AGE AND SIZE-SELECTIVE PREDATION ON LARVAL FISHES - THE BIGGER-IS-BETTER HYPOTHESIS REVISITED

A combined mesocosm and microcosm approach was employed to analyze the impact of age, and of size independent of age effects, of yolk-sac capelin larvae Mallotus villosus on their vulnerability to predation. The impact by both visual (3-spined stickleback Gasterosteus aculeatus) and non-visual (jellyfish Staurophora mertensi) predators was evaluated. We found, as predicted from the literature, that the probability of capture by both predator types decreased with larval age. However, contrary to the current view, when the predators were offered a mixture, and hence a choice, of larvae separated in age by 2 d, the probability of death by predation was higher for older larvae. We also examined size effects, independent of age effects, by generating large and small larvae of identical age through artificial fertilization of eggs from selected capelin females. We found no difference in the probability of capture of small versus large larvae of the same age. Yet, when the predators were presented with a mixture, and hence a choice of small versus large larvae, the larger larvae experienced higher mortality. This, too, is counter to current thinking. We show that being smaller at a given age may actually confer a survival advantage to larvae. These outcomes are discussed in terms of the interacting probabilities of encounter, attack and capture which define the probability of larval death due to predation.