Tuna often associate with large marine species such as dolphins and whale sharks; tuna fishermen sometimes rely on sightings of these species to indicate the presence of large schools of tuna. Once tuna have been sighted, a net is set to encircle the shoal and the bottom of the net is drawn together, like a purse, to trap the fish. Owing to the fact that there are often large marine fauna close by, these animals can also become trapped along with the tuna.
This study used log book data and observer data to assess the number of whale sharks sighted, caught and killed in purse seine nets in the Atlantic Ocean and the Indian Ocean. The biggest incidence of whale shark mortality occurred in certain hotspots where whale sharks overlapped with the purse seine fishers. Overall the incidence of whale shark mortality in the Atlantic and Indian Oceans was relatively low - from 1995 to 2011, 2 whale sharks were reported to have died out of 186 encircled by the nets. In recent times, the purse seine fisheries have implemented new strategies for ensuring the survival of whale sharks caught in nets, such as the cutting of the net to allow the shark to escape.
This data is in stark contrast to whale shark mortality in the Pacific Ocean provided by the Western and Central Pacific Fisheries Commission (WCPFC). Between 2007-2009, 56 whale sharks died in purse seine nets - the most common reason cited was that the fishermen were not aware of the presence of the shark in the net before mortality occurred. As a result of this, the WCPFC has banned the setting of nets around whale sharks since the beginning of 2014.
Additionally, both the Indian Ocean Tuna Commission (IOTC) and the WCPFC have admitted that accurate data on the number of whale sharks encircled and killed by purse seine fisheries is difficult to obtain. The data for this study relied on the accurate reporting of whale shark mortality by purse seine skippers and observers - which only cover 10% of the European fleet.
More analysis of the tuna purse seine fishery and better observer coverage is needed in order to obtain a better picture of the mortality of large pelagics in the tuna purse seine industry.
