Other than an affinity for open water, seabirds, sea turtles and marine mammals such as dolphins may not appear to have much in common. The sad truth is that they are all unintended victims – bycatch – of intensive global fishing. In fact, accidental entanglement in fishing gear is the single biggest threat to some species in these groups. A new


 co-authored by Stanford Woods Institute Senior Fellow 

Larry Crowder

provides an unprecedented global map of this bycatch, starkly illustrating the scope of the problem and the need to expand existing conservation efforts in certain areas.

“Some of the earliest bycatch issues involved marine mammals taken in purse seines (large nets with floats) that pursued tunas and shrimp trawls that drowned sea turtles,” said Crowder, who is science director at the

Center for Ocean Solutions

and the Edward Ricketts Provostial Professor of biology at Stanford. “The problems were ultimately solved by scientists who clearly defined the problems and by managers and fishermen who sought innovative fishing methods to allow fisheries and protected species to coexist.”

During the past half-century, fish hauls around the world have increased from about 19 million tons to more than 154 million tons per year. Beyond the problem of overfishing – more than 60 percent of sea creatures brought up are classified as overfished or collapsed stocks – bycatch can lead to “major changes in ecosystem function and process,” the study’s authors write. It also damages fishing gear and wastes fishermen’s time and money. Accurate data on global bycatch are hard to find because of the need for trained on-ship observers across vast oceans.

To fill the gap, Crowder and Rebecca Lewison, lead author of the paper and co-leader of the study directed a research team that looked at hundreds of peer-reviewed studies, reports and symposia proceedings published between 1990 and 2008 to obtain a global perspective on what kinds of animals were being caught, where they were being caught and the types of gear in which they were trapped. They then compiled all of this information into a single comprehensive map and dataset.

“It highlights the importance of looking at the bycatch issue across different species, fishing gears and countries,” said Lewison, an ecology professor at San Diego State University. “When you do that, it makes it clear that to address bycatch, fishing nations need to work together to report and mitigate bycatch. No single country can fix this.”

The study revealed bycatch hotspots and gaps in available data, such as the lack of information on small-scale and coastal fisheries and many ocean regions that are heavily fished by commercial fleets. Among the findings:

  • Marine mammal bycatch is highest in the eastern Pacific and the Mediterranean.
  • Sea turtle bycatch is most prevalent in the southwest Atlantic, eastern Pacific and Mediterranean oceans.
  • Seabird bycatch is highest in the southwest Atlantic and Southern Indian oceans.

Expanding the use of bycatch mitigation tools such as turtle excluder devices is only part of the solution puzzle, according to the study’s authors. Community engagement is key in less-regulated, small-scale fisheries. To make meaningful headway, researchers and managers need more data on the extent and distribution of global fishing. “Reducing bycatch levels in coastal fisheries sectors will require coordination across national boundaries using integrated approaches that link conservation action with sustaining human livelihoods, incentives and commitment to protecting the environment,” the study’s authors write.

This study is part of a research initiative funded by the Gordon and Betty Moore Foundation.

Read about a Stanford-related sustainable fishing project that aims to reduce bycatch.