To feed or not to feed : the ontogeny of foraging and predator avoidance trade-offs in larval cod (Gadus morhua)

Leader, Leslie L. (1994) To feed or not to feed : the ontogeny of foraging and predator avoidance trade-offs in larval cod (Gadus morhua). Masters thesis, Memorial University of Newfoundland.

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Abstract

Animals are often forced to make ecologically important decisions when faced with potentially conflicting behavioural alternatives. For larval fish, starvation and predation are thought to be the two major causes of mortality. A larva's ability to initiate and maintain exogenous feeding, while at the same time avoid predation, often produces a situation where two necessary but incompatible behaviours conflict. The manner in which foraging behaviours are compromised under risk of predation should reflect the degree of predation threat encountered. Because the larval period is one of rapid growth and development, vulnerability to both starvation and predation will change with larval size and developmental state. -- In this study, experiments were conducted which offered cod larvae the opportunity to forage in the presence and absence of a predator to determine if larvae would trade-off foraging for predator avoidance. Larvae were reared at low, medium, and high prey densities to examine if food availability influenced the timing and magnitude of this trade-off. -- Larval cod displayed threat-sensitivity in their foraging activity, as trade-offs and active predator avoidance behaviours were only observed in high risk areas of the experimental aquaria. Larvae reared with high prey densities grew faster, survived longer, and in the presence of a predator showed reduced foraging and swimming behavior at week three post-hatch. In medium food densities, larvae grew slower and did not trade-off foraging and swimming behaviour until they were four weeks old. In addition, these larvae were less likely to trade-off foraging for predator avoidance as compared to those reared at high food densities. At low food densities, larvae were not observed to trade-off foraging for predator avoidance and total mortality occurred after week two post-hatch. -- In all three prey densities, yolk-sac and first-feeding cod larvae were not highly responsive to visual attack cues. However, responsiveness increased with growth and development. The timing of foraging trade-offs in larval cod were highly correlated with increased activity levels and the development of body pigmentation. In order to counter the effects of increased visibility, it may become increasingly important for larval cod to reduce foraging activity in the presence of predation threat. These results suggest that the observed timing and magnitude of trade-off behaviours in cod larvae may be size (i.e. developmentally) related.

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