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Supplementary Figure 1a. A visual representation of how shoal cohesion was calculated. The test aquarium was subdivided into 4 equal sized sections (each measuring 15cm (L) × 30cm (W) × 30cm (H)). Shoal cohesion was calculated by counting the number of focal fish in each zone at 3 minute intervals (3 minute intervals for a 30 minute observation equals 10 zone counts). The standard deviation was taken from each time interval zone count. An average shoal standard deviation was then calculated from the 10 × time intervals zone counts, to produce one standard deviation per shoal. The average of the AM and PM observations were taken. This value was the categorised as either high or low (low 1). A high standard deviation indicated high cohesion amongst the shoal, whilst a low standard deviation indicated low cohesion. Figure 1b: Histogram showing the frequency distribution of mean dispersion values, indicating clear bimodal distribution.