Habitat transitions in marine fishes-expanding the otolith toolkit

Abstract

Most marine teleost fishes suffer high mortality during the larval phase, therefore small differences in larval mortality can have big impacts on population and stock dynamics. Larval fish may move large distances, potentially experiencing a range of conditions which can influence mortality. Tracking the migration of fish larvae and predicting how changes in ocean conditions may influence migration routes (and potentially larval mortality) is challenging, but essential for stock management. Recent advances in tag technology have allowed monitoring in smaller species, but small body sizes and high larval mortality rates limit available data. Individual-based migration models have been developed to predict potential migration pathways but still present inherent uncertainty due to assumptions that are difficult to verify empirically. Otoliths, continually growing, paired calcium carbonate structures formed in the fish's inner ear, can potentially reveal individual migration patterns by recording ambient water chemistry experienced during growth, related to hydrography. However, linking otolith compositions to a specific spatial origin is challenging due to the unknown environmental factors and difficulties in otolith sampling techniques. This study aims to evaluate the potential of using otolith chemistry, specifically stable oxygen isotopes coupled with individual-based migration models, to reconstruct the migration history of individual small fish and to establish a connection between the theoretical movement predicted by the model and the empirical data obtained from the otolith isotope-based method. The study includes three main approaches: (1) a case study of passive drift of North East Atlantic mackerel, using simulated ocean model currents and hydrography to predict otolith isotope records for individuals following a range of pathways around the continental shelf; (2) analysis of stable isotopes of otoliths to infer aspects of thermal physiology in juvenile mackerel; (3) the application of the model-otolith isotope combination analysis method to a broader range of open-sea eel larvae for a range of hypothetical swimming behaviours. In (1), the potential use of otolith δ18O profiles, as accurate and low-cost "natural tags" for stock discrimination and broad-scale geolocation of fish, are first evaluated. In (2), estimates are obtained of the thermal performance curve of field metabolism in a wild juvenile fish, indicating that ecological factors such as food availability or competition for resources may play a more significant role in shaping an organism's overall metabolic rate than temperature alone. In (3), extension of drift analysis with different swimming vectors further explores the potential of using otolith δ18O values to distinguish between successful and failed drifts, and to verify the hypothesis that NAO affects eel migration. This thesis thus provides insights into the potential of using otolith chemistry and individual-based migration models to improve our understanding of the early life history of small fish and their migration patterns, which can have important implications for the conservation, management, and recovery of fish stocks.

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

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