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Volume 12, Numbers 1  • February-March 1994
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Blue Crab in Winter

Battle over Blue Crabs: Capping the Last Great Fishery

Changes in Regulations: Recent and Proposed

Managing Striped Bass

Also visit:
Bay Commission Asks: Are Blue Crab Stocks Stressed?
(Marine Notes Summer 1995)

SPOTLIGHT ON RESEARCH:

The Blue Crab in Winter

By Michael W. Fincham

Perhaps the only man legally fishing in Maryland's icy waters for blue crabs early this winter was Tom Maurer. A research assistant with the Chesapeake Biological Laboratory, he began his crabbing in mid-December this year, usually meeting up with Lonnie Moore at his workboat on calm mornings and traveling with him out to the mid-Bay mainstem or up some of its major tributaries. Moore is a watermen used to making a living catching blue crabs, but he knows he won't see any of this winter catch ever make it to market.

[blue crab]

For this kind of crabbing, they aren't carrying crab pots or trot lines or long-handled nets-the tools of the summer crab trade. They work instead with an iron dredge -the weapon of choice for winter crabbing. Twenty times a day under leaden skies and light to moderate winds, Maurer has to muscle the dredge up to the stern and heave it overboard. Relaxing, he takes a LORAN reading, then watches the cable whine out until he can feel the teeth bite in and the line go taut. Gunning the engine, Lonnie Moore starts gouging a bumpy swath across 100 yards of Bay bottom. They are dragging a dredge that measures 6 feet wide at the mouth with knife-like teeth 4 to 7 inches long. Inserted in the dredge is a fine net with 15 mm mesh for holding tiny crabs. On more than half the hauls-after clawing through an entire football field of mud-the dredge comes up empty of crabs. The rest of the time, the catch averages about 10 crabs. The highest haul for a single tow in Maryland waters has been 60 crabs.

Since dark comes early these months, the researcher and the watermen are usually tying up at a dock around 4 p.m. From one winter's day on the Bay, they typically bring back only 200 crabs-and a notepad full of numbers.

Counting Crabs

Where do blue crabs go in winter? This question has bewitched biologists for decades. But it's another kind of question that bedevils nearly every waterman every winter, and it goes like this: come summer, how many big, catchable blue crabs will there be in Chesapeake Bay this year? The answer for last year: there were probably 653 million catchable crabs swimming around in the Chesapeake Bay during crabbing season in 1993. The year before there were only 440 million. And the year before that, 1991, there were as many as 893 million. These population estimates include all the crabs in the Bay down to half an inch in size that are overwintering at depths of five feet or deeper. These estimates are among the findings recently reported from the largest survey to date of the Chesapeake blue crab in winter.

Scientists now believe they have more reliable numbers to chart the up-and-down populations of blue crabs in the Bay-thanks in part to all that cold-weather dredging by Maurer and Moore. Their work is just one part of an ambitious Baywide survey. While Maurer continues collecting crabs from the mid-Bay, a crew from the Maryland Department of Natural Resources will start dredging the upper Bay in February, and a crew from the Virginia Institute of Marine Sciences will be carrying on their all-winter work in the lower Bay. By the time these three groups wrap their work in March, they will have dug crabs from nearly 1,000 sites in the Bay.

This winter survey is using crude iron dredges and a complex sampling design to address some basic questions asked by biologists and watermen. Directing the project are Brian Rothschild and Jon Volstad of the Chesapeake Biological Laboratory and Rom Lipcius of the Virginia Institute of Marine Science, who is coordinating winter work in the southern Bay. With three years of data they say they can now come up with a good answer to the waterman's perennial question: how many crabs this year?


[crab sketch]

"The day before the beginning of the season, we could have an estimate of the number of crabs that could be caught on the first day of the season," explains Brian Rothschild, a fishery scientist and chief designer of the sampling scheme. On day one, "we know how many crabs in the Bay are above the minimum size and how many are below the minimum size," says Rothschild. He even has maps showing where all those crabs spent the winter.

