Infection of brook trout fry, Salvelinus fontinalis, by ectoparasitic copepods: the role of host behaviour and initial parasite load

Ectoparasitic copepods tend to aggregate on their fish hosts, with few fish harbouring many parasites and most fish harbouring few or none. In laboratory experiments, it was determined whether individual differences in behaviour among uninfected hosts, and parasite-induced behavioural changes in infected hosts, act to generate aggregated distributions of the copepod Salmincola edwardsii on brook trout fry. Fish were individually exposed to parasites twice, first to numbers ranging from 0 to 80 copepods, to produce variability in initial parasite load, and then to 40 copepods. Fish behaviour was significantly affected by the presence of copepods in the surrounding water in both exposures. However, there was no clear relationship between copepod density and host behaviour. Among uninfected fish exposed to copepods for the first time, the number of copepods acquired by a fish was inversely related to the time it spent motionless. Prior infection increased the probability that a fish would acquire further copepods during a subsequent exposure. This may be due to copepod-induced behavioural changes, since the time spent motionless by fish decreased as the number of copepods harboured increased. This suggests that behavioural differences among uninfected hosts, and greater risks of acquiring further parasites through behavioural changes in infected hosts, could lead to few fish harbouring most parasites in natural populations.