The effects of biotic and abiotic factors on fish swimming performance

Abstract

Mobility is the key to success for many animals. Movement allows organisms to make decisions about where to go, what to do and when to get there. Consequently, there is enormous selection pressure on behavioural, physiological and morphological traits related to locomotor performance. This thesis explores the effects of biotic and abiotic factors on the swimming performance of marine fishes focusing on two main questions: 1) Are fishes adapted to their local water-flow environment and if so, are these adaptations plastic? 2) Do ectoparasites affect aspects of steady and unsteady swimming performance and behaviour in their fish hosts? Chapter 2 describes and validates a non-lethal method for measuring pectoral fin aspect ratio, a common metric for assessing fin shape and swimming performance, on a variety of coral reef fishes. Chapter 3 describes intraspecific patterns in the fin shape and swimming performance of a widespread coral reef damselfish, Acanthochromis polyacanthus, across wave gradients on the Great Barrier Reef. I show that across smaller spatial scales, A. polyacanthus from sites experiencing differing water flow conditions vary dramatically in their fin shape, critical swimming speeds, maximum metabolic rates and aerobic scope as predicted based on their wave environment. Chapter 4 explores whether other species of widespread coral reef fishes from the Labridae, Acanthuridae and Pomacentridae families show similar diverging patterns in fin shape across localized flow gradients. I found that within a species, individuals with relatively more tapered fins are found in high flow habitats whereas individuals with relatively more rounded fins are found in low flow habitats. This trend is stronger for some species than others, and I discuss how behavioural differences and life histories may be driving the observed patterns. Chapter 5 asks whether the differences in swimming morphology and physiology in A. polyacanthus described in Chapter 3 are the result of phenotypic plasticity. I carried out a split-brood experiment on wild-caught juveniles spawned in either high flow or low flow habitats and reared them either in experimental wavy or calm aquaria for nine months. I found that swimming physiology was plastic in this species. However, using methods developed in Chapter 1, I found that fin shape did not show similar trends suggesting that there is selection for variation in fin shape within a brood. Chapter 6 examines the effects of a large isopod ectoparasite, Anilocra nemipteri, on its fish host, Scolopsis bilineata. I found that at high speeds, most of the additional energetic costs in parasitized fish are due to drag. However, parasitized individuals have higher maintenance costs (resting metabolic rates) than healthy individuals. Chapter 7 examines the unsteady burst swimming abilities of parasitized and healthy fish, and estimated their risk-taking behaviour in the field. Burst swimming performance was not impaired by parasites. However, parasitized fish allow a simulated predator to approach closer to them in the field before fleeing.