Historically, state agencies like Maryland's Department of Natural Resources (DNR) manage fisheries on a species-by-species basis to try to achieve maximum sustainable yield, or MSY, a goal that aims at yielding the largest long-term average catch without reducing the size of the population. DNR's Nancy Butowski says that "we're trying to move away from MSY - it's based on populations being at equilibrium and most populations are not." According to Butowski, we need to consider the ecosystem as a whole, to move toward multispecies management. Focusing on a single species may leave little room for margins of error that result from unexpected impacts to the ecosystem, such as the large areas of oxygen depletion in bottom waters seen in 2003, partly the result of unusually heavy rains, nutrient runoff and widespread algal blooms. In recent years MSY is judged to be what fisheries scientists call a "threshold," a level of catch at which a species' sustainability is threatened. "Consistently fishing beyond a threshold level would significantly compromise the ability of a fish stock to maintain a certain population size and ultimately lead to a decrease," says Butowski. We need to set "targets" that aim at achieving an optimum yield that often is considerably less than MSY, she says, which takes into account multiple species.

To more effectively calculate "thresholds" and "targets," 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. As UMCES fisheries scientist Tom Miller has pointed out, if two species are related as predator (e.g., striped bass) and prey (e.g., menhaden), removal of prey by harvesting will mean fewer predators can be supported; conversely, removal of predators will mean more prey can be supported. These outcomes become more complex with more competitors (e.g., striped bass, weakfish, bluefish) for the same prey or, as often is the case, several prey (e.g., menhaden, bay anchovy, blue crabs).

For striped bass, peak years of landings were from the 1960s to 1970s, Miller says; they coincided with low bluefish and weakfish catches. "Commercial landings of bluefish only peaked during the mid to late 1970s, when striped bass catches were declining rapidly and weakfish catches were at low levels." Is there a causal relationship? Did the reduced feeding pressure on menhaden by diminished striped bass populations benefit bluefish? Not necessarily, says Miller. "They are not simple replacements, although the same underlying fishing, trophic, and environmentally dependent mechanisms may drive the patterns."

Atlantic menhaden, a major schooling fish in coastal waters and Chesapeake Bay, are harvested and processed for meal and natural products, while the whole fish is used extensively as bait in blue crab pots. Over the 20 years before menhaden began to decline in the Chesapeake, their numbers and coastwide reproductive success were remarkably high, and consistently so, Houde says. Some people have argued that their diminishment has been caused by commercial harvesting, which is now more concentrated in the Bay than offshore.

Menhaden live in offshore ocean waters for the first 70 to 80 days of their life; they spawn on the near shore continental shelf where their eggs and larvae develop. Poor reproductive success and low recruitments could be attributable to failed survival in the coastal ocean rather than failed production in Chesapeake Bay. It has also been argued that increasing stocks of striped bass over the last decade have led to the decrease in menhaden. Harvesting and predation might be thought of as the two limiting factors on menhaden recruitment to the Chesapeake, says Houde, and could serve as an argument for reducing menhaden harvests in order to leave more for striped bass and other popular species. Or it might be argued that striped bass harvests should be increased to reduce the pressure on menhaden - after all, menhaden is also a commercial species. While this is a management-stakeholder decision, he says, "there is not a lot of evidence that increased menhaden harvests and striper predation have caused menhaden recruitment failure."

Further complicating the issue over such a multispecies management issue is the potential ecological role of menhaden, since they are significant consumers of phytoplankton, the single-celled algae that metabolize nitrogen and phosphorus. With nutrient overloading to the Bay and an overabundance of algae seen as key determinants in the decline of water quality, some have associated this decline with decreasing numbers of menhaden. It's an appealing hypothesis, yet there is very little evidence to support it, says Houde.

It's possible there's been some climate or weather change over the last 15 years that we haven't picked up that has caused lower survival of menhaden eggs and larvae in the coastal ocean, says Houde. But as he goes through a stream of speculative hypotheses, he is quick to add, "We can't say there is good evidence to support this either." We would have needed to set nets at the mouth of the Bay from January to April in a series of years and look at flux of young menhaden to know if there's been a decline in the input. That would be the ideal kind of study, he says. "It sounds simple but it's a big undertaking, and I don't know if anyone would want to fund it."

With all this uncertainty just over the role of menhaden, how are resource management agencies to manage multiple species at one time, let alone take into account ecosystem factors? Given that marine research cannot yet incorporate multispecies and ecological information into models to reliably forecast the implications of different management alternatives, Houde's answer is that the best course lies in adopting the ethic of doing the least amount of harm or no harm to the ecosystem in the face of great uncertainty.

This page was last modified June 07, 2004

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