Shoal Choice Behaviour in Fish: the Relationship Between Assessment Time and Assessment Quality

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Abstract
In this study, we investigated the role of assessment time in group size discrimination and in particular the trade-off between the time cost involved in gathering information and the potential benefits derived from the acquired information. In a first experiment, we presented individual chub, Semotilus atromaculatus, with a choice between 4 vs 4 and 8 vs 8 conspecific stimulus fish. After release we recorded the time taken by test fish to make a choice between the two stimulus shoals in the presence and absence of a fright stimulus. Test fish significantly reduced their response time in the presence of a fright stimulus and larger shoals (8 vs 8) were more quickly approached than smaller ones (4 vs 4). In a control experiment, chub were given a choice between an empty cylinder and a shoal (of 4 or 8 fish). By subtracting the response time in the control treatment from that in the choice treatment, we estimated the time test fish spent choosing between stimulus shoals to be 24-55% of the overall response time. These results indicate that choosing between different groups is associated with a significant time cost. In a second experiment, we presented test fish with stimulus shoals that differed in size: 4 vs 5, 4 vs 6, 4 vs 7 and 4 vs 8, to investigate how the response time of fish and their ability to distinguish between shoals of different size were affected by the magnitude of the shoal size difference and the presence and absence of a fright stimulus. The ability to discriminate between shoals of different size increased with increasing shoal size difference whereas response time decreased. Both response time and discrimination ability were significantly reduced in the presence of a fright stimulus. The latter suggests that the benefits derived from group size discrimination were increasingly outweighed by the time costs of making group size assessments in the presence of potential danger; i.e. making fast assessments became relatively more important than making correct ones.

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