Whale shark-tuna associations, insights from a small pole-and-line fishery from the mid-north Atlantic

Abstract

Fisherman have known and explored the association between tuna and whale sharks for centuries. Just as in the past, present day industrial fishers search for the presence of whale shark as cue to locate tuna schools. These whale shark-tuna associations have been usually assumed to result from the same ecological drivers that govern the attraction of tunas and other fishes towards floating objects or Fish Aggregation Devices (FAD). Yet, this assumption has never been explicitly investigated. As a result, the ecological (evolutionary) significance of the whale shark-tuna associations and the potential impacts of fishing these associations remain unclear. Here, we set out to test a set of predictions expected under the framework of the ‘indicator-log’ and the ‘meeting-point’ hypotheses, originally proposed in the context of the tuna-FAD associative behaviour, by contrasting inter-annual fishing patterns, over 16-years, across highly variable whale shark abundance periods and comparing whale shark associated tuna schools and free swimming schools in the Azores. In addition we also analysed the tuna fishing fleet behaviour over periods of high and low whale shark abundance across the archipelago.We found an overall south and eastward migration of fishing effort towards lower latitudes, where most whale sharks were reported, compared to the low whale shark occurrence period. Overall we found that the whale shark associated school differed from free swimming schools and fishing yields were also influenced by the presence of the whale shark. Specifically, we found that skipjack, bigeye, albacore and yellowfin tuna are caught in higher quantities when associated with whale sharks. Whale shark associated schools were more diverse and yielded smaller fish on average than free-swimming schools, as predicted under the theoretical framework of tuna-FAD associations. Our results highlight the possibility that whale shark behaviour may play an active role in the association dynamics, and that these associations may be mutually beneficial. We argue that whale sharks may represent a “meeting point” for the aggregation of small tunas and that this behaviour is more likely to be size dependent than species oriented. Finally, we discuss the potential implications of our findings for the management and conservation of both tunas and whale sharks.