 -  • O- - -DO (1.5mg/L)-mysid  48 52 -  B B I B I B I I 8 E I E I E I E I B ] I 8 E ) E ) B I E I B I B I  (a-B-fi) HI B l B l H H B l 13  Figure 26. Zone of tolerance for nerkids and mysids for Skaha Lake 2002. Temp. (C)/Oxygen (mg/L)  -K •  DO (4 mg/L)-0. nerka - -Temp (150C)-mysid  ' O- - -D0(1.5mg/L)-mysid B I B I B I i a B I B I B I B I B I B I E I B I B I  A-B-B-B-B-B-El  Figure 27. Zone of tolerance for nerkids and mysids for Osoyoos Lake 2002.  13  3.4 Predator-prey interaction between nerkids and mysids Stomach contents were sampled from 145 nerkids in Osoyoos Lake and 112 nerkids in Skaha Lake to determine whether mysids were present in the stomachs. The nerkids sampled were from a range of size classes. In addition, nerkids longer than 90 mm were aged using otoliths, while smaller fish were considered to be age 0+.  Stomach content samples were collected from age 0+ nerkids from Skaha Lake in July (2004), October (2002), November (2002), and February (2003), with 5%, 21%, 15%, and 15% of the samples containing mysids, respectively. These results suggest that as early as July age 0+ nerkids in Skaha Lake begin consuming mysids.  Stomach content samples were collected from age 0+ nerkids from Osoyoos Lake each month between August and November and in February. Mysids were present in the August (5 %) and the October 18 (17 %) and 30 (30 %) gut samples but not those from June 3 or 25, September, November, or February (Figure 28).  72  35  • Skaha  -I  • Osoyoos  30 2  25 •  "5 20 i_  17  5 15 ~s 10 •  5 0 -  21 •  15  •  15  5  -ifl-BO-f-fl—I  rB-0-  14-Apr- 24-May- 3-Jul-02 12-Aug- 21-Sep- 31-Oct02 02 02 02 02  10-Dec02  19-Jan- 28-Feb- 9-Apr-03 03 03  Date  Figure 28. Percent frequency of M. relicta in O. nerka stomachs (0 ) from August to February for +  Skaha and Osoyoos lakes. The lengths of age 0+ fry from Osoyoos Lake were measured from each of the three to five samples collected each month between June and November 2002 (Figure 29). However, only three samples were collected from Skaha Lake (August 30, November 1, and November 27, 2002). Nevertheless, nerkids in both lakes were of similar size so it is likely that the growth rate and pattern of nerkids in Skaha Lake would follow that in Osoyoos Lake. Newly emergent sockeye generally arrive in Osoyoos Lake in April/May (Hyatt & Rankin 1999; Lawrence 2003) at which time most mysids are adults between 14 mm and 20 mm long, with juvenile mysids in the 4-6 mm range available in mid to late May (Figures 16-19). Based on diet analysis, utilization of myids by nerkids occurs when sockeye are about 60-70 mm long and juvenile mysids are about 10-12 mm long.  73  90 80  E,  70 60 -  4  1)50§  40  [  9$  ID  Q  • Osoyoos  30-  S  ^  2 0  10  ,  XSkaha ho C 3  ho  >  W CD  c  en CD  O ,  o  co o O  o  6 o  Date  Figure 29. O. nerka (0+) lengths for Osoyoos and Skaha lakes 2002.  A l l 1+ and older age classes of nerkid in both lakes ingested mysids (Figure 30). Stomach content samples were obtained from 1+ and older nerkids from Osoyoos Lake in July and September, and mysids were found in 13% and 14% of the fish sampled respectively. The stomach contents from age 1+ and older nerkids from Skaha Lake were also sampled (August 21 and 30 and November), with the percent offish containing mysids increasing between August samples (13% to 73%) and remaining high (65 %) in November. This suggests that in August, older age classes of Skaha nerkid began to use mysids as a food source.  74  O  70  73  O  65  O Skaha • Osoyoos 20 H 10  J  •  O  13  •  13  14  0 4— 3-Jul  23-Jul  12-Aug  1-Sep  21-Sep  11-Oct  31-Oct  20-Nov  Date  Figure 30. Percent frequency of M.  relicta  in O.  nerka  stomach samples (1+ and older) from July to  November 2002 for Skaha and Osoyoos lakes.  In addition to checking for presence/absence of mysids in nerkid stomachs, other major food items were documented to compare nerkid and mysid diets (from other lake systems) (Table 6). Samples were separated into 0+ and older age classes of nerkids.  For 0+ nerkids from Skaha Lake, the main food items in October were cyclopoids, changing to  Epischura  and  Diaptomus  February. For 0+ nerkids from Osoyoos Lake,  in November, and,  Epischura  samples from August and September, changing to  and  Diaptomus  Daphnia  Diaptomus  Daphnia,  Diaptomus,  and  and cyclopoids in  were the main food items in  and cyclopoids in October and  November, Daphnia, Diaptomus and Cyclopoids in February.  For older age classes of nerkids, the major food items consumed in Osoyoos Lake in July and September were  Daphnia  and  Epischura.  For older nerkids in Skaha Lake,  Daphnia  was the major  food item for August, changing to Diaptomus and Epischura by October.  