Variation in the field metabolic rate of wild living plaice (Pleuronectes platessa) from the North Sea: constraining influences of body size, temperature, seasonal cycle, individual level growth rate and year of sampling

Abstract

Field metabolic rate (the combined energetic cost of performance when exposed to natural conditions) represents how an individual with a particular expressed genotypical and phenotypic respiratory potential reacts to the experienced environment. Population-scale measurements of field metabolic rate (FMR) can potentially identify the sensitivity of populations to environmental drivers such as temperature, and the capacity for climatic resilience through behavioural adaptation. To date, from a fisheries perspective, FMR is largely inferred or assumed from the relationship between environmental conditions and distribution, coupled with laboratory based respiratory potential experiments. Such studies have broad and untested assumptions, and as a result the accuracy of their findings has been questioned. In this thesis I apply a newly emerging proxy for estimating field metabolic rate in marine fishes, drawing on stable isotope values from otolith aragonite. Stable carbon isotope compositions are used to estimate the proportion of respiratory CO2 within the blood (and, therefore, the rate of oxidation of dietary carbon), while stable oxygen isotope values are used to infer experienced temperature. I apply the otolith metabolic rate proxy to otoliths recovered from a population of European plaice from the North Sea sampled during a period of rapid warming between the 1980s to the mid 2000s. I estimate time averaged FMR, experienced temperature and growth rate in 558 fish during various life stages and test three main hypotheses: 1) FMR covaries with environmental drivers influencing standard metabolic rate (body size and temperature), in a comparable manner to established metabolic theories of ecology: 2) variations in FMR over a seasonal cycle covary with seasonal bioenergetics rather than extrinsic thermal variability: and 3) individual level growth rate varies predictably with FMR. Within the sampled data set, FMR did not covary systematically with body size and temperature for adult populations. Temperature was shown to positively covary with FMR during juvenile life stages of the same individuals, suggesting that the energetic drivers forming FMR expression are not consistent throughout individual life history. Individual variation in FMR was conserved between juvenile and adult samples, and metabolic phenotype explained a greater proportion of among-individual variation in adult FMR than either body size or temperature. Seasonal variations in among-individual FMR appear to correlated to the migratory, feeding and breeding cycles of plaice, suggesting that FMR expression is a complex relationship between a combination of behavioural and physiological drivers. The inclusion of intrinsic factors, such as condition, explain a greater proportion of growth rate deviance within this data when compared to extrinsic factors. These findings imply that FMR trends are more complex than previously believed, and highlights the importance of incorporating individual behavioural patterns into predictive biogeographical and population output models. Stable isotope-derived estimates of field metabolic rate have great potential to expand our understanding of ecophysiology in general and especially mechanisms underpinning the relationships between animal performance and changing environmental and ecological conditions.

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Identifiers

ORCID for Clive Trueman: orcid.org/0000-0002-4995-736X

ORCID for Phillip Fenberg: orcid.org/0000-0003-4474-176X

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