Efficiency is not always the name of the game: Pile perch prefer to swim slower than their optimal speed

Abstract

The efficient and effective use of movement is essential for survival in mobile animal species. Among a plethora of other uses, movement is used to locate food, find mates, explore the environment, defend territories, and avoid danger (Pyke 1978, Gilroy and Lockwood 2012, Fortin et al. 2005, Rosell et al. 1998, Teyke 1989). The physiological and morphological parameters determining movement, and how different aspects of movement may be measured, are therefore significant areas of interest for animal biologists. In addition, knowledge of the movement of animals allows improvement of welfare and growth conditions in captive animals, and facilitates the study of other behaviours in both wild and captive settings. While animals move at a wide range of speeds in natural environments, it has been experimentally established that each individual has a speed at which their energy use is most efficient, after accounting for body mass and distance travelled (Wilson et al. 2015, Tucker 1970). This is their optimal speed, or Uopt (Weihs 1973, Pyke 1981). Uopt depends on metabolic rate, body size and shape, and likely other morphological and physiological parameters. Uopt therefore varies among species according to habitat and life history strategy (Weihs 1973, Tudorache et al. 2008, Wakeman and Wohlschlag 1982, Ware 1978). In temperate fish species, Uopt may vary from as low as 1 to as high as 5 body lengths per second (bl s-1) (Palstra et al., 2020; Tudorache et al., 2008; Tudorache et al., 2011). While Uopt may represent the most efficient speed of motion for an individual, the actual speed at which animals move is volitional and therefore linked to various other factors. Individual energetic needs, while determining physical parameters like Uopt, also contribute to behavioural factors such as individual “personality” traits. Individuals with greater metabolic rates may display greater tendency to exploratory behaviour (Fu et al. 2021, Binder et al. 2016), and therefore demonstrate a greater preferred swimming speed, or Upref, than would be predicted by their Uopt. Habitat and life history strategy may also contribute to Upref. Fishes found in complex habitats may tend to swim more slowly than their Uopt, in order to avoid obstacles (Priyadarshana et al., 2001). Fishes with hunting strategies such as “sit-and-wait” predators may spend the majority of their time completely immobile (Armstrong, 1986; Tolley & Torres, 2002). However, in pelagic and generalist fish species that spend the majority of their time moving it may be that Upref is predicted by Uopt. In the brook trout Salvelinus fontinalis, a pelagic migratory species, Upref strongly resembles Uopt at speeds greater than 25 cm s-1, or around 1 bl s-1 (Tudorache et al., 2011). The social environment may also determine the extent to which individual Uopt determines Upref. Individuals of social species will alter preference for abiotic parameters such as temperature in order to spend time with conspecifics (Cooper et al., 2018; Nay et al., 2021), however whether the presence of a conspecific disrupts the relationship between Uopt and Upref remains unclear. As different individuals are likely to have different Uopt and Upref values, determining whether a fish will sacrifice social interaction to swim at their preferred speed or will sacrifice swimming speed in favour of social interaction is a key consideration for social species (Jolles et al., 2017). Coincidence of Uopt and Upref has been previously studied in one species, S. fontinalis, in the context of determining whether Uopt or Upref may represent a better indicator of the best swimming speed to facilitate growth in aquaculture tanks (Tudorache et al., 2011). Uopt was found to be a good indicator of Upref, likely relating to this species’ migratory lifestyle. When travelling long distances with limited feeding opportunities is a key aspect of a species’ ecology, sustained swimming near Uopt is likely an energetically favourable strategy. Natural selection may then favour individuals whose Upref coincides with their Uopt. However, whether this relationship holds in non-migratory species is yet to be determined. Our study aims to build on this work by investigating the relationship between Uopt and Upref in a marine generalist species, the pile perch Rhacochilus vacca. In addition, we aimed to investigate how the relationship between Uopt and Upref may differ when this social species (Munsch et al. 2016) is alone versus in the presence of a conspecific. We predicted that Uopt would be positively correlated with Upref, though this relationship may be less strong in R. vacca as a marine generalist compared with previous studies on the migratory S. fontinalis. We also predicted that as a social species, R. vacca would alter Upref when swimming in pairs in order to remain with a conspecific.