75  Table 6. Main and secondary food items used by nerkids in the two study lakes.  0+ nerkids  Skaha  Osoyoos  Older nerkids  Skaha Osoyoos  Month  October November February August September October November February Date  Mean Length (mm) Main food item  72.3 74.7 76.3 71 79.5 76.5 76.3 75.6  August Illllllll October July llSllltf September  Daphnia. Diaptomus,  Secondary food item  cyclopoids  Epischura and Diaptomus Diaptomus and cyclopoids Epishchura & Daphnia Daphnia Diaptomus  and cyclopoids  cyclopoids Daphnia, Diaptomus,  Main food item  cyclopoids  Daphnia Diaptomus and Epischura Daphnia and Epischura Daphnia and  Epischura  Epischura  cyclopoids -  insects and cyclopoids Epischura Epischura  calanoids and  Daphnia  ^B^SllBllJB^BliiiliSlSllH -  In summary, nerkids from both Skaha and Osoyoos lakes only rarely include mysids in their diet between April and September, although consumption increases for the late fall in both lakes and through the winter in Skaha Lake. Older nerkids exhibit a higher frequency of mysid inclusion in their diets at all times of the year than 0+ nerkids. These results suggest that consumption of mysids by nerkids is controlled by the sizes of each group and by seasonal availability.  There is likely dietary overlap between nerkids and mysids in both study lakes. In Skaha Lake, cladocerans and Copepods were the major food item for the fall and copepods in February for 0+ nerkids. The major food items for 0+ nerkids in Osoyoos Lake were cladocerans in the summer, copepods in the fall, and cladocerans and copepods in February. Major food items for older age class nerkids in the summer were cladocerans in both lakes, and cladocerans and copepods during the fall in Skaha Lake. Since mysids prefer cladocerans (Rieman & Falter 1981; Spencer et al. 1999), dietary overlap is evident.  76  )  4. Discussion  Information was compiled to test hypothesized mysid-induced effects observed in other systems. The variables that were used included adult escapement, and taxonomic and seasonal zooplankton abundance. Changes over time were compared between a high mysid abundance (Skaha Lake) and low mysid abundance (Osoyoos Lake) lake. In addition, mysid and nerkid life history information was reviewed, movement patterns determined, and nerkid diet determined to indicate whether mysids are a net benefit as food or cost as a competitor to nerkids. Mysid-induced effects are evident in the seasonal zooplankton community composition since cladoceran diversity is lower and the cladoceran pulse later in Skaha Lake than in Osoyoos Lake. However, abundance of cladocerans has not changed in Skaha Lake but has decreased in Osoyoos Lake when recent samples were compared to those collected at the time prior to mysid invasion into the lake. Copepods have declined in both study lakes. In addition, hydroacoustic and nerkid diet analysis data suggest that mysids do interact with nerkids, not only do mysids and nerkids compete, but nerkids in both lakes also consume mysids. Mysid interactions vary between lakes, are seasonally different, and are influenced by physical factors such as temperature and dissolved oxygen.  Further work is warranted to quantify the interactions between mysids and nerkids, such as was conducted by Hyatt et al. (2004) on a 'trophic triangle' of pelagic species, in their case sockeye, Neomysis, and sticklebacks.  In Skaha and Osoyoos lakes, zooplankton are consumed by nerkids and mysids, and mysids are a competitor and food source for nerkids. The trophic triangle is driven by zooplankton production where competitive interactions between mysids and nerkids are negated at some level of zooplankton production. In Skaha and Osoyoos lakes, mysid effects didn't appear to influence zooplankton abundance, but did influence seasonal availability and diversity. In addition, zooplankton, mysids and nerkids (the "trophic triangle") are influenced by abiotic factors such as epilimnetic temperatures  77  and dissolved oxygen in the hypolimnion. In both Skaha and Osoyoos lakes, high epilimnetic temperatures provided refuge for zooplankton from mysid and nerkid predation. Although dissolved oxygen effects were not observed, low dissolved oxygen in Osoyoos Lake may seasonally eliminate mysid access to lake bottom food sources and refuges from pelagic predators including nerkids. Lastly, nerkid and mysid densities influence the trophic triangle. Hyatt et al. (2004) found that following a year of high nerkid densities, which cropped down the mysid population in the lake, the next nerkid brood year benefited from decreased competition for food. They concluded that fertilization should only occur when nerkid densities are high. Since nerkid predation may reduce the abundance of mysids in a lake for a year or more when nerkid densities are high care must be taken in comparing Osoyoos and Skaha lakes. The monitoring period for this project was during a series of relatively high nerkid abundance years in Osoyoos Lake, while for many years nerkids were generally in low abundance in Skaha Lake, except between 2002 and 2004 when there were high spawner escapements.  