Trophic status and rearing capacity of smaller sockeye nursery lakes in the Skeena River system

Abstract

The Skeena River system has one large sockeye (Oncorhynchus nerka) nursery lake (Babine Lake) and approximately 26 smaller lakes. In this study, we investigated 10 of the larger non-Babine nursery lakes in the system. We examined lake physics, chemistry, several trophic levels important to juvenile sockeye, and juvenile sockeye number, biomass, and diet. Collected data enabled us to document the lakes’ trophic status and the current state of their plankton and limnetic fish populations. The data also enabled us to estimate the numbers of juvenile sockeye the lakes could sustain when at rearing capacity. Primary productivity of the lakes varied widely (seasonal average photosynthetic rates ranged from 33-265 mg Cm^-2d^-1), but all lakes were oligotrophic with the exception of Kitwanga Lake, which was mesotrophic. Biomass and composition of the lakes’ zooplankton communities also varied widely. Seasonal average macrozooplankton (>250 µm in length) biomass ranged 2 orders of magnitude from 17 – 1,770 mg dry wt/m². Copepods were the dominant zooplankton in most lakes, although the large cladoceran Daphnia longispina was dominant in Kitwanga Lake, and small cladocerans (principally Eubosmina longispina) dominated Johanson and Sustut lakes. Factors limiting lake productivity ranged from glacial turbidity (Kitsumkalum Lake) to extremely low nutrient levels (e.g. Morice Lake). Limnetic fish densities were low (6,200/ha (stickleback (Gasterosteus aculeatus) made up two-thirds of this number). Size of age-0 fall sockeye fry ranged from 0.8 g in Morice Lake to 6.1 g in Lakelse Lake. Age-1 sockeye fry were captured only in Johanson and Morice lakes. Our data indicate that sockeye stocks in most of these lakes are recruitment-limited, and that current production of sockeye from these smaller Skeena lakes is