Individual variability in the behaviour and morphology of larval Atlantic cod (Gadus morhua, L.)

Forbes, Heather (2007) Individual variability in the behaviour and morphology of larval Atlantic cod (Gadus morhua, L.). PhD thesis, University of Glasgow.

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Individuals both within and between populations can vary markedly in a number of traits, including behaviour, life-history patterns and morphology. These differences do not simply reflect noise around a mean, but reflect real and important variability that can have a number of important implications. Such individual variability is especially prevalent among larval fish, which undergo significant changes in size, anatomy, physiology and morphology as they develop into adults.

The potential for fish to develop differing behaviours and morphologies has important implications in the aquaculture environment. For example, some fish may be aggressive and/or cannibalistic while others are not, or some fish may have a propensity to take risks while others shy away from risk. Elucidation of the mechanisms underlying these differences could enable the farmer to mitigate the development of behaviours and morphologies that are not conducive with welfare and production. Such information would be especially useful in the rearing of species such as Atlantic cod, which are highly cannibalistic in the larval and early juvenile period of development.

The purpose of the work outlined in this thesis was to elucidate certain aspects of individual variability in predominantly larval cod that relate to the culture of this species. An introduction to the subject area is provided in Chapter 1. Fish husbandry techniques followed the standard procedure employed at the two study sites and are described in detail in chapter 2. This chapter also describes the morphometric technique used to analyse morphology, principal component analysis of linear measurements, and discusses the reasons for adopting this technique.

Chapters 3 and 4 examine morphological development in larval cod reared under standard culture conditions and using common commercial feeds, for the purpose of elucidating developments in trophic morphology that could potentially relate to the development of cannibalism in this species. Chapter 3 specifically examines patterns of change in head shape in larval Atlantic cod and the extent to which head development varies within a cohort, while chapter 4 examines the effect of diet on the development of head morphology in larval cod.

The former of these studies identified clear and consistent patterns of growth in various measures of head structure (and especially eye diameter). This was with the exception of jaw width, which developed in opposition to these measures. Periods of rapid change in head morphology coincided with points at which the larval diet changed and may have been caused by this change. Growth of the head and the post cranium was highly variable, especially in the latter stages of larval development and investment in head growth relative to post-cranial growth increased over the first two thirds of larval development, remaining constant thereafter.

The second of these studies found that fish fed different prey types developed different head morphology. Specifically, fish fed small prey developed more fragile heads and larger eyes relative to jaw width than fish fed larger prey. Analysis of the head morphology of dead fish indicated that at least some of these differences resulted, not from the death of certain morphotypes, but from a phenotypically plastic response to the different diets. The morphology of a small number of cannibalistic larvae analysed during the study indicated that fish fed the larger prey developed morphology comparable with that of cannibalistic morphs.

In the study detailed in chapter 5, aggressive interactions in larval cod were quantified in order to determine whether these interactions represented an early form of cannibalism or a battle for resources. Attacks where characterised by brief, one-way, nips by an attacker to a victim. Fish also commonly exhibited a pattern of burst swimming (darts) that appeared to reflect a generalised escape response. This darting behaviour was not affected by the presence of food, but was more common in fish fed the higher prey densities. Conversely, overall levels of prey did not affect the incidence of aggressive attacks, although analysis was confounded by a decline in levels of aggression with increasing fish density. The frequency of nips was highest when food was absent and nips were preferentially directed at the tail of victims, to victims of a smaller or similar size than the attacker and to victims that showed abnormal body posture. These findings indicated that at least some attacks by larval cod represented an early attempt at cannibalism.

Chapter 6 details a study in which differences in the risk taking behaviour of one-year old cod of different stock and/or family origin were examined. Fish of North-eastern Arctic stock origin were found to be more prone to take risks than fish of Norwegian coastal stock origin. Furthermore, although there were no significant differences in risk-taking between families of North-eastern Arctic stock origin, a weakly significant difference existed between families of fish originating from coastal stock. The weight and condition of fish was significantly smaller in fish that emerged to escape than in fish that avoided risk and these factors may have contributed to the observed behavioural differences between stocks and families. Cortisol levels did not vary between risk avoiders or risk takers, but were significantly higher in control fish of North-eastern Arctic stock origin compared to control fish of coastal origin. These results provided evidence for a heritable component to risk-taking in cod.

The results of the aforementioned studies have important implications, particularly for the culture of cod and these implications are discussed in Chapter 7, together with a summary of the objectives and findings of each study. The future studies that are prompted by these findings are also considered.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Atlantic cod, Gadus morhua, trophic morphology, aggression, prey type, developmental plasticity, risk taking, boldness, aquaculture, cannibalism, individual variability
Subjects: Q Science > QL Zoology
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: Huntingford, Professor Felicity and Cutts, Dr Christopher
Date of Award: 2007
Depositing User: Heather Forbes
Unique ID: glathesis:2007-48
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 21 Dec 2007
Last Modified: 10 Dec 2012 13:15

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