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Fishing with Mike O’Brien… Butterfish: Little Fish Big Science

It is doubtful you’ve ever fished for or eaten butterfish. Butterfish are a thin, deep-bodied ocean fish that travel in large schools, typically offshore. These small, silvery species are a favorite food source for large game fish and are favorite bait for anglers in pursuit of tuna. I have also known anglers who were fishing the New Jersey Coast to catch striped bass and bluefish on live or cut butterfish, or imitation lures and flies.

National Oceanic and Atmospheric Administration (NOAA) Fisheries scientists keep track of their numbers, and the latest stock assessment has good news for fishermen. Butterfish are abundant, and the stock has been declared rebuilt. The more significant news is the science that went into it. For this assessment, scientists developed a new method for estimating fish abundance that takes changing environmental conditions into account — something that will become increasingly important as the climate changes and the oceans warm.

There is good news for fishermen. Al.though butterfish aren’t eaten much in the United States, they are a popular menu item in Japan, and the new assessment shows that the stock is healthy and can support a substantial commercial fishery. Following the re.cent assessment, the Mid-Atlantic Fishery Management Council proposed annual catch limits of roughly 20,000 metric tons.

Most stocks are declared re.built after a period of population growth. But in this case, it was NOAA’s scientific understanding of the stock that grew rather than the size of the stock itself. To estimate stock size, scientists do stock assessments, and the prior assessment for butterfish had a high degree of uncertainty. This made it difficult to set sustainable catch limits precisely, so to be conservative, they were set low.

Much of that uncertainty was caused by the fact that butterfish shift their distribution in response to changing bottom water temperatures. Those shifts can affect the results that scientists get when they survey fish at sea. The breakthrough came when scientists devised a way to ac.count for the effect of bottom water temperatures on the survey results. That reduced the uncertainty enough to set more precise, sustainable catch limits.

“By analyzing how butterfish respond to bottom temperature, we were able to nail the critical parameter for the stock assessment,” said Paul Rago, NOAA’s lead stock assessment scientist for the Northeast region. “That’s what allowed us to estimate the total size of the stock.”

Including environmental variables in a stock assessment might seem like an obvious thing to do. After all, fish populations, like all wild populations, respond to changes in the environment. But it’s often difficult to know which variable is driving the results. And even if the correct variable can be identified, measuring it accurately can be surprisingly difficult.

The first problem — identifying the key environmental variable — was solved thanks to a close research collaboration between scientists and commercial fishermen who joined together to form the Open Oceans Working Group. Based on their long experience on the water, the fishermen helped scientists determine that the key variable driving butterfish distribution was bottom water temperature.

The second problem — measuring that variable — was not nearly as simple as it sounds. Scientists needed to estimate bottom water temperatures at a fine spatial scale across much of the Northwest Atlantic. For that, physical oceanographers led by Josh Kohut at Rutgers University, ran computer simulations that produced maps of estimated bottom water temperatures going back 40 years. Scientists used those maps to re-analyze the results of past fish surveys and arrived at a reliable estimate of the size of the stock today.

“What we’re trying to do is to get the smartest people involved from academia, government, and the fishing industry, and to develop the science in a less adversarial way,” said NOAA Fisheries scientist John Manderson.

In the future, this type of environmental analysis might be more common, especially as fish populations shift in response to climate change. This will require new technology to monitor ocean conditions and enough computing power to crunch the data. But it will also require something more basic — old-fashioned collaboration between scientists and fishermen.

NOAA is an American scientific agency within the United States Department of Commerce that focuses on the conditions of the oceans and the atmosphere. NOAA Fisheries provides science-based conservation and management for sustainable fisheries and aquaculture, marine mammals, endangered species, and their habitats.

Mike O'Brien

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