Procedural effects of prey tethering experiments: predation of juvenile scallops by crabs and sea stars

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Abstract
This study examines the effects of an experimental tethering procedure often used in field predation experiments. In laboratory experiments, juvenile sea scallops Placopecten magellanicus, either free or tethered, were offered to predatory crabs Cancer irroratus and sea stars Asterias vulgaris. The effect of the tethering procedure on predation rates was specific to a predator-prey interaction and could be predicted based on an understanding of the underlying behavioural mechanisms. In crab-scallop interactions, encounter rate was a major determinant of predation rate. Since tethering did not affect encounter rates, it did not significantly affect predation rates by crabs. In contrast, in sea star-scallop interactions, the probability of sea stars capturing encountered scallops was a major determinant of predation rate. Tethering limited the scallops' escape response, which increased the probability of capture and, hence, predation rate. Therefore, assessment of the relative importance of these 2 predators in determining scallop survival in field experiments would be biased by the differential effects of the tethering procedure.

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