Uncertainty about food web dynamics and the effects of degraded environments continues to test fisheries managers on effective actions they can take, says researcher Ed Houde, shown aboard the Research Vessel Aquarius.

The return of striped bass has raised questions in recent years that weren't on anyone's mind two decades ago when rebuilding plans got underway. For example, how would increased striper stocks affect forage fish and other prey, particularly menhaden - the largest commercial fishery by weight in the Chesapeake? What would it mean for other popular species that also prey heavily on menhaden, for instance, bluefish and weakfish? What would it mean specifically for menhaden harvests, which account for 68 to 87 percent by weight of all commercial species, including blue crab, landed in the Bay?

Over the last 15 years, menhaden recruitments (i.e., the abundance of young produced) in the Chesapeake have declined substantially and remain low, Houde says. While dissections of striper stomachs reveal that menhaden still form a substantial part of their diet, they constitute less of that diet than they once did, he says. Striped bass are also consuming more of such prey species as anchovy, blue crab and other fish. Arguments have been put forth that the larger numbers of mature stripers and declines in menhaden have forced striped bass to feed on less nutritious prey - as a consequence, the argument goes, they are undernourished and susceptible to disease.

In recent years, large numbers of striped bass have been infected with Mycobacterium, a bacterial pathogen that can cause lesions and infect internal organs of what appear to be healthy fish. According to Andrew Kane, a fish pathologist at the University of Maryland College Park, as much as 70 percent of the adult stripers in the Bay may be infected. Where did Mycobacterium come from? It is sometimes found in fish raised in aquaculture systems, says Kane, and there has been some speculation that the microbe could have been introduced into the Bay years ago when cultured striped bass were stocked as one means of rebuilding populations. Or is Mycobacterium a natural constituent of the Bay? And if so, are stripers more vulnerable today than they were in years past because they are undernourished? There is no conclusive evidence so far that this is so, say both Kane and Houde.

Researchers need models that account for the complex food web relationships of each commercial or recreational species with their prey and between competitors for that same prey.

Uncertainty about such food web dynamics and the effects of degraded environments continues to test fisheries managers on effective actions they can take, says Houde. Added to these are changing impacts of climate and weather, from tropical storms to El Niño to global warming, all of which may affect spawning success or failure in any given year. Altogether, the challenges may seem insurmountable.

That's not necessarily so, says Houde. While fisheries managers do not manage the ecosystem, he says, they do manage fishing activities. "If we can regulate those activities well, we can contribute to making the ecosystem better with respect to its support of fisheries." In effect, it's a two-way street - for fisheries to be managed well, the ecosystem has to be healthy; for the ecosystem to be healthy, fisheries must be well managed. We're looking to do fisheries management more holistically, he says.

Changes that the EPA Chesapeake Bay Program has been making in trying to improve water quality in the Bay system demonstrate how fisheries ecosystem management may be better integrated in the future. The Bay Program, for example, has moved from a long-time goal of reducing nutrients 40 percent Baywide as a key means for improving water quality toward identifying specific water quality and sediment goals for specific habitats such as spawning and nursery areas, shallow-water Bay grass areas, and deep-channel refuges. According to the Bay Program, "different criteria would be applied to each use based on the species found there, for instance, adult fish in open water, oysters in deep water, Bay grasses in shallow water."

"I like to think of the ecosystem as the productive engine," Houde says. "We have to keep the engine in good shape, making sure that its productive capacity is maintained, such that wise stewardship benefits the fisheries." This means not only building age structure so that reproduction is not dependent only on one year class, as for striped bass, but it means "building biodiversity, building the biomass of a species, building predator-prey relationships, protecting habitat, and making sure that there is some reasonable part of the ecosystem that is protected from human activity." Setting aside areas protected from fishing can be controversial, though it is a spatial management tool that managers have been using for years. (See sidebar, Marine Protected Areas).

Biodiversity is one indicator of how healthy or robust the ecosystem is. Levels of production at higher trophic levels - layers in the food chain where one group of organisms serves as the source of nutrition for another group - are of special interest, Houde says, namely levels of predators (e.g., striped bass or bluefish) or prey (e.g., blue crabs or menhaden). "We want to maintain biomass diversity above threshold levels for those fishes we're likely to harvest."

This page was last modified June 07, 2004

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