UBC Theses and Dissertations
The feeding ecology of larval and pelagic juvenile redfish (Sebastes SPP.) on Flemish Cap (47 N, 45 W) Anderson, John T.
The copepod Calanus finmarchicus dominated the biomass of all invertebrate zooplankton species sampled on Flemish Cap during 1977-1983 as sampled by both coarse (0.333 mm and 0.505 mm) and fine (0.165 mm) mesh samples. Oithona similis and Oithoma atlamtica were numerically dominant in fine mesh samples. Significant differences were observed in the rate of C. fimmarchicus development and absolute abundance among the years sampled on Flemish Cap. Copepod eggs and nauplii were preferred prey of redfish larvae whereas Oithona spp. copepodites were not selected by larvae. The preferred prey of juveniles included Oithona copepodites, in addition to eggs and nauplii. This shift at metamorphosis -was a function of changing prey availability associated with the seasonal succession of zooplankton. Significant differences in diet between 1979 and 1980 compared to 1981 resulted from differences in prey availability which appeared to be due to differences in the seasonal development of C. finmarchicus. When copepod development started earlier and was faster, the diet of redfish was predominantly Oithona spp. Under those conditions, the redfish had lower amounts of food per stomach, lower relative condition and delayed age of metamorphosis at larger sizes. Higher feeding and growth rates occurred in 1980 in association with lower water temperatures and prey concentrations, compared to 1981. Differences in feeding and growth rates between years were related to a shift in available prey, where development of C. finmarchicus occurred earlier in 1981 and the zooplankton were dominated in May by Oithona spp. nauplii and copepodites. It appears that an earlier occurrence and subsequent decline in the abundance of C. finmarchicus eggs and nauplii was detrimental to the feeding and growth of redfish larvae in 1981. Higher growth and survival rates occurred in 1980 compared to 1981. Similar mean lengths of the population measured in August (Day 215) in 1980 and 1981, compared to different growth rates measured each year, indicates that only larger redfish survived in 1981. It is concluded that variations in physical circulation during this study, 1979—81, would not explain differences in annual variations of abundances. Comparison of relative redfish abundances at different ages demonstrated recruitment at age 5 was determined during the larval stage for the years 1979-81.
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