Figure 2.

Control for the overall activity of the contrasting stimulus shoals. To minimize differences in the overall levels of swimming activity between the shoals, the water temperature was varied. The large shoal was presented in water at 21°C in the stimulus compartment, whereas the smaller shoal at 29°C. The water temperature in the test aquarium, where the test fish were introduced, was at 25°C. In the control treatments one of the equally sized shoals was presented at 29°C and the other at 21°C. Proportion of time (preference index) spent by test fish in the 10 cm preference zone close to stimulus fish (mean ± SEM) is shown. Values above 0.5 indicate a preference for the more numerous shoal of stimulus fish or a preference for the more active, faster moving shoal when the stimulus shoals are of the same numerical size. Significant departure from the null hypothesis of no preference is indicated by asterisks: **P < 0.01, *P < 0.05.