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The influence of natural predation on the population dynamics of Pacific salmon : an experimental study

University of British Columbia

Predation is widely assumed to make an important contribution to the determination of year class strength in Pacific salmon. For the most part, this assumption is based on little in the way of a functional understanding of predator-prey interactions involving juvenile salmon. As a first step towards addressing this shortcoming, this thesis presents the results of a field experimental study that evaluates functionally the interaction between a predator, prickly sculpins (Cottus asper) and chum salmon fry (Oncorhynchus keta). The field research was conducted at an estuarine (Fraser estuary) and a freshwater (Rosewall Creek) site. In situ enclosures were used to ascertain the effect of prey density, prey and predator size and alternate prey on sculpin predation rates. Field and laboratory observations of predator distribution, abundance and feeding behaviour were used to complement the enclosure experiments. For the most part the experimental results were consistent with expectation. Predator search rates were far lower than expected, however; possibly due to partial spatial segregation of the predator from its prey. As well, the data did not fit a simple stochastic model of predation based on random search and encounter, suggesting that the predation process is far from random. Alternate prey strongly influenced predation on chum fry, as did predator size. Larger sculpins had greater search rates and maximum ration levels. In contrast, prey size did not appear to influence predation. The experimental results were combined with other field data to develop crude estimates of the overall impact of sculpin predation on chum fry in the two systems studied. In the Fraser estuary, the impact estimates are particularly uncertain, due to inadequate knowledge of sculpin abundance and chum fry residence times. Nevertheless, plausible estimates are high enough to be considered significant (6-81%). For Rosewall Creek, a simulation model was developed and used to explore the effect of changing fry production on sculpin predation rates. The key result was that although the model predicted compensatory mortality to occur as fry abundance increased, it was not sufficient to regulate the prey population. An additional unexpected result of the field experiments was gorging of fry by sculpins at very high prey densities. This behaviour is not predicted by conventional predation models, although it has been observed in many predator-prey systems. Two alternative models are proposed that might account for this phenomenon, one based on digestive physiology and the other on optimal foraging theory. Evidence is presented to suggest that the former is unlikely to be true, while the latter may offer a realistic generalization of conventional predation models that accounts for extrinsic restrictions on the predator's foraging behaviour.

Text

2010-08-06

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http://hdl.handle.net/2429/27116

Zoology

University of British Columbia

Graduate