Predation by Jellyfish on Larval Fish: An Experimental Evaluation Employing in situ Enclosures

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Abstract
Large in situ enclosures were used to investigate the mortality of yolk-sac larval capelin (Mallotus villosus) by four species of jellyfish predators. Daily instantaneous larval mortality increased linearly with predator density but was independent of initial larval density, indicating a linear functional response of the four jellyfish species. The addition of alternative prey did not modify the linearity of the functional response and did not significantly reduce or increase larval mortality imposed by two jellyfish species. The evidence of a linear response in large enclosures contrasts with the curvilinear (Type II) response previously reported in studies conducted in small laboratory containers. Larval mortality was strongly dependent on predator size within jellyfish species and increased with initial larval size. Larval capelin appeared more susceptible to predation at the time of transition from endogenous to exogenous feeding. Daily larval mortality rates due to predation varied between 0.015 and 1.58 depending on jellyfish species and appeared more strongly determined by body mass than differences in feeding appendages of jellyfish. Our results indicate that reduced larval mortality to jellyfish predation is achievable only by timing larval release in water mass where jellyfish numbers are naturally reduced. We estimated that macro invertebrate predation has the potential of being the primary regulator of larval survival and we hypothesized that large intra- and interannual variation in larval survival may be due to the variability in the emergence timing of capelin larvae relative to the growth trajectory of jellyfish.