Parasitism, oddity and the mechanism of shoal choice

When choosing between shoals differing in Schistocephalus solidus infection status, uninfected test sticklebacks Gasterosteus aculeatus showed a preference for joining uninfected conspecifics when shoal sizes were equal, but reversed this preference when the relative size of the infected shoal was increased by a factor of 3. When given a choice between a shoal composed of size-matched minnows Phoxinus phoxinus and a shoal composed of the same number of all uninfected or all S. solidus -infected sticklebacks, test fish always preferred the sticklebacks, regardless of their infection status, over the minnow shoal. These observations suggest that species, parasite status and shoal size are all of importance when fish decide which shoal to join.