Counting the Fish in the Sea
New evidence about menhaden could inform new approaches to managing them
School of menhaden. Photograph, NOAA Fisheries/Jerry Prezioso
Atlantic menhaden form large schools in the Atlantic Ocean and Chesapeake Bay that are sought after by fishers and predators. Photograph, NOAA Fisheries/Jerry Prezioso

OILY AND BONY, they measure only about a foot long as adults. But Atlantic menhaden have been called the most important fish in the sea, with good reason.

They are among the favorite dinner choices of striped bass, the finfish species highly valued by commercial and recreational anglers in the Chesapeake Bay. Menhaden (Brevoortia tyrannus) are fished in their own right and processed into fish oil, fertilizer, and animal feed, among other products. This commercial fishery is the largest by quantity on the East Coast.

For a fish as important as menhaden, scientists and fishery managers want to know as precisely as possible how many live in the Chesapeake and the coastal Atlantic Ocean and where they are. The regulators are responsible for regularly conducting stock assessments of the menhaden population and determining how many fish can be harvested without reducing their numbers below sustainable levels.

To estimate the menhaden population, it helps to know not only how many are born but also how many die annually, not just from fishing but also from being eaten by larger fish. Maryland Sea Grant recently funded research projects that examined in new ways the menhaden death rate and other questions, yielding some unexpected results.

The timing for an expanded understanding of menhaden is good: regulators are considering whether to use new methods to adjust their existing approach to managing the species. Researchers hope to help answer a long-standing question: do existing harvest levels leave enough menhaden in the Chesapeake Bay and Atlantic Ocean as food for predator fish, such as striped bass, that we humans in turn like to eat?

Measuring Mortality

No one can count every fish in the sea, so fisheries scientists have long relied on mathematical models to estimate numbers of menhaden and other species.

Those models are based on lots of data and assumptions about the many factors that collectively determine population size. One factor is fishing. Today, the East Coast's commercial menhaden industry is dominated by a single company, Omega Protein, which operates a fishing fleet and processing plant based in Reedville, Virginia. Other fishers catch menhaden for bait.

Another important influence on the population is natural mortality. How many menhaden die each year from natural causes, such as disease? How many are eaten by bigger fish like striped bass or by raptors like ospreys? It's notoriously difficult to quantify precisely the natural mortality of fish in the wild, but better estimates can give fisheries managers more accurate estimates of menhaden survival. If the remaining fish outnumber those lost to natural causes and fishing, the population should tend to be sustainable over time.

About five years ago, Mike Wilberg, a fisheries scientist at the Chesapeake Biological Laboratory (CBL), began thinking that the existing official estimate of menhaden's rate of natural mortality was due for an update. That meant taking a fresh look at a large set of data about menhaden collected 50 years ago.

The Atlantic States Marine Fisheries Commission, which regulates menhaden and other commercial species along the U.S. East Coast, uses a model of the menhaden population to set harvest limits, and its model includes an estimate that 42 percent of adult menhaden die each year. The estimate was developed based on studies of menhaden's biology and life cycle and also on findings from a research project on menhaden movement and mortality conducted from 1966 to 1969 by the National Oceanic and Atmospheric Administration's Beaufort Laboratory in North Carolina. During those years, researchers caught approximately one million adult menhaden, injected them with metal tags, and released them into the ocean along the East Coast. It was a massive undertaking to tag so many individual fish by hand. "I can't imagine anyone suggesting doing that study today and not getting laughed at," Wilberg says.

Some of those tagged fish were recovered when commercial fishermen brought their menhaden catches to processing plants. The companies participated in this "mark-recapture" study by installing magnets above the conveyer belts that moved large masses of menhaden through the plants; the magnets scooped up the metal tags to be collected for analysis. In all, more than 100,000 tags were recovered. By knowing how many tagged fish were released and later recovered, researchers were able to estimate how many died.

Wilberg and his colleagues wondered if they could obtain a better estimate of the natural mortality rate if they analyzed this historic data using contemporary statistical methods. They also hoped to learn new details about the migration patterns of menhaden, another important consideration in managing the species.

First the team needed the numbers in a form they could analyze. An electronic copy of the data set was lost during the 1990s, but the information had lived on, recorded on voluminous paper print-outs stored in binders stacked six feet high. Help came from the Atlantic States Marine Fisheries Commission and Maryland Sea Grant. The commission paid to digitize the data, and starting in 2014, Sea Grant funded a graduate fellow, Emily Liljestrand, who took a lead role in analyzing it.

What the scientists found was striking and challenged some long-standing conclusions about menhaden. Based on earlier studies, fisheries scientists had believed that much of the Atlantic menhaden population migrated south for the winter to warmer waters off the Carolinas.

