More harm than good from crab ban, experts say

For scientific reasons, they say harvest moratorium will yield diminishing returns

  • A Maryland blue crab takes a defensive posture after being pulled onto afishing boat.
A Maryland blue crab takes a defensive posture after being pulled… (Barbara Haddock Taylor…)
August 21, 2014|Dan Rodricks

Angus Phillips, an inveterate Annapolis-area crabber, joined my call for a moratorium on the harvest of blue crabs from the Chesapeake Bay. "The time has come," he wrote in The Washington Post last month, "to stop pussyfooting around and shut down crabbing for a few years, to give the delectable crustaceans a chance to recover the way geese, yellow perch and rockfish did."

Phillips wrote about fishing and hunting for 30-plus years at The Post before retiring; he's clocked a lot of hours trotlining for crabs, too. On the water this summer and last, he saw firsthand what the Maryland Department of Natural Resources reported in May: Blue crabs are in serious decline, with the number of females just below the level believed necessary to sustain the overall population. Last year's harvest was the lowest on record.

Phillips and I both invoke the rockfish moratorium of the 1980s — a five-year prohibition that brought the exploited stripers back from the brink. Similar measures gave the yellow perch a chance at recovery. Same with Canada geese.

So it's just common sense: Let's give the crabs a break.

But Maryland's crab experts disagree.

On my radio program, Brenda Davis, manager of the DNR's Blue Crab Program, said a moratorium wouldn't help; there was "no guarantee" of more crabs if we banned the harvest generally — or specifically prohibited the taking of females — for just one year.

Her reasons are as complex as the life cycle of the blue crab, so I submitted follow-up questions. Davis handed them off to Glenn Davis, her husband of 19 years and the "blue crab statistics guru" at DNR. Mr. Davis provided the following points against a moratorium:

1. Flaw in rockfish comparison: The "rock," or striped bass, is often held up as the poster child for the effectiveness of a moratorium. "However," Davis says, "the substantially different life histories of striped bass and blue crabs invalidates any comparison."

Blue crabs don't live that long, maybe three years, sometimes four. Rockfish, on the other hand, live much longer — three decades, in some cases — and produce more eggs as they age.

So while a moratorium could help a finfish population grow, that kind of stock rebuilding won't work for blue crabs.

"Blue crabs are a tropical species, and are susceptible to [winter] mortality, more so the older they get," says Davis. "Every three to five years, we experience a colder-than-average winter. That results in a large percentage of adults being killed, at times upward of 50 percent in Maryland. This means there are two years of adults in the population at any one time, with the first-year (and youngest) adults making up the vast majority. So stock rebuilding — allowing more and more adults to live longer, thus having [more] older individuals in the population — won't happen with blue crabs."

2. The longer blue crabs live, the less fertile: "Unlike striped bass and most other finfish," Davis says, "the spawning potential for female blue crabs decreases with age. Female blue crabs mate once in their lifetime. From that one mating, they store the sperm they received to fertilize eggs later."

Females produce from three to six egg masses, but the number of eggs in each successive brood usually drops. "There is an attrition of viable sperm over time, as well," Davis says. "The single-mating makes blue crabs different from other crustaceans, like opilio crab, king crab, lobsters, which live to be much older and are capable of mating each year after maturity."

3. Diminishing returns: Davis says that having more spawners doesn't necessarily equal more adult crabs for harvest. In fact, some years just the opposite happens. "This is due to competition for resources — more energy spent fighting and finding food equals less energy to make eggs — and increased natural mortality. When young crabs are very dense, they are more available to predators and they compete and fight with each other for food and habitat. Crabs are also very cannibalistic. ... We run into the law of diminishing returns because natural mortality increases when crabs become more dense."

4. Harming the economy: Crabs are a huge part of a Maryland seafood industry that the state estimates at $600 million annually. A moratorium, Davis says, would do more harm than good: "Given the diminishing biological returns we get from a moratorium, the potential benefit of a moratorium, which is not guaranteed, simply does not offset the detrimental impact of a complete ban on harvest."

Davis added this: "For years, people have complained about our fisheries being managed on the basis of politics and not science. Well, with the crab fishery, we have a management framework based on science."

And I respect science. I understand the points the Davises have made. But I'm still afflicted with the nagging intuition that a smaller harvest would mean more crabs in the bay, and more crabs, even for one year, would be a good thing.

I think I'll just eat more vegetables.

Dan Rodricks' column appears each Tuesday, Thursday and Sunday. He is the host of "Midday" on WYPR-FM.

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.