In a laboratory experiment, we investigated shoaling and bottom-dwelling behavior in the threespine stickleback, a teleost fish, after exposure to sublethal concentrations of the plasticizer, butyl benzyl phthalate (BBP), and compared our results with those from unexposed fish. The experimental fish were exposed to BBP for 26 consecutive days before experimentation began. Video recordings were used to measure their behavior. Two experiments were conducted, each consisting of 30 trials for controls and exposed fish. Each trial was videotaped for 10 min. In all trials, the fish were allowed to acclimatise before recording began.

1. Shoal choice. The test aquarium was divided into three compartments. In the two end compartments were placed one shoal of large fish and one shoal of small fish, respectively. The central compartment was defined as a neutral zone, where no choice was made. To start the experiment, one focal fish was placed in the central compartment and allowed to choose between the two shoals and the neutral zone. The amount of time it spent displaying interest in each zone was calculated from the videotape, along with the amount of time spent at the bottom of the tank. Neither exposed nor control fish preferred either of the shoals, but exposed fish spent less time in the neutral zone than in association with a shoal. Exposed fish also spent more time at the bottom compared to control fish.
2. Size-assortative shoaling, i.e. the segregation of a mixed-size shoal (containing small and large fish) into smaller groups. The test aquarium was divided into two compartments, separated by a removable partition. One shoal of mixed-size fish was placed in one compartment, after which the partition was removed and the fish could move freely. Using the videotapes, shoal composition was measured every minute for 10 min after the partition was removed. There was no difference in the ability of exposed and control fish to form size-assortative shoals. However, exposed fish tended to aggregate into fewer, larger shoals compared to control fish, which formed more, smaller shoals. The formation of smaller shoals by the control fish increased over time, but the exposed fish remained aggregated throughout.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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