The potential of parasitism in the structuring of a salt marsh stickleback community

Abstract

To determine if ectoparasites have the potential to play a role in structuring a salt marsh stickleback community, we measured abundance of the copepod Thersitina gasterostei and fish louse Argulus canadensis in the field, and quantified their effects on the condition and survival of juvenile fish. We also determined if differential host selection occurred among three sympatric host species of sticklebacks, Gasterosteus aculeatus, Gasterosteus wheatlandi, and Pungitius pungitius. The abundance of T. gasterostei was highest from September to November. They were overdispersed on their hosts, so that only a few fish harboured large numbers of parasites. Prevalence and intensity of T. gasterostei were higher on P. pungitius and G. aculeatus than on G. wheatlandi. There was no evidence that larger fish harboured more T. gasterostei than smaller fish. Abundance of A. canadensis peaked in August. Heavy loads of A. canadensis decreased the survival of G. aculeatus and G. wheatlandi in aquaria. In laboratory experiments, P. pungitius acquired more A. canadensis, followed by G. wheatlandi and G. aculeatus, respectively. Argulus canadensis had a greater attack success and faced less risk on P. pungitius, while its attack success was lowest and risk highest on G. aculeatus, with values for G. wheatlandi being intermediate. Host size and presence of T. gasterostei on hosts had no influence on host selection by A. canadensis. An increase in host density resulted in a decrease in the mean number of A. canadensis per fish. We suggest that parasitism could play a role in structuring this stickleback community through its differential effects on the three host populations.