Wild zebrafish exhibit a wide range of behavior. We found abundant wild zebrafish in flowing rivers and still water, in large, tightly-knit groups of hundreds of individuals, as well as in small, loose shoals. In two still-water populations, zebrafish were quite small in body size, common, and in tight groups of up to 22 fish. As in earlier laboratory studies, these zebrafish exhibited very low levels of aggression. In slowly flowing water in central India, zebrafish were relatively rare and gathered in small shoals (4-12 fish), often with other small fish, such as Rasbora daniconius. These stream zebrafish were larger in body size (27 mm TL) and much more aggressive than those in still water. In a second river population with much faster flowing water, zebrafish were abundant and again relatively large (21 mm TL). These zebrafish occurred in very large (up to 300 individuals) and tightly-knit (nearest-neighbor distances up to 21 mm) groups that exhibited collective rheotaxis and almost no aggression. This remarkable variation in social behavior of wild zebrafish offers an opportunity for future studies of behavioral genetics, development, and neuroscience.

Behavioral responses are shaped by a complex interplay of extrinsic factors, such as predation and flow conditions, and intrinsic properties such as metabolic state. We investigated variations in boldness among four populations of zebrafish collected from widely varying habitat conditions across India. We measured feeding latencies of fish in four contexts: in a novel environment; in an environment with refuge and predators; in the presence of a shoal; and in the presence of a shoal and predators. The population from a high-flow and high-predation habitat was significantly bolder than populations from low-predation and low-flow/stagnant habitats. Individuals were significantly bolder in the presence of shoal and predators than in the other contexts. In general, males were significantly bolder than females. Smaller individuals took greater risks to feed than larger individuals only within two populations. The high-predation, high-flow habitat fish also showed significantly lower between- and within-individual variation in boldness than all the low-predation and low-flow populations. Furthermore, we found significantly repeatable responses across contexts within all populations, indicating behavioral consistency. Differences in response to variations in experimental contexts and stress and individual differences in metabolic rates could possibly explain the observed between- and within-individual variation among populations.

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