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Assessment of land use impacts on shore-spawning kokanee abundance and habitat in Okanagan Lake, British Columbia Wong, Cecilia

Abstract

This study examined the possible impact of land use changes on shore-spawning kokanee habitat to explain the decline in Oncorhynchus nerka at Okanagan Lake, British Columbia. Four objectives of the study were: 1.) to characterize spatial and temporal trends in fish abundance using enumeration data provided by the British Columbia Ministry of Environment, Lands and Parks Regional Fisheries Branch for the period from 1973 and 1997; 2.) to characterize spatial and temporal trends in land use from aerial photographs taken in 1963, 1971, 1981 and 1994-6; 3.) to characterize spatial differences in habitat characteristics from field measurements in the fall of 1997 and spring of 1998; and 4.) to examine associations and causative relationships among land use, habitat features and kokanee abundance. Between 1973 and 1997, the shore-spawning kokanee population decreased by 90.5%. The highest fish counts were reported in 1974 while the lowest were reported for 1993. Within each year, spawning activity was most variable in the southeast quadrant of the lake. Over years, spawning activity also decreased the most in the southeast quadrant. Significance tests revealed that fish enumeration reaches with historically high spawning activity experienced the greatest declines. The current and historical distribution of land use differed between 27 shore segments used for detailed analysis and land use in a 500m buffer zone around the entire lake. Among the 27 study units, forested lands were dominant while sage grasslands dominated the entire lake buffer zone. Changes in land use between 1963 and 1996 were also different. Land use data at the 27 study reaches indicated an increase in urban development and forested lands, and a decrease in agriculture and sage grasslands. Lake-wide data revealed an increase in urban development and a decrease in all other land uses. Despite differences in current and historic land use distributions, temporal trends consistently indicated urban areas increased as agricultural areas decreased. Shallow shoreline areas, shoreline slopes and substrate depths differed significantly among lake quadrants but not study reaches. Shoreline areas were larger in the southeast and southwest quadrants of the lake. Shoreline slopes were steeper and substrate depths were greater in the northwest quadrant. Steeper shoreline slopes, greater substrate depths and more angular substrate characterized enumeration reaches with higher current spawning activity. Shoreline areas with gradual slopes, more shallow substrate depths and less angular substrate characterized areas with a higher density of buildings and docks along the shoreline and larger areas of agriculture and sage grasslands within the 500 m buffer zone. These observations suggest that habitat features may facilitate interactions between land use and spawning activity. Interactions between fish abundance and land use including the area of development, agriculture and sage grasslands were demonstrated, however the associations were weak. Dock density was a better predictor of reduced spawning activity than the areal index of land use adjacent to the lakeshore. The current study examines only the shore-spawning stage of the kokanee life history. Other shorerelated stages that may be impacted by habitat changes associated with land use likely include egg incubation, alevin maturation and fry emergence stages. Additional studies are recommended to examine the relative contributions of these stages to overall fish abundance. At the same time, it is recognized that some shoreline issues may already be addressed with conscientious water level management and control of lakeshore development.

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