He is less confident about describing what happens on day two and after. With the coming of warm weather crabs start moving and growing and dying. As the population starts to change, life gets more complicated for fishery biologists. They are looking for ways to estimate how fast the small crabs grow into legal crabs. "From that," he says, "we would know the rate at which new crabs would be pumped into the fishable population each day."

Figuring out those growth rates for crabs will take more time and perhaps new techniques. Estimating even the age of crabs is notoriously difficult: fish tags tried on crabs usually drop off when crabs moult, and after females stop moulting they stop growing.

Scientists on the survey are hoping to explain and perhaps predict the sudden surge of new crabs that often enters the crab fishery in August and September-sometimes glutting the market and dropping the dockside price, especially after Labor Day when the summertime demand slacks off. "If we had ten years of survey data," says Jon Volstad, "then we could work out better estimates."

For six winters now, the crab survey has been largely experimental, as researchers focused on designing and dredging, then re-designing and re-dredging to devise a sampling plan that would give a good preseason prediction of the Bay's blue crab populations. According to Rothschild the survey is now ready for operational use as an annual management tool.

Running the Numbers

Scientists go dredging in the dead of winter because they know blue crabs are easier to find. That's the time of year when these beautiful swimmers hunker down in the mud and sand at the bottom of the Bay. "If you are sampling a fish population, you usually have a big problem because they are swimming around," explains Rothschild. "But crabs are like trees, because they are fixed. And it is a lot easier to count the number of trees in the forest than it is to count the birds-particularly if the birds are migrating."


With three years of data [biologists] say they can now come up with a good answer to the waterman's perennial question: how many crabs this year?


[crab boat and pot]

Even with these winter crabs rooted in place, the sampling can get complicated. The key features in the dredging scheme designed by Rothschild and his colleagues include:

  • Random samples dredged from different depths in the mainstem and major rivers of the northern Bay, the mid Bay and the southern Bay.

  • Experiments documenting how each dredge tow captures only 15% of the crabs in its path (approximately one in seven).

  • Nearly 1,000 tows a year, each running approximately 100 meters as verified by LORAN or GPS readings.

  • A variety of statistical techniques to account for the patchy nature of crab settlement patterns (half the tows come up empty; others have hundreds of crabs).

  • An annual index giving the average number of crabs dredged out of out of every 1,000 square meters of bottom. For 1991 the index was 13.4 crabs and the Maryland harvest was 91.5 million pounds. In 1992, the winter index was only 6.6 crabs and the year's harvest dropped to 54.7 million pounds.

  • Multipliers that convert this annual index into an annual population count.

One result of all that dredging and multiplying has been an index and a population count that roughly tracks with harvest totals over the last several years. (See accompanying box.) Another result has been new insights into where blue crabs go in winter.

Winter Survey Estimates of Blue Crab
Population in the Chesapeake Bay



Year

Total Number of Crabs in Bay

Total Number Harvested

Exploitation Rate
1991 893,000,000 274,500,000 30.7
1992 440,000,000 164,100,000 37.3
1993 653,400,000 ? ?

Playing the Numbers

These new population counts could become an important part of the growing debate over how best to manage blue crabs in the Chesapeake. There are now at least six crab indexes that help predict blue crab harvests, but this pre-season index and population count may prove among the most helpful for quick management response to changes in the fishery. Because of fears of overfishing, resource managers and lawmakers at both ends of the Bay are considering controversial changes this winter that could cap fishing effort in this historic fishery.

And there is in the winter survey some evidence that a cap could make sense. According to the survey, exploitation rates in the crab fishery are increasing, while crab populations are not.

  • In 1991, for example, the survey estimated a population of 883 million crabs, and watermen that year caught 274 million of those crabs. That gives an exploitation rate of 30.7% for commercial fishing. Add in another 70 million crabs caught by recreational fishermen-a rough, but conservative estimate -and the rate jumps above 38% for 1991.

