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Studies examining the effects of alternative prey on the risk of predation to fish larvae have found a reduced risk to larvae, but only when in the presence of a relatively high number of alternative prey. However, all studies using zooplankton as alternative prey have used zooplankters smaller in body size than the fish larvae. In this study, we provided three-spined stickleback Gasterosteus aculeatus with the opportunity to forage on 5 to 7 d old cod larvae Gadus morhua, presented alone and in the presence of alternative prey of different sizes and numbers relative to that of the larvae. In one experiment, larvae were presented with an equal number of Artemia s a h a smaller, equal and larger in body size than the cod larvae. In a second experiment cod larvae were presented in the presence of a naturally CO-occurring zooplankter of equal size, adult Calanus finmarchicus. Larvae were presented at ratios of larvae to Calanus of 1:1, 3:l and 5.6:l. In both experiments, the presence of alternative prey of equal or larger size significantly increased the latency (in seconds) until a cod larva was captured and decreased the overall proportion of cod larvae captured. In the second experiment, this was true even at a ratio of larvae to copepods of 3: l . Protection to the larvae appeared to be the result of selection by the predator of the alternative prey. Results are discussed in hght of existing ideas on larval size and predation risk, and larval survival with respect to temporal and spatial overlap between larvae and zooplankton.

Risk of predation to fish larvae in the presence of alternative prey: effects of prey size and number