4.1 Recent changes  A summary of the major variables investigated (Table 7) indicates that in Skaha Lake between 1969 and 2002; kokanee adult returns have progressively decreased between 1969 and 2001 then recently increased in 2002 (and subsequently in 2003 and 2004), mysid densities have increased, cladoceran densities have remained relatively unchanged, copepod densities have decreased, and phosphorus levels have decreased. During this period for the Osoyoos Lake rearing populations, sockeye adult returns to its inlet have been highly variable with a long term decreasing trend, mysids are now present in low densities, cladoceran densities have declined slightly, copepod densities have decreased, and phosphorus levels have decreased due to conversion to tertiary treatment of sewage at major centres in the Okanagan. Cladoceran diversity appears to have decreased in both lakes, although this is more evident in Skaha Lake. In addition, in Skaha Lake there appears to be a delay in  78  the cladoceran pulse, perhaps due to a mysid influence. Additional information to the benefit of mysids as a food source compared to the competitive interactions from mysids in both study lakes will aid in assessing the likely future impacts from mysids (to be discussed in section 4.2).  Table 7. Summary of variables of interest for Skaha and Osoyoos lakes.  Year  1969  1971  1977  1978  2001  2002  Lake  O. nerka  Escapement  Skaha  -50  -50  L  L  12  86  (thousands)  Osoyoos  80.5  50.7  23.2  12  26.6  34  A/L/M/H*  Skaha  43.5  140.7  brood year  M . relicta  liiiiiiiliKhii  A  A  A  A  35.5  10.5  Skaha  5.5  10.7  5.5  8.2  11.3  5.6  Osoyoos  8.3  5.9  16.3  12.5  2.7  1.7  Skaha  248.4  190.7  116  175.9  53.5  45.9  Osoyoos  153.3  70.1  86.6  87.5  35.4  42.5  Skaha  31  20.7  11  18  11.2  8.5  Osoyoos  26.9  228  21  23.5  15.2  9  Osoyoos  Cladoceran  Copepod  Spring Total  #/cm  #/cm  ug/L  2;  2  mm  P r  (April/May)  *A-absent, L-low, M-moderate, H-high  The kokanee decline in Skaha Lake supports the widespread western North American evidence that limnetic fish, in this case kokanee, generally decline after the establishment of mysids (Andrusak et al. 2004; Beattie & Clancey 1991; Bowles et al. 1991; Goldman et al. 1979; Lasenby et al. 1986; Rieman & Falter 1981). However, between the years 2002-2004, adult returns have recently increased.  Prior to mysid invasion the greatest impacts to kokanee populations were from losses of spawning habitat, namely from the channelization of the Okanagan river between Okanagan and Skaha Lakes,  79  where less than 1% of high quality habitat is remaining (Long & Newbury 2003), the channelization of Ellis and Shingle creeks, and also channelization in most parts of the Okanagan River between Vaseux and Osoyoos lakes. Habitat enhancement work was conducted through the addition of spawning gravels to a section of the Penticton channel, which is where the majority of kokanee now spawn. However, since channelization of the Okanagan River was completed in the 1950s and there were consistently 50,000 kokanee spawners from Skaha Lake in the early 1970s, a question arises about where they spawned prior to the addition of the enhancement gravels? While, Ellis Creek, a tributary to the Penticton Channel, was also channelized in the 1950s likely resulting in the loss of spawning habitat, Shingle Creek flows through Penticton Indian Band reserve lands and was not channelized in the 1950's. Channelization work on Shingle Creek did not occur until the late 1970s, with the resulting loss of kokanee spawning habitat (Louis 2002, personal communication). Thus it is likely that the kokanee population decline through most of the past thirty years is due largely to spawning habitat loss.  Another potential factor for decline after the 1970s is implementation of the Okanagan Basin water agreement. One of the goals of the agreement was to reduce phosphorus levels in the mainstem lakes (Jensen & Epp 2001). Since phosphorus has been used as an indication of limnetic fish production in other lakes (Downing et al. 1990; Hanson & Leggett 1982; Hyatt & Rankin 1999; Stockner & Shortreed 1985), a decrease in phosphorus would likely result in a decrease in fish production, in this case, kokanee. In addition, local fishermen attribute the decline to the kokanee egg take in the 1970s, when it is estimated that 50% of the kokanee were removed for hatchery production, with the resulting fry transplanted elsewhere (Summerland hatchery files).  