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Shoaling behaviour is generally described as a trade‐off between the anti‐predator benefits of living in groups and the costs of increased foraging competition. An individual's fitness varies as a function of shoal size and shoal composition, and this relationship is potentially body length dependent. As teleost fishes show indeterminate growth, many populations exhibit a broad range of individual body lengths. The latter is used as a criterion in active choice of shoaling companions, and shoals are often size‐assorted. This reduces predation risk through minimizing phenotypic oddity, and may reduce competition between size‐classes. There is some evidence for a positive relationship between shoal size and the body length of shoal members, although it remains unclear whether this is a result of active shoal‐size choice or a by‐product of the body length distribution of the population. Shoal membership is highly …