Interannual variation in the availability and utilization of euphausiids as prey for Pacific hake (Merluccius productus) along the south-west coast of Vancouver Island

The species and size composition of euphausiids consumed by Pacific hake ( Merluccius productus ) along the south-west coast of Vancouver Island were monitored during the 1989, 1990 and 1995–97 summer feeding seasons. Thysanoessa spinifera and Euphausia pacifica were the only euphausiid species eaten by hake. Hake persisted in selecting krill longer than 17 mm and consistently preferred T. spinifera, even though there was a substantial reduction in euphausiid abundance, a change in species composition and a shift in the size distribution to smaller individuals after the 1992–93 warm water years. The biomass of euphausiids available to hake after 1993 averaged 27% of that for 1991, the pre-ENSO year, and was 12% of the 1991 value in 1996. Hake predation was not responsible for the reduction in euphausiid abundance, as it was already low at euphausiid sizes smaller than those eaten by hake.     

An investigation of the biological basis of return variability for sockeye salmon (Oncorhynchus nerka) from Great Central and Sproat lakes,Vancouver Island

We tested whether variations in stock characteristics (spawner and smolt abundance) and biotic conditions (prey variability, predation, competition) during the early marine period explained variations in the return of sockeye salmon (Oncorhynchus nerka) to Great Central and Sproat lakes, adjacent lakes on the west coast of Vancouver Island. There are two freshwater age groups in each lake; fish spend 1 or 2 yrs in freshwater after hatching. We tested the influences of stock and biotic factors on the return of each of the two age groups from each of the two lakes. Results of regression analyses showed that prey biomass variability best explained the variation in return for all lake‐age groups. Euphausiid (Thysanoessa spinifera) and cladoceran (Evadne) prey biomass variability explained between 0.75 and 0.95 (adjusted R²) of the variation in return. There appear to be instances of a mismatch between the seasonality of prey productivity and the apparent critical period of feeding for juvenile sockeye.