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Temporal trends and variability in the spatial distribution of European tropical tuna purse-seine fishing in the Atlantic and Indian Oceans
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It is useful to complement more sophisticated stock status estimations based on stock assessment models with simpler approaches based on analyses of raw catch-effort data to maximize the probability of detecting overexploitation and hyperstability as early as possible. Here we develop a series of annual indices for the spatial distribution of catch over 1991-2019 by European purse seine vessels of the three major tropical tuna species as a function of ocean and fishing mode (floating object or free swimming fish schools). Time series of these indices are examined to identify temporal patterns with a focus on any long term trends that might be indicative of declining stock status or hyperstability. Spatial indices are also calculated for important bycatch species over 2011-2019 from observer data for French vessels. In general, results indicate a relative stability in the spatial distribution of catch over the last 30 years, though major perturbations, such as Somali piracy and major El Niño events, are identifiable. Nevertheless, recent decreasing trends in the presence of bigeye tuna and certain bycatch species merit further investigation.
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