Preference responses of zebrafish to lo-'. 10 M alanine (Ala) were concentrationdependent. Behavioural responses to copper (Cu) and Cu + Ala mixtures were also assessed.Zebrafish avoided 100 and 10 pg Cu I-', but not 1 pg I I . Mixtures of lo-' M Alaf 100 pg Cu I I and 10 M Ala+ 1OpgCu I -' were avoided as intensely as was Cu alone. Responses to lo-' M Ala+ 10 or I pg Cu I-' and 10 'M A h + 1 pg Cu 1 ' did not differ statistically from controls (no detectable preference or avoidance). These results demonstrate, firstly, that a concentration of a pollutant avoided by itself (10 pg Cu 1 ') may not be avoided when encountered with an attractant chemical stimulus (Ala) and may suppress the preference for an attractant stimulus, and secondly, that a concentration of a pollutant not avoided by itself and not considered deleterious ( I pg Cu 1 I) suppresses attraction to Ala (an important constituent of prey odours for many fishes).
Previous studies have examined the effects of grouping on the locating (search) phase of foraging and feeding behaviour in fishes. Few studies have examined whether schooling in fishes may facilitate individual foraging by enhancing a group's responsiveness to food odours. The purpose of the current study was to assess the effect of increasing group size on the responsiveness of zebrafish. Eruchydunio rerio (Hamilton Buchanan), to L-alanine, an amino acid which is an important constituent of prey odours for many fishes. Based on the results of previous studies, either an increasing or decreasing linear relationship or a unimodal (convex or concave) relationship between responsiveness and group size was expected; the results, however, were bimodal.Groups of four fish were most responsive to alanine, as determined by the mean percentage of occurrences offish in the area of a behavioural arena (an octagonal fluviarium) into which alanine was infused (at lo-', or lo-' M). Groups of two, six and eight fish were significantly less responsive ( P < 0.05) than either groups of four fish or individual fish. The responses of groups of two, six and eight fish were not significantly different from each other.
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