By contrast with a multitude of laboratory studies on the social organization of fish, relatively little is known about the size, composition and dynamics of free-ranging fish shoals. We give an overview of the available information on fish shoals and assess to what degree the predictions made from laboratory studies are consistent with field data. The section on shoal choice behaviour in the laboratory is structured so that the evidence for different shoaling preferences is discussed in the context of their mechanisms and functions. Predictions based on experiments in captivity regarding preferences for conspecifics, individuals of similar body length and unparasitized fish were highly consistent with field observations on free-ranging shoals whereas preferences for familiar conspecifics and kin remain to be conclusively demonstrated in the field. In general, there is a shortage of studies in which shoaling preferences have been investigated both in the laboratory and the field, and field studies have so far been largely descriptive revealing little about the underlying mechanisms of observed patterns. Given the great importance of fish shoals both in fundamental and applied research, an advancement of our knowledge of their social organization should significantly contribute to a better understanding of a whole range of topics including reciprocal altruism, group-living and self-organization.

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 dynamic and individuals may maximize their fitness by switching frequently between groups of varying size and composition in response to changes in their physiological stage and the external environment. Fish shoals provide an excellent opportunity to investigate the functions and mechanisms of group living, and future studies should aim to take an integrated view of individual behaviours, group size and phenotypic composition when investigating group choice decisions. 2000 The Fisheries Society of the British Isles

The amount of intraindividual genetic variation has often been found to have profound effects on life history traits. However, studies concerning the relationship between behaviour and genetic diversity are scarce. Aggressiveness is an important component of competitive ability in juvenile salmonids affecting their later performance and survival. In this study, we used an experimental approach to test the prediction that juveniles with low estimated genetic diversity should be less aggressive than juveniles with high estimated genetic diversity in fry from a highly endangered population of land-locked salmon (Salmo salar). This was achieved by using a method enabling the accurate estimation of offspring genetic diversity based on parental microsatellite genotype data. This allowed us to create two groups of offspring expected to have high or low genetic diversity in which aggressive behaviour could be compared. Salmon fry with low estimated genetic diversity were significantly less aggressive than fry with high estimated genetic diversity. Closer analysis of the data suggested that this difference was due to differences in more costly acts of aggression. Our result may reflect a direct effect of genetic variation on a fitness-related trait; however, we cannot rule out an alternative explanation of allele-specific phenotype matching, where lowered aggression is expressed towards genetically more similar individuals.

No abstract

The ability to distinguish among chemical cues from multiple predators is of key adaptive value for many prey fish. We examined the attractiveness and repulsiveness of chemical stimuli from different coexisting fish species fed on different diets on the behaviour of hatchery reared Arctic charr young in a Y‐maze fluviarum, where the charr could choose between two sides either with control water or stimulus water with fish odour. We used stimuli from (1) matching sized conspecifics, large (2) Arctic charr, (3) salmon, (4) brown trout and (5) brown trout fed on Arctic charr fry. Other salmonids were given pellet food. Additional fish odour treatments included piscivorous (6) pike and (7) burbot. In the control trials both sides received control water. Arctic charr young were expected to respond adaptively to the stimuli from coexisting piscivorous fish. The charr most strongly preferred water with the odour of their matching sized conspecifics, which was the only fish odour they were familiar with before the experiments. They also showed significant preference for other salmonid odours, even though these fish are potential predators on small charr. Chemical stimuli from pike and burbot, on the contrary, were strongly avoided, and burbot odour even prevented the charr to swim and enter the lateral halves of the fluviarum. Moreover, odour from brown trout fed on Arctic charr fry was avoided when compared to stimuli from trout fed on pellets. Although the Arctic charr young were completely naive regarding piscivores, the fact that they could distinguish between different predator taxa and diets on the basis of chemical cues only reflects the long coevolutionary history of these fish populations.