Behavioral responses to light gradients, olfactory cues, and prey in larvae of two North Pacific gadids (Gadus macrocephalus and Theragra chalcogramma)
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The growth and survival of larvae can be significantly enhanced through close association with patches of high prey concentration. However, the taxis and kinesis responses used by larvae to locate and maintain residence in micro-patches remains poorly understood. In this study, the behavioral responses of Pacific cod (Gadus macrocephalus) and walleye pollock (Theragra chalcogramma) larvae (45–100 dph) to light, prey scent, and prey were examined. Both species displayed an ontogenetic shift in response to a horizontal light gradient, with small larvae (11–13 mm SL) exhibiting a positive phototaxis and large larvae (23–32 mm SL) exhibiting a negative phototaxis. Whether this reversal is related to ontogenetically appropriate foraging cues or some other aspect of the environment remains to be determined. Neither species displayed significant behavioral responsiveness to the introduction of olfactory prey cues at either size. The aggregating (taxis) response of large larvae to introduction of live prey was stronger than that of small larvae, possibly due to increased reaction distances and encounter rates. In addition, both species exhibited a kinesis response of reducing the frequency of swimming bouts in response to introduction of live prey. These results suggest that the scale of prey patchiness and the physical factors that determine patch encounter rates are a significant determinant of larval growth and survival in the early feeding stages of marine fishes.
KeywordsForaging cues Larval behavior Gadus macrocephalus Theragra chalcogramma Patchiness Kinesis Taxis
We wish to thank Michelle Ottmar, Scott Haines, Ben Laurel, Louise Copeman, and Matthew Hawkyard for assistance with larval rearing. Cliff Ryer provided assistance with light measurements in experimental tanks. Ben Laurel and Michael Davis offered advice throughout experimental design and analysis. Jim Ruzicka, Michael Davis, Allan Stoner, and two anonymous reviewers provided valuable comments on this manuscript. Fish culture was supported, in part, by a research grant from the North Pacific Research Board (#R0605 to B. Laurel et al.). A.R.C. was supported through funding from the AFSC diversity panel.
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