The otolith-isotope method: an opportunity to examine field metabolic rate as an in situ indicator of climate change within and across juvenile Atlantic cod populations (Gadus morhua)

De Groot, Valesca (2023) The otolith-isotope method: an opportunity to examine field metabolic rate as an in situ indicator of climate change within and across juvenile Atlantic cod populations (Gadus morhua). Masters thesis, Memorial University of Newfounland.

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Abstract

Individual metabolism is a unifying variable in animal ecology, influencing all aspects of performance including growth rate, energetic efficiency, and mortality. As abiotic factors continue to fluctuate due to climate change and anthropogenic disturbance, it is becoming increasingly important to measure an individual's metabolic rate in its natural environment to assess critical energetic tradeoffs. Field metabolic rate (FMR) is the metabolic rate measured from a free-ranging organism, taking into account an individual’s specific dynamic action (SDA), standard metabolic rate (SMR), and activity, yielding a holistic estimate of energy intake and expenditure. Unlike with terrestrial animals, our knowledge of the physiological processes of fishes to date is largely based on extrapolation from observations of historic populations, or from laboratory-based measures of respiratory potential. Here, I describe a newly developed proxy for FMR which pairs the stable isotope composition of the otolith with estimates of oxygen consumption and experienced temperature to yield the full temporal history of the energetic costs associated with environmental change in free-ranging fishes. I outline the opportunities this method presents to make macroecological connections between individual metabolic rate and greater ecosystem interactions, and pair gaps in knowledge in the fields of conservation, ecology, and physiology with important research questions. I also apply the otolith-isotope FMR methodology to juvenile Atlantic cod (Gadus morhua) populations from the Northwest Atlantic, an iconic and economically important species, to examine the relationship between vulnerable early-life stages, decreasing ocean temperature, and energy costs. I compare the FMR-thermal sensitivity and the mean FMR between life history stages within the Newman Sound population (age-0 pulse 1 and pulse 3) and between populations (Newman Sound, Newfoundland; Skagerrak Coast, Norway) to identify pulse- and population-specific behavioural and physiological tradeoffs. I discover pulse-specific differences in mean FMR, and provide insights on the pathways of energy allocation. At the population-level, I recover in situ physiological trends consistent with the metabolic cold adaptation hypothesis, suggesting population-specific thermal adaptation in juvenile Atlantic cod from two populations on opposite sides of the North Atlantic. This research contributes to expanding the current knowledge base of in situ physiological performance for an important fisheries species in the face of changing oceanic conditions. We hope future studies can build upon this research to bridge the gap between data and policy, incorporating field metabolic rate into stock assessments to better inform recruitment forecasting, ensuring a long-term, sustainable Atlantic cod fishery.

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