Relationships between tail-beat frequency and swimming speed in free-swimming fishes

Abstract

Understanding the metabolic costs of locomotion is key for understanding the energy budgets of wild animals, yet can be challenging to quantify, especially for aquatic animals that may be difficult to observe or recapture. Here we quantified relationships between tail-beat frequency and swimming speed in free-swimming fishes at a large pelagic display aquarium at the Nordsøen Oceanarium, Denmark. Using stereocamera footage of free-swimming fish, we were able to determine the swimming-speed and body size of individuals from four study species: Atlantic mackerel, horse mackerel, needlefish, and the starry smooth-hound shark. While there were strong relationship between tailbeat frequency and swimming speed, the steepness of this relationship varied among species and between swimming types. Furthermore, for Atlantic mackerel, the relationship between swimming speed and tailbeat observed in free-swimming individuals differed from that measured for individuals swimming within a swim tunnel. This may be due to differences in how momentum can be used to maintain speed while freely swimming, or the energetic saving that can be gained by swimming in schools, and suggests caution be used when applying relationships between oxygen uptake and swimming speed derived from swimming tunnel respirometry to free-swimming fishes. Overall, more work examining the swimming behaviour and bioenergetics of free-swimming fishes is required to understand energy use in differing environments in the wild.