During the 1990s, the prevailing scientific wisdom was that Chesapeake Bay blue crabs could not be overfished. They spawn at the mouth of the Bay, and their larvae drift out to sea where they spend a month or so before being swept back inshore by easterly winds and currents. The wind and tide, therefore, were assumed by scientists to control blue crab reproduction, and nothing we did could affect it.
We have since found that there is another side to blue crab biology. Yes, they mature quickly and produce a lot of eggs, and they depend on favorable currents to complete their life cycle. But even the hard-to-overfish crab has a minimum number of eggs that must be produced to provide a reasonable chance for a good recruitment of young crabs to the population each year. After a dozen years of poor recruitment, we finally bit the bullet two years ago and cut way back on the crab catch to boost the number of eggs produced. As a result, the population has already doubled in size.
The established scientific viewpoint is the same for Atlantic menhaden: They spawn offshore and produce a lot of eggs, and their larvae depend on currents to sweep them into coastal bays and estuaries. Scientists say the number of adult fish in the population doesn't affect the number of young fish produced each year. They say environmental factors must be the cause of the 15 years in a row of poor recruitment. It's the same thing scientists used to say about blue crabs.
The Atlantic States Marine Fisheries Commission has just completed a new assessment of Atlantic menhaden and an independent scientific review of the results. The assessment (a mathematical modeling of the population) estimated the egg production and fishing rate for menhaden, and compared them to threshold levels previously adopted. The conclusion: In 2008, the fishing rate was below the threshold for overfishing (barely) and the egg production was above the threshold for an overfished population. Superficially, one could assert in a commentary that there is no problem with the menhaden population ("There's no problem with the menhaden population, only what is believed about it," February 2010), but there is much more to the story.
First, the fishing rate in 2008 was so close to the threshold that there is a good chance it actually did exceed it. Furthermore, the assessment uncovered a historic pattern in which the threshold was exceeded in 32 of the last 54 years. Thus, the assessment executive summary expressed "concerns about frequent overfishing in the past and potential overfishing in 2008." More importantly, the independent scientists concluded that the threshold levels being used did not provide enough protection for the menhaden population-they noted that the current population has only 5-10 percent of the reproductive potential of an unfished population-and they recommended more conservative thresholds.
Probably the most sobering output of the assessment is the finding that there are now fewer menhaden in the Atlantic Coast population than any time in the last 54 years. (See graph on this page.)
It is hard to understand how an industry press release after the assessment could declare that Atlantic menhaden were "abundant." There are fewer menhaden to feed striped bass, weakfish, bluefish and many other predatory fish; and there are fewer menhaden for ospreys, loons, pelicans and gannets.
It is well-documented that striped bass diets have shifted over the last 20 years from one dominated by menhaden to one dominated by alternative prey. And scientists have now demonstrated that poor diet affects the progression and severity of mycobacteriosis in striped bass, a normally fatal disease that afflicts up to 70 percent of striped bass resident in the Chesapeake Bay.
Everyone seems to talk about science underpinning fisheries management, but that is easy to say. Science is not science if it is used selectively. Nor is science infallible, the biological sciences in particular.
The best available science is never enough, but waiting for better science is a decision in favor of the status quo. Species like menhaden that have always been managed to maximize catch without regard to their important ecological role cannot afford the status quo. The precautionary principle, in which managers err on the side of the resource when science is limited, has become an international standard, and it must be adopted for menhaden.
The experience with Chesapeake blue crabs should be a lesson for menhaden and many other species: Make the tough decisions when the science suggests it even if the science isn't perfect. That some scientists say the number of adult menhaden doesn't affect the number of young fish produced each year is at least counterintuitive. But the independent scientists who reviewed the assessment noted that the natural variability in annual menhaden reproduction may be masking the fact that the number of adults really is important. Maybe this is the beginning of a turnaround like the one we have seen with blue crabs.