Effects of gelatinous zooplankton on fish early life abundance in the Barents Sea

Fluctuations in fish populations may reflect variability in predation pressure on vulnerable early life stages. Gelatinous zooplankton are potential predators of fish eggs and larvae, and changes in the abundance of jellyfish may therefore result in variable survival of early life stages of fish. We tested whether fluctuating jellyfish presence in the Norwegian and Barents seas is associated with larval cohort sizes of fish. Yearly indices of fish larva abundances and occurrence of ctenophores and cnidarian medusae were derived from Soviet-Russian ichthyoplankton data sampled in spring and summer from 1959 to 1993. A generalized additive model was fitted to describe the association between the fish larva index in summer, and summer and spring occurrences of ctenophores and cnidarian medusae. Spawning stock biomass and mean age of spawners were included in the model as proxies for the initial number of eggs and potential density-dependent mortality. Cod Gadus morhua and haddock Melanogrammus aeglefinus larva abundances were negatively associated with summer ctenophore occurrence, while haddock larva abundance was positively associated with summer medusa occurrence. The negative association between fish larvae and summer ctenophores is consistent with predation and competition. The difference in effects on cod and haddock might stem from differences in food preferences, timing and location of spawning, developmental rates, association, and ocean drift patterns. We have documented significant interactions between fish early life stages and gelatinous zooplankton, consistent with predation, competition, and other interactions that ultimately may affect the population sizes of the fish.