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

Abstract

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.

Understanding how animal groups achieve coordinated movement

“…Instead, faster, larger birds emerge as leaders (Pettit et al, 2015, 2013; Herbert-Read, 2015). Similar results have been found in other taxa – larger roach ( Rutilus rutilus ), for example, occupy positions at the front of groups (Krause et al, 1998; Krause et al, 2000). Therefore, differences in individuals' body size and self-assortment as a result of preferred speed can lead to certain individuals disproportionally influencing others' movements.…”

Kinematics and energetic benefits of schooling in the labriform fish, striped surfperch Embiotoca lateralis

“…Although positional preferences have been observed in schools in relation to the physiological state of each individual (e.g. hunger level, Krause et al 1998), schooling is a dynamic behaviour, and most fish continuously change positions from trailing to leading (Krause 1993(Krause , 1994Domenici et al 2002). A relatively constant reshuffling rate was found in schools of herring Clupea harengus which decreased when fish were exposed to hypoxia (Domenici et al 2002).…”

“…The positioning of each fish in a school is the result of a number of trade-offs. For instance, leading positions may be chosen by hungry individuals, since they would react more strongly to smells or other cues from food sources, although they may also experience a higher risk of predation (Krause 1993, Bumann et al 1997, Krause et al 1998. Energetically, trailing individuals can enjoy energetic advantages, although in the case of relatively large schools, trailing individuals may experience a further reduction in oxygen level due to the oxygen consumption of fish in the front (McFarland & Moss 1967, Domenici et al 2007).…”

Leadership, consensus decision making and collective behaviour in humans

“…Reebs (2001) also found an influence of body size on leadership with large knowledgeable fish being readily followed by small uninformed fish whereas small knowledgeable fish were less readily followed by larger individuals. Krause et al (1998) found that front positions tended to be occupied by larger fish and food-deprived fish. Bumann et al (1997) showed that individuals in front positions in shoals of roach R. rutilus led the group, finding that just a single individual in a front position could have a strong influence on an entire shoal of 16 fish.…”