Contradicting the hypothesis of a general decline in nerkid populations after invasion by mysids are the most recent spawner escapements for 2002 to 2004, when the average escapement is an order of magnitude greater than the average between 1989 and 2001. Clearly, kokanee populations are highly variable, as is the population in Nicola Lake (Northcote 1964; Northcote & Lorz 1966). Further  80  investigation of the reasons for such a rapid increase is warranted. Possible reasons for the increased escapements may include: improved in-lake survival of kokanee, tributary contributions from Ellis and Shingle Creeks, and recently corrected errors in adult escapement estimation. While spawner escapements increase dramatically, there was not a corresponding increase in in-lake estimates from DFO's surveys, which suggests that there may be generally poor egg to fry survival and periodically high in-lake survival (Rankin 2003, personal communication).  Over the years that sockeye escapements have been enumerated in Okanagan River, the population has been persistent but highly variable and, in recent monitoring years, limnetic fish abundance has correlated well with escapement numbers. There are numerous factors contributing to long-term declines in Okanagan sockeye, including habitat loss, dam construction, high water temperature, reduced discharges, and exotic competitive and predatory species. The addition of mysids now may be or may become an additional factor. However, not all large lakes display a declining nerkid population trend following mysid introduction (Lasenby et al. 1986), so further analysis of these issues is warranted.  The similar densities of historical and present cladoceran populations in Skaha Lake doesn't support the hypothesis that mysid invasion decreases cladoceran densities. In addition, a decrease in cladoceran density has been observed in Osoyoos Lake where mysid abundance is currently low. Copepod densities and overall zooplankton abundance have decreased in both study lakes. However, in Skaha Lake cladoceran diversity has decreased and it appears that the seasonal abundance has been affected by delaying the seasonal increase in densities, which supports the hypothesis that mysids delay the.cladoceran pulse. Cladoceran diversity has also decreased in Osoyoos Lake but not to the same extent as in Skaha Lake.  Other factors that may affect zooplankton abundance in both lakes are the reduction in phosphorus concentrations following tertiary treatment of sewage in the 1970s by the main Okanagan  81  communities and the implementation of point source pollution reduction (Jensen & Epp 2001). Phosphorus concentrations in Skaha and Osoyoos lakes have decreased from 31 ng/L and 26 u.g/L respectively in 1969 to 8.5 ug/L and 9 u.g/L respectively in 2002.  While phosphorus levels have decreased, they may not be low enough for mysid predation to exceed cladoceran production in Skaha Lake. In Lake Tahoe cladocerans disappeared following the introduction of kokanee and mysids in the early 1970s (Goldman et al. 1979), but have been returning with gradual cultural eutrophication of the lake (Byron et al. 1986). The high productivity of the Skaha Lake is evidenced by the large size of nerkids relative to other interior lakes in B.C., including Osoyoos Lake, which has only a low abundance of mysids. There has been a slight decrease over the long-term in cladoceran abundance in Osoyoos Lake, which is likely due to decreased nutrient levels, although in high density years nerkids may cause a temporary marked reduction in cladoceran densities (Burgner 1991). This is what was most likely observed in Osoyoos with the reduction in cladoceran densities due to juvenile sockeye predation. There appears to be a change in cladoceran species composition with a decrease in species diversity but not abundance. This was observed in both lakes and may be due to nutrient reduction or to preferential consumption by some predators, which suggests that cladoceran abundance depends on both nutrient inputs (bottom up control) and interactions with predators (top down) (i.e. mysids and nerkids). However, this doesn't explain why copepods have decreased in abundance while cladocerans have not but may be due to little interactions (i.e. predation) with predators and only the reduction in nutrient inputs.  Yet another possibility for the lack of observed changes in the Skaha cladoceran population is that mysids are a preferred food source for kokanee, which at some time during the year switch from cladocerans to mysids, as seen in Okanagan Lake (Levy 1991). Nerkids as young as 0+ have been shown to consume mysids (to be discussed later).  