Intra‐school positional preference and reduced tail beat frequency in trailing positions in schooling roach under experimental conditions

Abstract

Using three different swimming velocities and a school size of eight roach Rutilus rutilus, individual intra-school position and tail beat frequency were examined in a flume tank. Tail beat frequency was determined in defined leading and trailing positions. Individual roach showed consistent intra-school positional preferences which implied a sustained positional pattern where certain individuals took up front positions whereas other individuals swam in the rear part of the school. The positional preferences could not be attributed to inter-individual differences of the eight roach in terms of total length, mass or condition factor. At the tested swimming velocities of 2, 3 and 4 L T s À1 , roach in trailing positions swam with tail beat frequencies reduced by 7Á3, 11Á9 and 11Á6%, respectively, compared to roach in leading positions. These results suggested that roach situated in trailing positions experienced energetic savings due to hydrodynamic interactions at a wide range of swimming velocities. This may be important during migrations or when a school of roach is holding its position against the current in a lotic habitat. The observed sustained positional pattern combined with a hydrodynamic advantage in trailing positions would indicate that these energetic savings might not be evenly shared among schoolmates of roach. A positive correlation between swimming velocity and stride length was found. The present study, however, does not support any conclusions concerning these findings.

Critical swimming speed in groups of Atlantic salmon Salmo salar

“…Several previous studies have shown that swimming in a group decreases the cost of transport (Hartwell & Otto 1978, Fields 1990, Herskin & Steffensen 1998, Svendsen et al 2003, and a longer flume length increases U crit by allowing transition between swimming gaits (Peake & Farrell 2006, Tudorache et al 2007, 2010. Therefore, it is not surprising that U crit was significantly lower when post-smolts were tested individually in a smaller swim tunnel.…”

“…U crit is normally measured using swimming respirometers containing individual animals. However, in a natural setting many species of fish form groups, which may significantly reduce the cost of transport; for example, the time to exhaustion increased 10-fold when swimming 6 men haden Brevoortia tyrannus together compared to swimming them alone (Hartwell & Otto 1978), while a reduction in tail-beat frequency of fish in trailing school positions has been observed in several studies (Fields 1990, Herskin & Steffensen 1998, Svendsen et al 2003. Thus, the U crit traditionally obtained from individual fish might not be a representative indicator for swimming capabilities in species forming schools or groups.…”

Sustained swimming capacity of Atlantic salmon

“…Therefore, small groups of Atlantic salmon swimming together in a larger volume may reduce stress levels during experiments. Swimming in schools is also likely to reduce the cost of transport and therefore improve the swimming performance in some species (Hartwell & Otto 1978, Herskin & Steffensen 1998, Svendsen et al 2003.…”

From behavioural analyses to models of collective motion in fish schools

“…Tail beat frequency, which has been demonstrated to be positively correlated with oxygen consumption for solitary fish [35], is used as a proxy of the effort of the fish. Recent experiments on schools of roach [36] and seabass [35] found a significant reduction in tail beat frequency of the trailing fish compared with those of the front. However, because of the indirect nature of the measurement, there is still no conclusive proof of an energetic saving when swimming in school, and it is worth mentioning that many previous studies were confounded by the fact that fish in large groups tended to be less stressed, and thus may be expected to use less oxygen.…”