How strong are familiarity preferences in shoaling fish?

Individual fish of certain species show preferences for rejoining shoals of familiar individuals, suggesting that the development of familiarity within groups may be beneficial to group members. However, the relative value of shoaling with familiar individuals compared to, for example, joining a larger or more phenotypically matched group, is not known. We first confirmed, in separate experiments, that European minnows, Phoxinus phoxinus, prefer to join shoals of familiar individuals (with which they had been kept for 14 days) over unfamiliar ones, and show an increasing preference for the larger of two unfamiliar shoals presented in numerical size ratios of 1:1, 1:1.2, 1:1.9 and 1:4. In the latter experiment, test fish showed a marginal preference for the larger shoal at size ratio 1:1.2, and significant preferences in the 1:1.9 and 1:4 trials. To examine how test fish traded off familiarity against group size, we used the same shoal size ratios in a third experiment, this time with the smaller shoal being composed of individuals familiar to the test fish. In these trials, preferences for larger (nonfamiliar) and smaller (familiar) groups were balanced at the 1:1.9 shoal size ratio, and test fish significantly preferred the larger shoal only in the 1:4 trials. This suggests that the fish perceive the value of shoaling with familiars as equivalent to the benefits gained by doubling shoal size. Our results also indicate that preferences for familiar shoalmates are sufficient to offset defection to slightly larger groups. We discuss how this may stabilize group composition in natural habitats.