82  Lastly, abiotic factors may have influenced the effects of mysids during the late summer/fall sampling. During this time, the thermocline in both study lakes was at least 12 m deep in 2002 (Figure 26 and 27). High summer water temperatures may permit cladocerans to grow and reproduce with little predation by mysids and nerkids, which are more temperature sensitive (Chess & Stanford 1999; Chipps 1998; Goebel et al. 1995; Northcote 1991; Rudstam et al. 1999), although mysids have been observed in the warm epilimnion for short periods of time (Whall & Lasenby 1999, MS).  The delay in the pulse of daphnid zooplankton in Skaha Lake compared to Osoyoos Lake supports the hypothesis that mysids delay the pulse of cladocerans after invasion into a lake. This change in seasonal pattern after mysid invasion will likely persist, as is evident in recent studies in Lake Pend Oreille where zooplankton abundance, composition, and seasonal patterns have remained relatively unchanged for the past 30 years since establishment of mysids (Clarke & Bennett 2003). A delay in the cladoceran pulse may reduce the survival of newly emergent nerkids in Skaha Lake, although studies of newly emergent kokanee in small enclosures with a range of zooplankton abundances found no effect on kokanee growth (Clarke and Bennett (2002). In addition, a comparison of the lipid content in newly emergent kokanee showed little difference between fish from lakes with and those without mysids (Clarke et al. 2004). However, the possibility remains that mysids reduce the amount of food available to juvenile nerkids throughout the growing season thereby decreasing their overwinter survival, a potential limiting factor for kokanee production in Okanagan Lake (Andrusak et al. 2001).  83  4.2 Potential future effects  Results of the diel vertical migration monitoring show that mysids undergo a typical migration, migrating up during sunset and back down prior to sunrise. In addition, results in both lakes show that abiotic factors such as temperature and dissolved oxygen influence mysid behaviour. Mysids generally did not move into areas generally above 17°C in either lake (Figure 25, 26 and 27). In addition, age 0+ nerkids in both lakes also underwent a diel vertical migration as in other lakes (Levy 1989; Narver 1970; Northcote 1964; Northcote & Lorz 1966). Overlap is occurring between mysids and nerkids in both lakes, with more overlap in Osoyoos Lake.  It appears that mysid size and abundance is influenced by nerkid predation and physical factors in Osoyoos Lake, but not in Skaha Lake. Osoyoos Lake is more productive than Skaha Lake and has a lower abundance of mysids that are significantly smaller in size than in Skaha Lake.  Seining data showed that nerkids are not present when water temperatures are above 17°C, which corresponds well to results found in other studies of Osoyoos Lake (Wright 2002; Wright & Lawrence 2003).  Female mysid life history stage densities and size frequency distributions for both lakes suggest a one-year life cycle for mysids, as would be expected for such productive lakes.  Gravid female mysids release juveniles in spring, which by late summer can be distinguished by sex. B y late fall females become sexually mature and are gravid through the winter, releasing their offspring in spring. This is different than the life history observed in Okanagan Lake, where mysids have a two-year life cycle (Andrusak 2000, MS). Skaha and Osoyoos lakes appear productive enough to support mysids with an annual life cycle, similar to the pattern found in a productive bay of Lake Tahoe (Morgan 1980). '  84  Mysids in Skaha Lake are significantly larger than in Osoyoos Lake, which is not what would have been expected, as Osoyoos Lake is more productive and has a lower abundance of mysids. The main reasons for this difference may be: (1) the limnetic nerkid population was at a much lower density in Skaha Lake than in Osoyoos Lake during the years monitored, (2) temperature and dissolved oxygen conditions are more severe in Osoyoos Lake and smaller mysids are better able to live under these conditions and thus evade predation, and (3) temperature and dissolved oxygen conditions limit mysid access to epilimnetic and lake bottom food resources, especially access by large mysids.  