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Volume 13, Numbers 4  • September-October 1995
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Contents

New Oyster Wars

Science and Ethics

Sturgeon -- Looking Ahead

Student of the Chesapeake

SPOTLIGHT ON RESEARCH:

Multispecies Management in the
Chesapeake Bay -- A Far Future?

By Merrill Leffler


[Commercial fish harvest]

Bluefish, croaker, sea trout, striped bass, menhaden -- as one species comes and another goes, does this signal the effects of natural cycles of the impact of fisheries management efforts, or both?

Ten years ago, Maryland banned the taking of striped bass from its portion of Chesapeake Bay -- at least until clear signs indicated the fishery was recovering, Maryland's controversial action was part of a Congressionally backed restoration plan that required states from Maine to North Carolina to make major changes in striper fishing in their coastal waters. Those changes included raising the minimum catch sizes -- in some cases from 18 inches to 32 inches -- and reducing the daily creel limit. The aim was to allow striped bass to mature so they could return to their native rivers in the Bay to spawn. And they have.

Today striped bass are back, and the state has declared the fishery recovered. A rare success story -- and a heartening one at a time when the Chesapeake's oyster fishery has been nearly eliminated and concern is mounting over a possible decline of blue crabs. Only recently, however, have questions been asked about the implications of the striper recovery for the productivity of the Bay ecosystem as a whole, for the abundance of other popular species, bluefish and weakfish, for example. Does the increasing abundance of stripers mean anything with respect to the prey they all go after -- anchovy and menhaden and crabs, for that matter?

Some fishermen have speculated that diminishing crab stocks -- if in fact they are decreasing -- could have more to do with voracious predation by stripers than human predators. This conjecture was based on large numbers of small crabs found in the stomachs of some striped bass.



Does a boom for one species mean a bust for others?

There has not yet been evidence to support the argument that stripers have put a dent in the crab population, at least according to studies by the Maryland Department of Natural Resources and the Virginia Institute of Marine Science. Though those studies are probably right, says Ed Houde, a fisheries scientist at the Chesapeake Biological Lab, part of the University of Maryland Center for Environmental Science (UMCES), striped bass might be having an impact on crab dynamics. "Historically, you would guess that they [stripers] had little impact," he says. "But sea grass habitats have been disappearing in the Bay for years, and young crabs rely heavily on those grasses. Crabs may now be more vulnerable to predation, perhaps far more than they would have been."

Whether or not further research turns Up a significant relation between striped bass and crabs, the issue raises a question that offshore fisheries managers from Georges Bank to the Bering Sea have been struggling to answer for years -- namely, does a boom for one species mean a bust for others? How does the harvesting of one fish species affect the abundance and dynamics of other species? And do we need to account for such interactions in fishery management plans in the Chesapeake?

Multispecies Management

Ed Houde and Tom Miller, also at UMCES' Chesapeake Biological Lab, have undertaken a literature review and synthesis to ask just that question. With support from the Chesapeake Bay Program's Scientific and Technical Advisory Committee (STAC), they are focusing on the multispecies nature of Bay fisheries and possible interactions. "The goal," says Miller, "is to ask whether there is even compelling evidence that we need to adopt a multispecies approach in the Chesapeake."



Multispecies models often predict results contrary to conventional wisdom -- a problem for fisheries managers.

Some years ago, Harley Speir of the Maryland Department of Natural Resources examined a set of sportfishing data collected in the same location to see if he could detect an inverse relation between striped bass and bluefish catches. If bluefish catches were up, he asked, were striped bass down or vice versa? He found no such connection. For the most part, though, such investigations in the Bay have been cursory, mostly localized, and they've played no part in fisheries management. "We've primarily considered the effect of removal rates of one species on the abundance of that species," says Speir. "We've never examined to see if removal of that species will affect other ones." Until now, no multispecies studies, let alone management practices based on multispecies models, have been undertaken in major estuaries like the Chesapeake. Most research has been done in offshore fisheries, for example, Georges Bank and the North Sea. In the North Sea multispecies models -- such as the Multispecies Virtual Population Analysis -- have had an impact on how quotas are set.

(That commercial fisheries have been collapsing is not a shortcoming of multispecies models, says Miller, as much as the general failure of management practices -- simply too many fish being caught.)

"What a multispecies approach should presumably allow you to do," Miller says, "is to modify the catches of individual species based upon the removal of other components in the food web." Are there ecosystems where these ideas are firmly in practice? Not fully, says Miller, who points out that trophic food webs are extraordinarily complex and depend not only on predation and competition among species for a variety of prey, but a host of other factors -- climatic conditions, physical properties, let alone human issues of commercial and sportfishing, together with their political implications.

[crab harvest]

What is the effects of the Bay's changing ecosystem on blue crab stocks? Concern over habitat changes and overfishing have led Maryland Governor Parris Glendenning to institute new harvest regulations specially aimed at protecting female crabs.

A symposium on multispecies management several years ago in The Hague pointed to these problems directly. From a fishery manager's perspective, while species-interaction models may be conceptually more realistic than managing single species alone, their complexity and the results they produce may run counter to conventional wisdom. As Ed Houde points out, multispecies analysis in the North Sea shows clearly "that most fish are killed by other fish" through predation and that "how you manage" top predators such as cod and whiting can have major impacts on other fish in the system." Multispecies models," however, often lack sufficient data, so as realistic as they may be conceptually, they may not adequately represent the ecosystem because of their incompleteness. In the North Sea, for instance, the multispecies model considers only nine species and then only from the post-larval stage -- meanwhile, the model treats the North Sea itself as a single box.

