Devices used to catch tuna are killing hundreds of thousands of sharks per year in the Indian Ocean, researchers estimate.

Tuna tend to gather around objects floating in the sea. So over the last two decades, fisheries have attracted tuna by leaving masses of bamboo poles and netting, called fish aggregating devices (FADs), adrift in the ocean. Later, fishing boats return to catch the assembled fish.

To find out if sharks could become tangled in the nets, the study authors attached satellite tags to 29 silky sharks in the Mozambique Channel and near the Republic of Seychelles. The movements recorded by the tags suggested that four of those sharks became caught in the nets near the surface, the team reports in Frontiers in Ecology and the Environment. Divers also visited 51 FADs and found that 35 percent of them had ensnared one or more silky sharks.

Extrapolating from that data, the team estimates that FADs in the Indian Ocean kill about 480,000 to 960,000 silky sharks per year. Fisheries should replace the netting with biodegradable ropes, the authors say. — Roberta Kwok | 4 July 2013

Source: Filmalter, J.D. et al. 2013. Looking behind the curtain: quantifying massive shark mortality in fish aggregating devices. Frontiers in Ecology and the Environment doi: 10.1890/130045.

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