The monitoring years for the two study lakes were during two large brood years of sockeye in Osoyoos Lake so one might expect to see increased predation on mysids (although not seen in diet analysis) and increased competition for food, which could limit the growth of mysids. This was not seen in nerkids in Osoyoos Lake as underyearling sockeye were still large compared to nerkids in other interior B.C. lake systems. Thus, one would predict that during a period of low sockeye abundances in Osoyoos Lake, the lengths of mysids would increase to be equivalent to that in Skaha Lake.  A second explanation may be due to smaller mysids being more tolerant to warmer temperatures (Chipps 1998; Rudstam et al. 1999) and lower dissolved oxygen levels (Sherman et al. 1987), which may mean that the larger mysids in Osoyoos Lake are more susceptible to predation because they cannot move as far as the small mysids into the high temperature and low dissolved oxygen area of the lake and thus evade predation. The relatively high temperature tolerance by smaller mysids is supported by the observed vertical location of mysid concentrations in Osoyoos Lake compared to the concentrations in Skaha Lake.  In addition to reducing predation, the increased temperature and dissolved oxygen tolerance afforded to smaller mysids may enable them to access more food. The temperature and oxygen limits in  85  Osoyoos Lake are well known for sockeye (Hyatt & Rankin 1999). It has been suggested that at temperatures higher than 15°C, it is bioenergetically costly for mysids to feed, even though they will continue to do so at temperatures up to 18°C (Chipps 1998). In addition, dissolved oxygen levels limit access to lake bottom food resources (Sherman et al. 1987). Thus since Osoyoos generally has a deeper thermocline than Skaha Lake, it may be advantageous for mysids in Osoyoos Lake to be smaller and thus be able to access more of the lake.  Overlap is occurring between mysid and nerkids in both lakes with more overlap in Osoyoos Lake than in Skaha Lake during the night. However, since sockeye are visual feeders and may not be feeding at night (Foerster 1968), the increased overlap in Osoyoos Lake may not be a factor in the ability for sockeye in Osoyoos Lake to predate on mysids. The lack of mysids is evident in the diet analysis, but may be a possible explanation for the reduced mysid mean length in Osoyoos Lake when compared to Skaha Lake.  Preliminary diet analysis identified that age 0+ sockeye and kokanee ingested variable amounts of mysids during the year and in the two lakes. Clearly mysids are a food source in both lakes, and a greater proportion of 0+ nerkids eat mysids in Skaha Lake than in Osoyoos Lake. This was not expected, as we would have predicted that abiotic factors that influence mysids by limiting their vertical migrations would have made mysids more available as prey in Osoyoos Lake. However, the difference between consumption rates in Skaha and Osoyoos lakes could be due to differences in mysid densities, not overlap in position in the lakes. Skaha Lake has a higher density of mysids than Osoyoos Lake.  A n increase in nerkid abundance in Skaha Lake through reintroduction of sockeye may benefit subsequent brood years through a year-after-year reduction in competition with mysids as they are cropped down by the sockeye and kokanee. The benefit of mysids as a food resource may decline when their abundance levels are reduced to those observed in Osoyoos Lake. Alternatively, i f nerkid  86  abundance were to increase the competition between mysids and nerkids for food may be more costly than the value of corouming mysids.  If mysid densities were to increase in Osoyoos Lake, increases would likely be accelerated in years of low limnetic nerkid abundance since high densities of nerkids already stretch the food resource, as evidenced by density dependent growth in the lake (Hyatt & Rankin 1999). However, i f abiotic factors influence mysid growth and predator evasion, as has been suggested here, the prey benefits of mysids may exceed the costs of competition. Also, in years of high limnetic nerkid abundance, the mysid population may be reduced for one or more years, which may influence the growth and survival of subsequent brood years of nerkids (Hyatt et al. 2004).  Overall, since autumn sizes of underyearling, 1+ and 2+ nerkids are in the larger size range compared to other interior lakes with mysids, it appears that there are no density dependent growth effects from mysids; however such effects may become evident if mysid and/or nerkid populations increase in the lakes. Both competitive costs and prey benefits from mysids are evident on nerkid populations in both study lakes, but detailed energetic analyses are required to quantify the potential outcomes of increases in nerkid (Skaha Lake) and mysid (Osoyoos Lake) abundances.  