Nevertheless, what makes the idea of multispecies management attractive, says Houde, is its taking into account interactions among species and the effects of fishing on relative abundance, predator-prey relationships and competitive relationships among species. For example, the harvest of a top predator (like striped bass or bluefish) may allow numbers of its prey species (like menhaden) to increase, affecting populations not only of prey species, but also of species it consumes in the food chain, what ecologists call a cascade effect.

If multispecies management has a future for the Chesapeake, the review and synthesis by Houde and Miller could provide a stepping stone. The two researchers will scour historical records in trying to detect trends over the years among important popular and commercial species, among them, striped bass, bluefish, flounder, sea trout, croaker and blue crab. They will be looking for cycles of abundance and occurrences of explosive growth and collapse. "We will look to see if there are clear indications of variations in abundance between species that suggest predator-prey interactions," says Miller. "For instance, if bluefish are abundant, are menhaden less abundant?"

Menhaden and anchovy are major prey of bluefish, striped bass, weakfish and other finfish -- they are the two most abundant fish in the Bay and on the whole east coast. Unlike anchovy, which travel in small schools and for which there is no commercial fishery, menhaden are the major commercial finfishery in the entire Chesapeake, some 200,000 tons a year.



Bluefish catches are down along the east coast -- multispecies modeling may help to explain why.

Bay harvests of menhaden have remained steady, though there are no limits on those harvests -- on the other hand, bluefish landings along the east coast have declined sharply in recent years. Is there any relationship? Right now, it is impossible to say.

Teasing out underlying causes is an important goal of their study, says Miller: "Can we find something in the data that suggests we need to consider them in unison rather than in isolation -- if not for the whole Bay ecosystem, at least for key components?"

[menhaden catch]

The Bay's biggest catch -- beyond striped bass, blue crabs and bluefish -- is menhaden, some 200,000 tons a year.

For example, he asks, do we see variations in catch related to phytoplankton productivity of the Bay if it is a particularly wet or dry year? "Certain things we can factor in quantitatively," he says, "like river runoff, seasonal water temperature, estimates of productivity and sea grass coverage over time." In other words, not all shifts in abundance will be due to multispecies effects of fishing, predation and competition."We're" not trying to find a single smoking gun," Miller says.

One problem they face is that the historical data he and Houde will be using -- records of abundance, for example -- weren't collected with their interests in mind. While there are statistics on commercial landings, landings are a measure of catch, not necessarily of abundance. Furthermore, the records on recreational catch are poor to non-existent. "We will have to assume fishing effort [the total fishing gear in use over a period of time] was constant or, in scaling, somehow, convert catch statistics to species abundance."

For now, that's the best they can do.

Fisheries Management and Ecosystem Stability

There is another view to multispecies management beyond predator-prey and competitive relationships among fish, one that has to do with the effects of fisheries on the health or the stability of the ecosystem itself. Just how important is the ecological role of certain species -- of oysters, for instance; perhaps of menhaden? Such multispecies management considerations are a first step on a long road that could lead to ecosystem-based management of our fisheries.

We have already learned much, in the past two decades, about the interconnectedness of the Chesa peake Bay ecosystem. The widespread decline of underwater grasses and an increase in summer depletion of oxygen in bottom waters, for example -- both attributed to excessive nutrients entering the Bay -- have meant the loss of healthy habitat for young crabs, oysters and other species. Overnutrification has overfertilized these waters and led to explosive algal growth far beyond the Bay's assimilative capacity. Consequently, a major goal in the cleanup of the Chesapeake Bay is the reduction of nutrients by 40 percent -- such reductions, it is hoped, will significantly lower algal production and such consequences as oxygen depletion.

The Bay's poor assimilative capacity, some have argued, may also be due to the near-elimination of oyster stocks and oyster reefs. Several years ago, Roger Newell of the UMCES Horn Point Environmental Lab estimated that oyster stocks in the late 19th century could have filtered, on average, the entire Chesapeake Bay in three to five days; in 1988, it would have taken more than a year. With the further decline of oysters since then as reflected in lowered harvests and monitoring, such filtering comparatively would take even longer. Newell's estimates helped catalyze concern over the oyster's ecological role -- and while oysters continue to be managed primarily for commercial use, for the first time resource managers have begun to acknowledge and take into account the oyster's ecological importance.

Menhaden may also have an important ecological role in the Bay by sequestering nitrogen in the enormous amounts of phytoplankton they consume: since they move out into coastal waters, they become potential exporters of nitrogen. A number of UMCES scientists have argued that menhaden could sequester and export much more nitrogen by limiting the commercial catch to fish aged at least three to four years. (Menhaden generally do not reach sexual maturity until age two; the largest menhaden on record, at three pounds, was eight years old.) That is not the case now. "In the lower part of the Bay," says Ed Houde, "a fishery is hammering on them at most of the ages, from juvenile stages through age one and two."

We are still a long way from using commercial and recreational landings as a tool for managing nutrient levels or other measures of Bay health. The complexity of trophic food webs, let alone the political and social issues at stake, may make that more an ideal than a reality. But before they can become practical tools, such issues as multispecies and ecosystem-based management must first become part of the ongoing conversation -- and that has begun. The far future may hold surprise connections between striped bass and crabs, between oysters and menhaden that we cannot yet foresee.




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