Temporal trends and variability in the spatial distribution of European tropical tuna purse-seine fishing in the Atlantic and Indian Oceans

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.