87  5. Conclusions and Recommendations on the outlook for Skaha and Osoyoos lakes  5.1 Conclusions/Recommendations  The kokanee decline in Skaha Lake is not easily attributable to mysids. Between 1969 and 2001 kokanee escapements declined concurrently with the establishment of mysids in the lake; however kokanee escapements between 2002 and 2004 were an order of magnitude greater than those in the previous decade and were larger than pre-mysid escapement estimates. So, while escapements are highly variable, there is no clear cause of the variability.  Further work into reasons for the recent increased escapements is recommended. In addition, it would be useful to obtain historical data and standardize them into escapement estimates that can be directly compared to recent estimates, and to move forward with a set enumeration methodology that will improve long-term trend analyses. A standardized data set and methodology for future enumeration surveys would enable effective monitoring of the Skaha kokanee population during sockeye reintroduction.  As with kokanee, Okanagan sockeye have a persistent population and numbers are highly variable, reflecting the many factors that influence the population both within and outside the Okanagan basin. Okanagan sockeye are known to exhibit alternating periods of high and low production lasting several years in a row. The mechanisms driving these variations are poorly understood but likely involve both freshwater and marine conditions and events. During some periods sockeye numbers have been low enough to cause concern amongst fisheries managers that the stock may be extirpated. The recent establishment of a persistent population of mysids in Osoyoos Lake adds to concerns for this sockeye stock because mysids compete for food. Consequently, it is important to determine whether the net effect of mysids on sockeye in Osoyoos Lake is a benefit (food source) or detriment (competitor). Results provided in the current study serve as a basis for some initial conclusions on  88 f  when, where and how mysids are likely to interact with juvenile sockeye salmon. However, further work will be required to determine the net effect of these interactions.  Late summer/early autumn samples from selected years between 1969 and 2002 suggest that there has been little change in the cladoceran population in Skaha Lake and a small decline in Osoyoos Lake. This does not support the hypothesis that mysids caused a change in zooplankton abundance. This lack of change in cladoceran abundance may be because these zooplankton escape from predation by mysids by moving into high temperature and low dissolved oxygen zones in the lake at some times of year, including the period of sampling for this project. Copepod abundance, on the other hand, has declined in both lakes, especially in Skaha Lake. This decline may be due to the advent of tertiary sewage treatment in Okanagan communities over the past thirty years. The decreased copepod abundance has resulted in an increase in the cladoceran/copepod ratio for Skaha Lake, but little change for Osoyoos Lake. Cladoceran species diversity has decreased in both lakes, especially in Skaha Lake, while copepod species diversity has declined slightly in both study lakes, which supports the hypothesis that mysids change zooplankton community composition. There is a delay in the cladoceran pulse in Skaha Lake when compared to Osoyoos Lake, which supports the hypothesis that mysids delay the cladoceran pulse. It is recommended that continued monitoring for seasonal and total abundance of zooplankton be continued. Since mysids have only recently invaded Osoyoos Lake there is an opportunity to monitor zooplankton responses to this invasion. In addition, Skaha Lake provides an opportunity to monitor zooplankton responses to an increase in nerkid production due to the reintroduction of sockeye.  The recent large kokanee escapements in Skaha Lake, the large size of nerkids in the study lakes compared to nerkids in other B.C. interior lakes with mysids, and the lack of change in late summer/fall cladoceran abundance over the period since mysids were introduced suggest that food supplies are not currently limiting kokanee populations in Skaha Lake. The high return years and  89  subsequent low in-lake nerkid abundance suggest that egg to fry survival may be the main limiting factor for the Skaha Lake kokanee population.  Mysids in Skaha Lake are larger than those in Osoyoos Lake, which may be a result of competitive interactions with the large nerkid population in Osoyoos Lake during the two years of monitoring (2001 and 2002) and/or may be the result of abiotic influences.  