Positioning Behaviour in Roach Shoals: The Role of Body Length and Nutritional State

This study investigated the positioning behaviour of individual fish in small shoals of 2 and 10 roach, Rutilus rutilus. In pairs consisting of one small and one large fish with an average length difference of 10 mm, no difference was observed in the number of initiations of shoal movement between small and large fish. However, once both fish were moving, large fish took up the front position more frequently than small ones. A subsequent experiment showed that body length was positively correlated with swimming speed which could explain the leading position of large fish. In a second experiment, using pairs of size-matched fish we found that in moving shoals fish which had been deprived of food for 7 days were more likely to take up front positions than well-fed conspecifics. No difference, however, was observed in the frequency of initiations of shoal movement. A final experiment with pairs and shoals of 10 fish showed that fish in the front position (and in the front half of the shoal for shoals of 10 fish) were much more likely to obtain food particles than fish in other shoal positions when resources were limited. This could provide a functional explanation for the occurrence of food-deprived fish in front positions. Our results suggest that the initiation of new swimming directions in stationary shoals does not follow the same principles as positioning behaviour in shoals that are already on the move.