But the new analysis by Wilberg's team indicated that about 50 percent of the menhaden in the northern part of its range — from Cape Hatteras, North Carolina, past the Chesapeake Bay to New England — remained there over the winter. That behavior may persist today. A more recent study by a master's student working with Wilberg found large amounts of menhaden larvae in the northern area, from Virginia to Rhode Island, during wintertime.

Also striking was the team's new estimate of the natural mortality rate of adult menhaden — that 69 percent of adult menhaden die each year, or more than two out of three. That was significantly higher than the estimate of 42 percent in use by the Atlantic States Marine Fisheries Commission.

Both findings could influence the future monitoring and management of menhaden. For now, the commission said in August of this year, harvest levels continue to be sustainable and the menhaden population is not overfished. But the commission has also called for considering in its next stock assessment, scheduled for 2019, whether using a better estimate of menhaden's natural mortality could improve the model.

Wilberg and other scientists say an alternative method, one that would reflect what actually happens in the ocean, would use a new approach to estimating the rate of natural mortality: it would assume the rate varies over time. The commission's existing model for menhaden stock assessments assumes that this rate doesn't change.

"For a fish like menhaden, we expect the natural mortality to be driven a lot by predation," Wilberg says. "And we know that some of their predators, like striped bass, have had major changes in abundance [up and down] over the past 50 years." As a result, Wilberg says, the annual mortality rate of menhaden probably has risen and fallen, too. The Atlantic Ocean and Chesapeake Bay are ever-changing ecosystems, so assuming that the rate of natural mortality for menhaden has remained unchanged may yield an incorrect estimate of the size of the menhaden population today.

Commerical fishing. Photograph, Gordon Campbell / At Altitude Gallery
A commercial "reduction" fishing industry nets menhaden to be processed as nutritional supplements and animal feed. Photograph, Gordon Campbell / At Altitude Gallery
Considering the Ecosystem

The fisheries commission has for years expressed similar concerns and is examining new approaches for estimating the menhaden population size to address this shortcoming, approaches that could showcase a new style of fisheries management.

Genevieve Nesslage, also a fisheries biologist at the Chesapeake Biological Laboratory, is supporting this effort in a separate, innovative project funded by Maryland Sea Grant.

She is developing a new mathematical model for menhaden, a hallmark of which is that it assumes the rate of natural mortality, including predation, varies over time. The model estimates future changes in the population's total biomass or fish flesh — the grand total you'd get if you counted up all the individuals in the population and weighed them one by one. Biomass can provide a more complete picture of a population than abundance alone because older fish tend to be bigger and bulkier than younger ones of the same species.

The fisheries commission spent several years working to include a variable rate of natural mortality in its mathematical model of the menhaden population. In the end, however, the agency decided not to do so because scientific reviewers weren't convinced that the approach produced reliable-enough results that improved the existing model. Nesslage has designed her model to avoid some of the technical difficulties the commission encountered. So far, she says, her model has shown promising results in tests of its ability to provide reliable estimates of menhaden biomass.

The Atlantic States Marine Fisheries Commission is considering a plan to complete by 2019 a scientific review of Nesslage's model and other approaches that use different methods to estimate the natural mortality of the menhaden population and how many can be sustainably fished. The commission would then decide whether to use any of the new approaches to help it set menhaden harvest limits.

The commission says a better model of the menhaden population should not only account for the effects of predation. It would also make useful predictions about how changes in the menhaden population conversely affect the predators that depend on them for food. Besides commercial fish like striped bass and bluefish, these predators include seabirds, such as ospreys, and marine mammals, such as humpback whales.

Genevieve Nesslage and Mike Wilberg. Photograph, Nicky Lehming
Scientists Genevieve Nesslage and Mike Wilberg (above) have been studying new ways to estimate the menhaden population. Photograph, Nicky Lehming

Managing the menhaden population with those concerns in mind reflects a new style of fisheries regulation called multispecies or ecosystem-based fisheries management. It represents a departure from the existing approach, called single-species management, which largely focuses on how to maximize the fishing harvest of a single species without reducing its population to unsustainable levels. The ecosystem approach presents technical challenges and requires deep understanding of many moving parts in the natural world, and so this method has yet to be implemented fully in the Chesapeake Bay region or for that matter anywhere in the United States. But Atlantic menhaden could be the first case.

Nesslage says she expects her work will shed light on the commission's first question, how do predators affect the menhaden population? But not on the second, how many menhaden should be left in the water to feed the predators?

Answering the second question, Nesslage says, "requires a ton of data, and it's still very cutting edge to even attempt to do it." But her model is a step in that direction. "We need to start incrementally moving, even if it's just small baby steps, towards a more ecosystem-based approach for managing these forage fish," she says. "Even if our approach is very menhaden-centric, it's a complement to the current model from an ecosystem point of view because it's saying that things aren't static in the environment, things are changing and affecting menhaden dynamics. So you want to try to manage them sustainably despite that fact."

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