Why it matters

We know that many sea creatures, including endangered species, are killed by overfishing, accidental catch and entanglement in fishing gear. More overlap between wildlife and fishing boats means that those harmful impacts are more likely to happen.

Even considering only observable fishing boats broadcasting their positions, the presence of boats signals considerable risk for marine life. For example, California sea lions forage in Pacific coastal waters from the Canadian border to Baja California and are accidentally caught by boats fishing for hake and halibut. We found observable fishing activity in over 45% of the sea lions’ habitat.

In another example, migratory salmon sharks feed on salmon near Alaska’s Aleutian Islands during the summer and breed in warmer waters off the coasts of Oregon and California during the winter. Along their journey, salmon sharks are accidentally caught in fishing nets and longlines. We detected observable vessel fishing activity in nearly one-third of salmon shark habitat.

Our findings indicate that such threats are higher when dark fishing boats are present. Estimates of risk to California sea lions and salmon sharks increased by 28% and 23%, respectively, when we accounted for dark vessels.

This information could affect fishery regulation. For example, regulators use risk information to set catch limits for species such as tuna; higher risk could mean that catch limits need to be lower.

For species such as sea lions and salmon sharks that are accidentally caught by fishermen, higher risk levels could indicate that fishing boats should use more selective gear. California is currently acting on this issue by helping fishermen phase out use of large-mesh drift gill nets in state waters. These nets, which hang like curtains in the water, catch many other fishes along with the target species.

Accounting for dark vessels is particularly important in international waters where boats from multiple countries operate, because AIS data is one of the most complete sources of fishing activity across nations. Tracking dark vessels can help make this information as comprehensive as possible and provide insights into the multinational impacts of fishing.

Our study does not account for vessels that do not use any vessel tracking system, or that use systems other than AIS. Therefore, our risk calculations likely still underestimate the true impact of fisheries on marine predators.

What’s next

The world’s oceans are rich in life but poor in data, although this is changing. High-resolution satellite imagery may soon offer even more information on risk from dark vessels.

President Joe Biden and other global leaders have pledged to protect 30% of the ocean by 2030. Better data on human-wildlife interactions at sea can help ensure that new protected areas are in the right places to make a difference.