A very high proportion of modern slavery occurs on fishing vessels, and it is a major cause of the collapse of marine fisheries. andrey_I/Shutterstock

Slavery and overfishing are both nearly universally condemned these days, even as both continue. What is less acknowledged is the two are inextricably linked, with the destruction of the oceans made possible in part by the failure to tackle slavery on fishing vessels.

The world's fisheries are in trouble as deep as the oceans in which the fish swim. Global catches of wild ocean fish have been falling by more than a million tons every year for two decades, even while fleets expand. Many fish stocks will take years to recover, even if we were to cease fishing now.

Some of the reasons for this are well recognized, with a lack of international regulation creating a classic “tragedy of the commons”, where fleets acting in their own short-term interests undermine the welfare of all. However, a paper in Nature Communications shows there is a substantial relationship between the prevalence of modern slavery in a fishing nation and the amount of unreported (and therefore unsustainable) fishing catch.

Other factors associated with a high rate of labor abuses in a nation's fishing fleets, as estimated by activist organizations, include low prices, a high proportion of catch coming from outside a nation's exclusive economic zone, and government subsidies, usually in the form of cheap fuel.

“As a result of subsidies to build fleets we have twice as many fishing boats as we need,” the study's first author, University of Western Australia PhD student David Tickler, told IFLScience. Subsidized fuel, Tickler added, “keeps you out fishing, but doesn't help with crew costs.” As the fish get harder to find, but competition keeps prices low, profitability depends on squeezing labor costs.

In some countries, this has been dealt with through mechanization, but elsewhere vessel owners have responded with conditions classified as modern slavery. Crew may find themselves unpaid if they're allowed to leave ships at all, even being murdered when they complain. Nations richer than their neighbors recruit migrants that “often lack status and formal paperwork making them particularly vulnerable,” said Tickler. Unfamiliarity with the language of the vessel's home nation makes complaints harder still.