The influence of body size on the intermittent locomotion of a pelagic schooling fish

Abstract

There is a potential trade-off between grouping and the optimizing of the energetic efficiency of individual locomotion. Although intermittent locomotion, e.g. glide and upward swimming (GAU), can reduce the cost of locomotion at the individual level, the link between the optimization of individual intermittent locomotion and the behavioural synchronization in a group, especially among members with different sizes, is unknown. Here, we continuously monitored the schooling behaviour of a negatively buoyant fish, Pacific bluefin tuna (N ¼ 10; 21.0 24.5 cm), for 24 h in an open-sea net cage using accelerometry. All the fish repeated GAU during the recording periods. Although the GAU synchrony was maintained at high levels (overall mean ¼ 0.62 for the cross-correlation coefficient of the GAU timings), larger fish glided for a longer duration per glide and more frequently than smaller fish. Similar-sized pairs showed significantly higher GAU synchrony than differently sized pairs. Our accelerometry results and the simulation based on hydrodynamic theory indicated that the advantage of intermittent locomotion in energy savings may not be fully optimized for smaller animals in a group when faced with the maintenance of group cohesion, suggesting that size assortative shoaling would be advantageous.

Habitat use and movement patterns of small (age‐0) juvenile Pacific bluefin tuna (Thunnus orientalis) relative to the Kuroshio

“…In the East China Sea, young PBF were found to prefer frontal regions of the Kuroshio, but never entered the fast‐moving current (1.3–1.7 m/s), which moved faster than the swimming speed of the PBF (1.1–1.4 m/s ß ) (Kitagawa, Sartimbul, et al., ). Juvenile PBF have been found to swim slower (0.6–0.7 m/s) (Noda et al., ) than the current velocity along the southern coast of Japan (1.0–1.5 m/s) (Nitani, ). Tagged PBF likely preferred the stable environment of the coastal waters over the offshore Kuroshio because of the reduced energetic costs of swimming in the former.…”

“…(, ) and Itoh et al. () applied these tags to age‐1 PBF (45–78 cm FL at release), and other recent studies have deployed archival tags and accelerometers on small, juvenile PBF to describe their vertical movements in the wild (fish ranging from 20.5 to 26.5 cm FL) and swimming behaviors in an open‐sea net cage (fish ranging from 21.0 to 24.5 cm FL) along the southern coast of Japan (Furukawa, Fujioka, Fukuda, Tei, & Ohshimo, ; Noda et al., ). However, these studies did not address horizontal habitat use by free‐ranging individuals in natural waters.…”

“…The accuracy of these location estimates can be improved by integrating sea surface temperatures and bathymetry parameters into position-estimation models (Galuardi et al, 2010;Lam, Nielsen, & Sibert, 2008;Teo et al, 2004). Kitagawa et al (2000Kitagawa et al ( , 2004 and Itoh et al (2003) applied these tags to age-1 PBF (45-78 cm FL at release), and other recent studies have deployed archival tags and accelerometers on small, juvenile PBF to describe their vertical movements in the wild (fish ranging from 20.5 to 26.5 cm FL) and swimming behaviors in an open-sea net cage (fish ranging from 21.0 to 24.5 cm FL) along the southern coast of Japan (Furukawa, Fujioka, Fukuda, Tei, & Ohshimo, 2017;Noda et al, 2016). However, these studies did not address horizontal habitat use by free-ranging individuals in natural waters.…”

Physiological mechanisms underlying animal social behaviour

“…Hence, equal proportions of gliding during locomotion of different-sized animals would lead to fission of the group. Instead, smaller animals show greater proportions of active propulsion than larger animals, thereby incurring a greater energetic cost [26]. Physiological differences between individuals may represent a similar proximate mechanism to size differences in causing tradeoffs, and groups may segregate on the basis of their physiological capacities, thereby reducing the benefits of group membership.…”

“…If activity patterns of two individuals are different, one or both may experience a cost if they are obliged to synchronize their behaviour [10]. For example, Pacific bluefin tuna (Thunnus orientalis) adopt intermittent locomotion, where active swimming episodes are interspersed with gliding, resulting in overall lower energetic costs of locomotion compared with continuous active swimming [26]. The speed of gliding, however, depends on animal size, and smaller animals glide more slowly than larger animals.…”