By Karen Zusi
BU News Service

The prospect of casting a fishing line and snagging a shark might sound exciting to some young anglers, but for the sharks, getting accidentally caught could be a death sentence.

A study published online this summer in Global Ecology and Conservation investigated the survival rates of twelve different North Atlantic shark species in commercial longline fishing and the potential impacts on their populations. Researchers from the University of Miami found significant differences between each species; their work can be used to inform future conservation targets for sharks in the region.

Unintentional hooking of animals is a major issue in commercial fisheries management. Longlining, a method in which fishing lines with multiple baited hooks per line are released from a boat for later retrieval, often doesn’t include modifications to prevent other species from taking the bait. These longlines can catch unwanted or endangered animals like birds, sea turtles, and sharks (called “bycatch”) as well as commercially-valuable swordfish and tuna.

The shark researchers looked at seventeen years’ worth of longline fishing bycatch data to determine when each species was caught and whether an individual was dead when the line was retrieved. They then investigated factors such as water temperature and hook depth to determine which specific variables might affect each species.

“One species may have 80% survival, but if it has 60% survival when you fish in much deeper water, that’s important to consider,” explained Austin Gallagher, a PhD candidate at the University of Miami and the study’s lead author. “That can be used to shed light on mechanisms that lead to mortality.”

The researchers also collated published details of shark physiology to get a picture of each species’ overall reproductive potential. By combining these datasets, the team was able to assign vulnerability scores to different types of sharks. They based the scores on how likely the sharks were to be caught in the first place, how likely they were to die when they were hooked on the line, and how well their populations could bounce back from longline mortality.

The study found that some shark species are more resilient to longlining than others. Tiger and blue sharks had over an 80% survival rate regardless of other factors, while populations of scalloped hammerhead, dusky, longfin mako, and bigeye thresher sharks were highly vulnerable under certain conditions.

Gallagher commented, “This is a huge global problem for the species that aren’t surviving. It’s a really critical thing that we have to start getting our fingers on and developing policies for.” He added that sharks serve a critical function in the environment by “eat[ing] the dead, dying, and diseased fish. They essentially clean the environment out.”

Conservation in longline fisheries is a high-stakes balancing act between protecting the animals and protecting the livelihoods of the fishers. Jordan Watson, a PhD candidate at the University of Alaska working with the National Oceanic and Atmospheric Administration to investigate new mitigation strategies, commented on Gallagher’s study and the importance of tailoring these methods: “It’s good that there are people still collecting the traditional ecological data and figuring out the biology so that we can use the mitigation measures.”

Watson’s recent work has focused on fishing gear modifications that reduce bycatch without severely lowering the catch of desired species. He continued, “I think bycatch reduction is going to be a story of small victories, and hopefully we can continue to make enough small victories that it will make an impact on the battle.”