Parasites and size-assortative schooling in three-spined sticklebacks

Recent field studies carried out in lacustrine environments have suggested that fish schools may be assorted with respect to parasite infections, but the generality and strength of parasite-assortative schooling in fish is unclear. In this study, data are presented on the body size and externally visible macroparasite load of 366 individual three-spined sticklebacks Gasterosteus aculeatus L. (Gasterosteidae, Teleostei) from six naturally formed schools captured in the Gullmarsfjord on the west coast of Sweden. Length - frequency analysis suggested that two separate year classes (presumed 0 + and 1 +) were represented in the sample. Individual schools were assorted by body size, and contained largely fish of a single year class. The number of fish in the school was negatively related to median body size of members. Probability of infection increased significantly with fish length for two parasites (Cryptocotyle lingua (Trematoda) and Glugea anomala (Microsporidia)), and marginally for a third (chalimus larvae of Caligus sp. (Copepoda)). After correcting for body-size effects, the proportion of school members infected with C. lingua and G. anomala did not differ significantly from a model of random assortment by infection status. However, two of the six schools were found to have significantly more, or significantly fewer, members infected with Caligus sp. chalimus larvae than predicted by the random model. Significant inter-school differences in the intensity of C. lingua infection were also detected, but these resulted from the combined effects of size-assortment and a strong correlation between body size and infection intensity. The results of this study provide only limited support for the existence of active parasite-assortative schooling in marine sticklebacks, and are discussed in relation to recent studies of parasite assortment amongst host fish in other ecosystems.