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Abstract

The Azores archipelago lies at the limit of whale shark (WS) distribution in the North Atlantic, where sporadic sightings of “pintado” (local name) date back to the early tuna fishing operation from the mid XX century. From 1998 and 2008, only nine adult whale shark sightings had been confirmed by the Azores Fisheries Observer Program. However, from 2008 onwards there has been a dramatic increase in the numbers of reported adult individuals around the most southern island of Santa Maria. A recent study found that this increase in sighting frequency was related to an increase in local SST (above 22.5°C), and decrease in chlorophyll-a, with local variation around the large seamounts in the area. While whale sharks and tuna have long been known to associate, the nature and motivation of these associations remains unclear. Moreover the WS-tuna associations in the region remain to be characterized as well as the potential impact of fishing these associations. In this study, we analysed a 13-year observer dataset (1998–2011) from the pole-and-line tuna fisheries from the Azores EEZ to investigate the regional behaviour (distribution in catch abundance and composition) of three tuna species, skipjack, bigeye and albacore during this period. We also examined how these patterns have responded to the environmental drivers, the distribution and abundance of whale sharks and compared whale shark associated fishing events and events targeting free swimming schools. We also detailed an elaborate protocol to guide future research to investigate the impact of fishing the WS-tuna associations and test the “functional bycatch” theory by combining state of the art electronic tracking and behaviour technology with physiological proxies of stress and condition of both whale shark and tuna. Changes in the distribution the tuna species were evident with the centre of gravity found closest to the whale shark aggregations in the latter years (2008 in particular) and was independent of fishing effort. Between 2008 and 2011 for the areas where whale sharks have been found we find the associated whale shark fishing events have significantly more individuals of skipjack and bigeye tuna and the area surrounding the eastern islands (Santa Maria etc.) also yielded the highest catches per fishing event. The albacore did not show any significant variation in CPUE for either predictor. Overall, catch rates of tuna were higher and multi-species aggregations were more likely when associated with whale sharks. Preliminary results suggest that the presence of WS during the tuna fishing season may have an effect on fishing yields and tuna species composition of catches. It is also apparent that the fishing fleet behaviour could be responding to WS distribution and abundance. Our results support the need to investigate the potential detrimental effects that fishing WS-tuna associations may have on both the WS and tuna.

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/content/papers/10.5339/qproc.2016.iwsc4.17
2016-05-15
2019-09-20
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http://instance.metastore.ingenta.com/content/papers/10.5339/qproc.2016.iwsc4.17
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