Mysids are a competitor for the food resources in Skaha and Osoyoos lakes as they both consume zooplankton, however, it is unknown i f they consume the same species of zooplankton. It is recommended that detailed diet of nerkids and mysids be conducted to determine dietary overlap. In addition, further work into how mysids influence nerkid diet is recommended. Four hypotheses identified to aid in providing further information are as follows: 1) The hypothesis that early emergent nerkids will exhibit different dietary patterns in the presence of  Mysis relicta  when compared to the absence of  M. relicta  in experimental  enclosures. 2) The hypothesis that early emergent nerkids will grow faster in the absence of when compared to the presence of  M. relicta  Mysis  relicta  in experimental enclosures.  3) The hypothesis that early emergent nerkids will survive at higher rates in the absence of Mysis relicta  when compared to the presence of  4) The hypothesis that, in similar concentrations of  M. relicta M. relicta,  in experimental enclosures. emergent nerkids maintained in  experimental enclosures will exhibit dietary patterns more similar to nerkids sampled at a similar time of year in Osoyoos Lake. Lastly, mysids are a food source for nerkids in Skaha and Osoyoos lakes. This food benefit varies by lake, age of nerkids, abundance of zooplankton, nerkid and mysid abundance, and by temperature and dissolved oxygen conditions. It is recommended that further work such as that done by Hyatt et al. (2004) be conducted to begin to quantify and model these competitive interactions between zooplankton, sockeye, and mysids.  90  5.2 Outlook on Skaha and Osoyoos lakes  A study in 1940 looking at sockeye growth rates and water temperatures in Skaha Lake, a "natural sockeye-producing area in the south Okanagan dry belt" (Donaldson & Foster 1940; Foerster 1968), found that sockeye grew at optimal rates and with low mortalities.  With the re-introduction of sockeye to Skaha Lake in varying densities, one might predict that when large numbers are planted in the lake mysids may be cropped down such that in-lake conditions may be more favourable for sockeye and kokanee fry the next year. However, kokanee may be periodically negatively impacted by competition with large numbers of introduced sockeye fry, especially if mysid populations are dense. Since sockeye fry and juveniles consume mysids, releases of cultured sockeye fry combined with kokanee in the lake may cause a decrease in mysid densities.  At some density the introduction of sockeye fry to Skaha Lake will decrease kokanee survival, as has been predicted by a simple life-cycle model developed by ESS A Technologies Ltd (Parnell et al. 2003). The model prediction is based on the amount of phosphorus available and a predicted fish biomass relationship, as was used in Hyatt et al. (1999). The model predicted that survival of kokanee fry to age 1.0 would be unaffected until the number of introduced sockeye reached about 1000 fry/hectare (i.e. about 2 million fry).  The reintroduction of sockeye will permit an evaluation of the rearing capacity of Skaha Lake, and the associated responses between kokanee, sockeye, and mysids. The recent large escapements of spawning kokanee have not resulted in high in-lake kokanee densities, which suggest that low egg to fry survival is limiting kokanee production.  Densities of mysids in Osoyoos Lake may increase significantly i f there is a series of years with low densities of nerkids in the lake to keep the mysid number depressed, which may result in a shift in the competitive balance in the lake between mysids and nerkids such that survival of nerkids is reduced.  91  On the other hand, when numbers of sockeye fry in the lake are high they may crop the mysids down such that mysid numbers may remain depressed for one or more subsequent years, thereby increasing fry survival in those subsequent years.  Other possible outcomes of increased mysid densities in Osoyoos Lake are an increase in kokanee size and survival, since mysids are a preferred food source for nerkids in the lake, and an increase in sockeye 'residualism' if conditions in the lake are beneficial enough to make migration to the ocean undesirable.  In order to better predict the outcome of reintroducing sockeye into Skaha Lake it would be beneficial to further study the interactions between zooplankton, mysids, and nerkids, such as was completed by Hyatt et al. (2004) in Muriel Lake, B.C. Among the issues that should be studied are the feeding habits of the organisms, their distribution in each lake by time of day and season, and competitive interactions by time of day, season, and population density.  92  Bibliography  Alexis, F., H . Alex, S. Lawrence, C. Bull, and H . D. Smith-Editor. 2003. 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