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Male and female zebra fish Danio rerio were given choices of shoals that differed in sex and size. Male zebra fish preferred to associate with female shoals over male shoals, but had no preference when given a choice between a mixed-sex shoal and either a male or female shoal. Female zebra fish showed no significant preference when given a choice between male and females shoals, nor between mixed-sex shoals and either male or female shoals. When given choices between shoals of differing size, females preferred to associate with the larger shoal, whether or not they were composed of males or females. Males, however, had no preference for larger shoals over smaller shoals, whether or not they were composed of males or females. These results showed that male zebra fish were capable of distinguishing between males and females solely on the basis of visual cues. Furthermore, these results demonstrated a significant difference between the shoaling choices of male and female zebra fish, which may indicate a difference in the function of shoaling for the two sexes. # 2005 The Fisheries Society of the British Isles

Aggression is a psychological construct that is commonly used to classify zebrafish behavior. Aggression is a complex trait that can be difficult to accurately measure. The literature on fish behavior describes many different methodologies to examine aggression, which, we believe, have not been compared in a formal manner. In this study we observed 19 individual zebrafish (Danio rerio) and quantified bites, lateral displays, charges, darts, and time near the stimulus in six common assays used to measure aggression. The methodologies included an inclined mirror assay, two flat mirror assays with different acclimation periods, a live conspecific assay, a clay model stimulus assay, and a video recording assay. Our results indicate high repeatability in most aggressive behaviors over time, which confirms the value of each assay to measure personality. However, our results also indicate significant differences between the assays. Specifically, assays using a flat mirror or live conspecific as a stimulus for aggression elicited more attempted bites than an inclined mirror, a clay model stimulus, or a video recording stimulus. Furthermore, the inclined mirror stimulus provoked more darts than any other assay. The results suggest the need for researchers to consider specific research goals when selecting the appropriate stimulus to provoke aggression in zebrafish.

Recent advances in the development and availability of genetically modified animals enable researchers to examine the effects of phenotypic characters on social behavior. In fish, shoaling behavior is known to be influenced by characteristics such as body coloration, striping pattern, body shape, and size. GloFishTM are genetically engineered zebrafish (Danio rerio) that express red fluorescent protein (RFP), resulting in on overall red coloration under the dark longitudinal stripes. The GloFish pattern is distinct from the light body coloration underlying the dark longitudinal stripes seen in wildtype zebrafish. We presented wildtype and transgenic RFP zebrafish with same‐sex shoals of both strains of fish in dichotomous choice tests. No preference for either of the shoals was shown, however, both strains showed significant preferences for swimming near shoals vs. swimming near an empty tank compartment. When presented with opposite‐sex individuals of both strains, no preference was shown by either sex of either strain. Thus, the red body coloration of transgenic zebrafish does not appear to affect choice of social partner, in either a shoaling or a potentially reproductive context.

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