NMFS Northeast Fisheries Science Center

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Woods Hole, Mass. – Leave more adult fish in the water today, and you increase the odds of having more young fish in the future. Take more of the breeding age fish out of the water, and you decrease the chances stocks will be abundant in the near future.

These may seem like obvious propositions, but until this year no one had produced compelling evidence that an increase in spawning fish is the key to rebuilding depleted fish stocks. In fact, some scientists and fishermen argue that when it comes to New England groundfish, the most important factor is not the number of spawners, but rather one or more environmental conditions that spawners, eggs and larvae encounter.

After analyzing data collected over nearly 40 years, three NOAA Fisheries scientists announced recently that they have found strong evidence of a link between spawning stock abundance and subsequent numbers of juvenile fish in New England groundfish stocks. Based on this relationship, the fisheries scientists are setting odds for success in rebuilding stocks under different management plans.

The connection between "spawning stock biomass" and the abundance of fish in the future is described in a paper published in February's Canadian Journal of Fisheries and Aquatic Sciences by Dr. Jon Brodziak, Dr. William Overholtz, and Dr. Paul Rago.

"In lay language, more spawning stock biomass means more sexually mature fish," explains Brodziak, the lead investigator for the study. "For most of the stocks we looked at, an increase in adult fish was more often than not followed by an increase in juvenile fish."

The finding should be good news for the groundfish fleet off the Northeastern U.S. The fleet has curtailed its effort since 1994 under a management plan designed in part to protect and increase spawning stocks. Under the plan, some stocks but not all of the depleted groundfish stocks have had increases in spawning stock followed within a few years by increases in numbers of juvenile fish.

Last year fishery managers asked the NOAA Fisheries biologists to predict the size of important groundfish stocks over the next ten years under different management plans that provide more-or-less protection for spawning stocks.

Predicting the size of fish stocks is complicated by impact of environmental conditions on fish eggs and larvae. Fish base their reproductive success on producing large numbers of offspring rather caring for a limited number of young. A single adult female can produce hundreds of thousands of eggs during a spawning, millions over the lifetime. Some years one or two in every hundred survive to become juvenile fish. Some years the odds of survival are far worse.

"With some animals, you might be able to predict how large the herd will be in two or three years based on the number of babies born this year," explains Rago, a fisheries biologist and co-author of the paper.

"But with fish, survival rates can vary enormously – the odds of going from egg to juvenile can vary from year to year by a factor of 100," Rago says. "As a result, you cannot predict very accurately the size of fish stocks X years in the future by simply by estimating how many eggs are in the water and multiplying by some survival factor."

On Georges Bank, large spawning stocks of yellowtail flounder (shown here) and haddock (shown at the top) are 20+ times more likely to result in good recruitment than small spawning stocks.

In an attempt to predict stock sizes over the upcoming 10 years under several fishing effort scenarios, the NOAA Fisheries team tested a half dozen fish population models to see which give the most accurate results with New England groundfish stocks. Some of these models emphasized the role of environmental factors, others the size of the spawning stock.

Models are sometimes tested by measuring their performance with a few years' worth of data. If, for instance, you wanted to know which model best predicts the size of the codfish stock on Georges Bank, you might run each model with data from the past five years and see which prediction most closely matches what actually happened. But if you look at only a few years' worth of data, you risk missing the real picture.

"Trying to understand what is happening with fish stocks by looking at a few years of data is like watching a parade through your basement window," Rago says.

Fortunately the team had a larger window through which to watch fish stocks rise and fall. NOAA's Northeast Fisheries Science Center has been collecting information about New England groundfish stock composition and abundance since the early 1960s. This remarkable 38-year record, which is the world's largest and oldest continuous fisheries biological database, contained information about spawning stock abundance and subsequent recruitment for all 11 groundfish stocks in question.

Testing the six different models with nearly four decades of information about the about stocks of cod, flounders, and hakes, the scientists found that no one model worked well for every stock. But spawning stock biomass models gave the best predictions for eight of the 11 stocks analyzed. In those eight stocks, higher recruitment is more likely when spawners are more abundant.

For instance, a larger-than-average spawning stock of Southern New England yellowtail flounder is three times more likely to produce a large number of juvenile fish than a smaller-than-average spawning stock. The odds with Georges Bank winter flounder are also 3:1 in favor of a larger spawning stock. For Southern New England winter flounder, the odds are 9:1. For Georges Bank haddock and yellowtail, the odds are better than 20:1 that a large spawning stock will outperform a small spawning stock..

"The results clearly demonstrate that if you want to maintain healthy, sustainable fisheries, it makes sense to leave a large biomass of spawners in the sea," Overholtz said.

"With fish stocks, there are still too many unknown factors and too much missing information to say, ‘If you do this or that, you are going to have this or that number of fish,'" Brodziak said. "But we can now say something really useful: we can give you the odds on what recruitment will be like depending on the size of the spawning stock."

It may be strange to think of scientists as odds-makers, but fishermen and fishery managers may find the odds analysis in the spawning stock study makes their jobs a little easier. If you make decisions that turn the odds to 3:1 in your favor, you won't win every year, but you'll be mighty successful over a lifetime of fishing.

Southern New England winter flounder offer 9:1 odds of a large spawning stock outperforming a small spawning stock.