Within- and among-individual variation in metabolic rate in juvenile Atlantic salmon, Salmo salar

Animals may adopt a range of energy strategies to persist and perhaps prosper under various prevailing environmental conditions. For example, they may achieve similar growth by investing heavily in maintaining a high capacity food capture and processing system or by using a lower capacity system that is cheap to run but which is less effective at accruing further resources. The way that individuals within species allocate energy resources is an intriguing issue that has implications for understanding competition, population structuring and the response of populations to environmental change. Energy budgets account quantitatively for the pathways by which food energy results in variation in somatic resources and constitute an important basis for evaluating links between behaviour and aspects of performance. Atlantic salmon have been an important model for exploring relationships between growth performance, metabolic strategies and individual behaviour. Juvenile salmonid fish have been a particularly useful subject for exploring individual variation in metabolism because it has been possible to relate standard metabolic rate (SMR) to behavioural traits and lifestyle within a species. SMR correlates with dominance status, which is reflected in the ability of fish to access high value food patches and may promote faster growth under some conditions. This thesis focuses on the within- and among-individual variation in SMR and looks at how a fish’s external environment and social interactions can influence its SMR, and whether this variation in SMR can affect other aspects of metabolism such as feeding. Experiments were carried out to determine: (A) whether the presence of a shelter reduces SMR; this may provide an additional reason for the extent to which these fish will compete aggressively for shelters when these are limiting (Chapter 2); (B) the extent of intraspecific variation in SDA (i.e. differences in the measured parameters that are used to define SDA), and to test whether the SDA for a given meal varies as a function of individual traits such as SMR. If SMR is directly related to scope for growth, then the speed and size of the SDA response should also correlate with SMR (Chapter 3); (C) whether ventilation rate is sufficiently accurately related to metabolic rate (MR) at a range of temperatures and activities so as to allow energy expenditure to be predicted outside a respirometer (Chapter 4); and (D) how visual isolation and the presence of a conspecific can affect SMR, using the method developed in the previous chapter (Chapter 5). Access to shelter was shown to have a significant impact on SMR, producing on average a 30% increase in metabolic costs in the absence of shelter. Therefore, the presence of appropriate shelter not only reduces the risk of predation but also provides a metabolic benefit to fish that is likely to have implications for growth performance and activity budgets. Variation in SMR was shown to affect the profile of energy consumption during digestion of a meal. The peak in post-prandial oxygen consumption, the duration of elevated metabolism and the overall magnitude of SDA all increased with the size of meal consumed. However, for a given meal size, fish with a higher SMR also had a higher peak and greater magnitude of SDA, yet experienced a shorter duration over which metabolism was elevated following the meal. Intraspecific variation in SMR is thus linked to variation in digestive strategies, although the costs and benefits of a given SMR are likely to vary with resource availability and predictability. Ventilation frequency was shown to be a good correlate of MR, since MR was found to correlate strongly with VR in all fish tested, at all temperatures and the relationship was independent of causal factor. The relationship was linear, and both the slope and corresponding intercept of the regression equation were strongly dependent on the fish’s body weight and the test temperature. Visual measurements of VR may therefore provide a highly accurate, cheap and non-invasive method of measuring the energy consumption of fish engaged in natural behaviours. Comparing SMR of individual fish when alone with response to presence and absence of physical shelters showed that all fish exhibited a reduction in SMR when provided with an appropriate shelter. However, when grouped, there were both positive and negative group effects on SMR depending on the relative size of fish. The consequences are that, although the group effect on SMR may be small on average, at the population level it is large, of variable sign and profoundly important in terms of the overall energy budget at the individual level. This thesis therefore shows that metabolic rates can vary extensively within as well as between individuals, depending on the context, and this variation will have profound effects on overall energy budgets.