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American fisheries are getting better at only catching the fish they actually intend to catch, rather than pulling in a menagerie of unintended victims. Around the world, the number of extra animals unnecessarily snagged during fishing—often called by-catch or discard—is decreasing. In the United States, by-catch rates have fallen from around 22 percent in 2002 to just over 10 percent in 2015, the most recent year for which figures are available. But these improvements have not happened evenly, and certain fisheries remain worse offenders than others.
In a recent study, researchers with the US National Oceanic and Atmospheric Administration (NOAA) analyzed more than 30,000 instances of by-catch recorded by 95 fisheries between 2010 and 2015.
The scientists assessed how each of the fisheries—which included the relatively modest California halibut trawl fishery, the mid-Atlantic lobster pot fishery, and the Bering Sea/Aleutian Islands pollock trawl fishery, one of the largest fisheries in the world—performed in terms of by-catch.
To compare by-catch rates across such diverse fisheries—industries that target different species in different places using various kinds of gear—the authors created a new metric called the relative by-catch index (RBI). This measure distills into one number a dozen different parameters, including how many fish and invertebrates each fishery discards; its effect on marine mammals, seabirds, and sea turtles; and the number of endangered species that it affects.
“We need tools like this because not all species are managed equally or even under the same law,” says study coauthor Elliott Hazen, an ecologist at the NOAA Southwest Fisheries Science Center in Monterey, California.
Rebecca Lewison, a conservation ecologist at San Diego State University in California who was not involved in the research, says the RBI is based on robust and intuitive criteria, including the focus on by-catch of protected species—a thorny problem for fisheries in the United States.
With a unified way to compare fisheries, the RBI let the researchers highlight trends in by-catch for fisheries across the United States.
One trend the tool highlights is that while by-catch rates for sea turtles in the Atlantic and Gulf of Mexico have fallen dramatically since the early 2000s, pelagic longline fisheries continue to catch turtles there in large numbers. Between 2010 and 2015, these fisheries caught roughly 4,200 sea turtles, including endangered loggerheads and leatherbacks, which gave them one of the worst RBI scores of all the fisheries studied.
“There’s a lot of fishing activity concentrated in areas that are apparently also good habitats for turtles,” says study coauthor Matthew Savoca, a marine ecologist at California’s Stanford University and the NOAA Southwest Fisheries Science Center.
“By-catch of vulnerable species is extremely problematic,” says Eric Gilman, principal scientist at the Pelagic Ecosystems Research Group in Honolulu, Hawai‘i, who was not involved in the study. “These fisheries can potentially put at risk the continued existence of some of these populations.”
Another trend is that fisheries are more likely to have high by-catch rates in areas with higher species diversity. Because of this, fisheries in the US Northeast, Southeast, and in the Pacific Islands have greater issues with by-catch compared with fisheries on the West Coast and in Alaska.
Non-selective fishing gear, including bottom trawls and gill nets that catch whatever runs into them, can also end up destroying habitats and disturb carbon stored at the bottom of oceans, Savoca says. Although more targeted fishing gear usually works better in terms of by-catch, he says, choosing the most appropriate gear type depends on several variables including region and time of year.
In general, however, by-catch rates across the United States seem to be improving. The reduction is due to changes in gear types and methods that increase selectivity during fishing, Gilman says. Other developments include fisheries using tools to dynamically monitor where in the water there are more likely to be by-catch species. Over the years, technologies such as turtle excluder devices and seabird-deterring streamer lines have helped lower by-catch numbers, Savoca says. “But there are also changes in markets, so species that may have been discarded decades ago now have market value and are retained,” says Gilman.
The decades-long reduction in by-catch rates is a result of “really strong efforts—both from fisheries management as well as the fisheries industry,” Lewison says. But she thinks there will always be some by-catch. “As long as we have commercial fisheries, we will have by-catch,” Lewison adds. “Which doesn’t mean that that by-catch number can’t be the smallest it possibly can be.”