Eyebrows went up from Cape May to Rockport back in January when the regional stock assessment workshop reported results of the first Atlantic mackerel status review since 1999. “The estimate for long-term maximum sustainable harvest is half or less of the estimate made in the last assessment. It’s a big change,” said Bill Overholtz, lead mackerel assessment scientist for the NEFSC.

If you’re on the other end of the pipe from Overholtz, fishing for mackerel, that also sounds like you might be harvesting less in the future, and that opportunity for expanding this fishery is more limited than previously thought. This, after planning out-year business based on much higher expectations for the long-term sustainable catch. Also, the stock is at record highs, having bounced back from record lows during the 1970s.

Atlantic mackerel at the underwater surface. Crown copyright (UK) used by permission of Fisheries Research Services, photograph by Tom McInnes

As the fishery has taken off, so has the need to better understand how this population responds to harvest. “That’s the biggest reason we’ve developed a more sophisticated model for mackerel--because the fishery has become more important in the last few years,” said Overholtz.

So what happened when the new model was put to use? The resulting picture is more reliable than that presented in the 1999 assessment, but it is also less clear: it estimated long-term sustainable harvest as a range, between 196 and 326 million pounds. So which is it? Or is it what the 1999 assessment indicated — 719 million pounds?

It’s almost certainly not 719 million pounds, said Overholtz, “You only need to look at history to see that.” Between 1970 and 1976, mackerel stocks were extremely large, as they are now, and international fleets took an average of 764 million pounds of the species annually, but the result was a stock collapse.

Productivity is what underlies the estimate of long-term sustainable harvest: how many fish of what size and age can be harvested annually such that the remaining fish can replenish the population through reproduction? To get a better grasp on the actual productivity of a big mackerel stock when it’s supporting a substantial fishery, researchers need more data.

“That includes biological information taken from the landings to better describe the characteristics of the current population, and a chance to observe how the population changes over a few years when significant landings are being made,” said Overholtz.

In February, Overholtz and NEFSC director John Boreman met with several mackerel processors and harvesters to explain more about the range of values presented in the new estimates, both for long-term sustainable harvest and for the total biomass necessary to support it.

“We planned to increase our own sampling effort, but we also thought that an industry-based sampling program might be an inexpensive and reliable complement to that effort,” said Boreman. Mackerel are processed at a handful of fish plants in the Northeast, where samples can effectively be obtained as a regular part of processing the fish.

Indeed, several processors were interested in supplying samples. A pilot project is now in place at four processing plants to provide more information on the lengths of fish in the landings . Participating plants collect samples twice a month, and report length and weight data for the fish. A “sample” is 100 fish per each market size category. Data collected at the plants will be a source of valuable information for improving the assessment.

NORPEL (Northern Pelagic Group, LLC), one of the largest pelagic fish processing companies in the U.S., just sent its first samples from the pilot project to researchers at the NEFSC. Other processors involved in the pilot are Cape Seafoods (Gloucester, MA), the shoreside processing vessel Atlantic Frost (Fall River, MA), and Lund’s Fisheries, Inc. (Cape May, NJ).

One thing the sampling program is expected to provide is more information on older, larger fish. “Mackerel can live to be 20 years old, but most of our samples come from medium and smaller fish,” said Overholtz. “We’re not suggesting that older fish aren’t in the stock, but they are an important part of the picture when it comes to figuring out productivity, since older, larger fish are usually the biggest contributors to the pool of new fish.”

Mackerel travel in very large schools, and tend to aggregate by size. “Right now we are not catching many larger mackerel in our research trawl surveys, and the fishery tends to stay on large schools of fish, even if the individuals aren’t the biggest fish that might be out there,” said Overholtz. Improved sampling might be one way to capture data when a trip of larger fish is actually landed.

The NEFSC’s developing hydroacoustic survey also holds potential for improving data on larger mackerel. In this kind of survey, the research vessel broadcasts an underwater sound toward a school of fish. The sound is bounced off the target fish and instruments aboard the survey vessel read the echo, creating a readable image of the school from which some physical characteristics, including size of individuals, can be obtained.

While we might not be too sure about the long-term sustainable harvest, there’s not much uncertainty about the overall condition of the stock. “It’s in great shape,” said Overholtz. The current biomass of fish large enough to spawn is probably greater than that observed during the last big mackerel boom of the 1970s. “The uncertainty is mainly over how productive the stock is, how much can be harvested without overfishing the resource,” he said.

Posted March 20, 2006