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The role of body size in the habitat choice and foraging behaviour of juvenile coho salmon under predation risk Reinhardt, Ulrich Georg


Models of behaviour in trade-off situations between foraging and predator avoidance predict that an animal's decision should depend on dynamic internal states, such as energy or hormonal levels. For example, it is predicted that willingness to risk mortality decreases with increasing assets of the forager. In this thesis, I used a combination of experimental manipulation and computer modeling to examine the influence of various dynamic states, particularly body size, on the risk taking behaviour of coho salmon (Oncorhynchus kisutch) fry. I showed that fry in semi-natural stream channels used protective cover opportunistically, associating with it only when it protected them from predators. In the laboratory, I observed habitat choice and agonistic behaviour of groups of coho fry under simulated predation risk. Bigger coho used a risky habitat less than smaller individuals. Bigger fry aggressively monopolized access to food in the absence of predation risk, but in the presence of risk, the size-dependent territorial hierarchy broke down and risk-prone small fish achieved relatively higher growth rates. Using a dynamic optimization computer model with parameters representative of coho foraging, I explored the relationship between body size and optimal foraging effort over the first feeding season. Like earlier dynamic optimization models, my model showed that the willingness to risk predation should decrease with size. In contrast to earlier models, my coho-specific model predicted that risk taking should be highest in the spring. In a final experiment, I examined the influence of feeding conditions, body size, and season on the willingness of individual coho fry to feed under simulated predation risk. As expected, bigger fish were more risk averse than small fish in the summer. This was not the case in the fall when the overall willingness to risk exposure to the predator was lower. I suggest that juvenile salmonids integrate information on their internal state and the environment into their foraging behaviour to a greater extent than previously thought. If increased risk aversion with size is common in nature, it may have important ecological consequences, for example, it may help to explain the widely reported lower mortality rate of bigger juvenile fish.

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