During two surveys in 2011 and 2013, we deployed pop-up satellite archival tags (PSATs) on subadult or adult porbeagles at the Bay of Biscay shelf break. We collected data that enabled the reconstruction of nine migrations (eight females, one male) that uncover the large spatial extent of these sharks in the Northeast Atlantic. The mean duration of each deployment was 292 d, with four reaching 365 d. The reconstructions show that, after migrations that extended up to 2000 km away from the point of release, the tagged porbeagles returned to their location of tagging. All the reconstructed migrations followed the same general pattern of a migration away from the Bay of Biscay in late summer, and a return in spring the following year. The total distance of the migrations was estimated at 5000–13 000 km for PSATs deployed for a full year (n = 4), with examples of migration to the Arctic Circle, southward to Madeira and westward to the mid-Atlantic Ridge. The observed site fidelity to the Bay of Biscay and the common migration pattern of all females provide evidence of complex spatial structure and dynamics that encompasses both the open ocean and heavily fished coastal areas, and highlights the challenge of assessing and managing the porbeagle stock in this area.
Besides understanding the effects of fishing on harvested fish stocks, effects on non-target species, habitats and seafloor integrity also need to be considered. Static fishing gears have often been mentioned as a lower impact fishing alternative to towed gears, although studies examining their actual impact on the seafloor are scarce. In this study, we aimed to describe fish trap movements on the seafloor related to soaking time and trap retrieval. Impacts on the seafloor of lightweight rectangular traps and heavier circular traps were compared. We used 3D video cameras to estimate sweeping motion on the seabed and penetration into the sediment during soaking time. The area and distance swept by each type of trap during retrieval was determined by a camera set up facing the sea bottom. The potential rotation of the traps around the mainline was assessed using an Acoustic Doppler Current Profiler. Results showed that no penetration and almost no movements could be detected during soaking time for either lightweight or heavy commercial traps, even for high tidal coefficient (maximum 6 cm). No rotation could be observed when the tide turned. The swept area covered by a trap during retrieval was low (maximum 2.04 m 2) compared to towed fishing gear and other static gear.
-In the north-eastern Atlantic, meagre (Argyrosomus regius) is one of the largest fish living on the shelf and this species has important commercial interest. Over the past two decades, large variations in meagre abundance have been observed with pluri-annual cycles but the factors involved in such variations are still unclear. Trophic interactions between meagre and other species (both prey and competitors) might be one explanatory variable of the observed variations in meagre recruitment and abundance. In the present study, we described the diet of commercial-size meagre in the Bay of Biscay from stomach content and stable isotope analyses, and explored its dietary ontogeny. We found that commercial-size meagres were mostly piscivorous with a diet dominated by clupeiform fish (mainly anchovy and sardine) completed by demersal fish (mainly pout and whiting). Cannibalism also accounted for a non-negligible part of the diet. Interestingly, stable isotope and stomach content analyses showed only a very slight increase in prey length and trophic level during the ontogeny of large meagre after 50 cm of total length and despite a 3 fold-change of the individual length in our sampling. Our results suggested that specific trophic interactions (i.e. bottomup control by clupeiform fish on meagre population, competitive effects on piscivorous populations or topdown control by meagre on clupeiform fish populations) may occur in the Bay of Biscay and can impact meagre abundance dynamics. Our study underlined the interest to enhance ecological knowledge of preypredator relationships in the development of ecosystem-based approach to understand trophic controls impacting aquatic living resources and fishery economy.
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