The value of thethering fishes (winter flounder and tautog) as a tool for assessing predation rates

Abstract

We evaluated the effect of tethering 2 species of juvenile fishes on their behaviour and susceptibility to predation under laboratory conditions in order to determine whether these fishes can be effectively used in field studies designed to assess predation rates. More specifically, this laboratory study investigated: (1) the effect of tethering, which required puncturing the body, on 2 mobile fish prey: the winter flounder Pseudopleuronectes americanus (42 to 85 mm total length, TL), and the tautog Tautoga onitis (47 to 91 mm TL); and (2) whether the predators, the summer flounder Paralichthys dentatus and the blue crab Callinectes sapidus, preferentially preyed upon tethered individuals. The behaviour of tethered individuals varied between species. Tethered P. americanus buried more extensively (i.e. greater percentage of body covered by sand) than untethered individuals. Therefore, tethering this species may reduce predation by visual predators because individuals are less visible. Tethered T. onitis were more likely than untethered individuals to occupy open areas of the experimental pool rather than to be found near shelter. In this case, tethering may increase potential predatory attacks because tethered fish are more accessible. Despite differences in behaviour caused by tethering P. americanus, there was no significant difference between the rates of predation by P. dentatus on tethered versus untethered individuals. However, as predicted by the change in T. onitis behaviour, P. dentatus preyed more frequently on tethered individuals than untethered ones. C. sapidus, a predator that uses olfactory cues, preferentially selected tethered P. americanus. The preference for tethered individuals may depend in part upon the release of prey body fluids and subsequent detection by predators that use olfactory cues. Furthermore, tethered individuals may change their shelter-seeking behaviour, and thus confound potential habitat-specific differences in relative predation rates. As a result, tethering as a tool to determine predation rates should be used cautiously for fishes. At a minimum, laboratory observations to detect tethering artifacts should be conducted before field experiments.