  • In 1992, when the population count was only 440 million crabs, watermen caught more than 164 million, giving a commercial rate of 37%. When the recreational catch is factored in, the total exploitation rate climbs above 46% for 1992.

  • Final harvest figures for 1993 are not yet finished in Virginia, but the evidence from Maryland shows that exploitation continues to rise. The population came in around 653 million crabs Baywide, a drop of 250 million crabs from 1991, but Maryland watermen by themselves were taking out, 153 million-nearly 10 million more crabs than they caught in the boom year of '91.

Though Volstad warns that exploitation rate is increasing, especially in Maryland, he is leery about adding rough estimates of recreational catch to fairly accurate estimates of commercial catch. "We know there is a substantial sport fishery for crabs because we see so many people out there fishing. But we don't have any realistic estimates (of recreational catch)."

From Virginia Rom Lipcius warns, "There is potential for major decline. We are in a low period right now, and when you continue heavy fishing pressure during a low period, you are in a position to cause a major decline-but not necessarily a collapse."

What the Numbers Mean

Our classic picture of the blue crab in winter has stood largely untouched for nearly 30 years. With the cool weather of fall, mature female crabs begin their passage south, headed for their hatching grounds near the mouth of the Bay where they crowd together waiting for spring. Male crabs, on the other hand, tend to linger in the tributaries and mainstem of the upper and middle Bay until cold weather sends them scuttling for Bay bottom where they can dig in, turn down their metabolism and hibenate over winter. With the new winter data, biologists are beginning to retouch this portrait, filling in empty spaces and even altering the outline with some unexpected findings about the species.

Large numbers of female blue crabs, it turns out, don't finish their fall passage before winter hits. These late-season travelers end up hibernating further north in the mainstem of the mid Bay and even the upper Bay. In the 1993 survey, more than 40% of the crabs found in the deep layers of the mid Bay by the 1993 survey were females who will hatch eggs next season. In the upper Bay nearly 10 percent of the deepwater crabs were maturing females.

Come spring, all those up-Bay females are at risk of capture as they head south for the spawning grounds at the mouth of the Bay. "Those females still have to run the gauntlet - the spring and summer crab pot gauntlet - in order to make it to the lower Bay and spawn," explains Rom Lipcius, a scientist who has studied Virginia's controversial winter dredge fishery. "It's fairly clear that if you want to protect the spawning stock, you cannot just emphasize the dredge fishery in the lower Bay. Many of the females who are going to spawn-sometimes as high as 50%-are still up throughout the Bay."

The winter dredge fishery in Virginia has always been controversial with many watermen in Maryland who read about Virginia dredges scraping up a catch that is 90% females. Virginia watermen, on the other hand, hear that some Marylanders are running between 800 to 1,500 crab pots a boat. During the fall run of sooks headed south and the spring run, all those pots also take a big bite out of the number of females who might end up hatching eggs that season.


[crab sketch]

The survey's findings about small blue crabs are reinforcing some ancient, instinctual beliefs about the value of seagrass beds. The VIMS crew sampled in shallow beds, not just with dredges, but with suction techniques, and they found high counts of small blue crabs nearly everywhere. "Seagrass is critical for the youngest juveniles, " says Lipcius because that is where they go to escape predators. "Blue crabs would survive without seagrasses, but not in large numbers."

Chesapeake Bay, he points out, is only three or four times larger than Delaware Bay, but the Chesapeake crab harvests are usually 20 times larger. The difference, says Lipcius, is the large seagrass beds that still remain in the lower Bay, beds that offer nursery grounds for millions of young crabs which will eventually enter the fishery. Preserving those habitats, he says, is one of the keys to preserving the blue crab bounty in the Chesapeake.

Insights like these about seagrasses and life cycles are the reasons for annual surveys like this. Winter surveys, with all the cold weather dredging and the number crunching that follows, are perhaps the grunt work of Bay science, justified because they bring new answers to old questions, questions like how many crabs do we have and where do they go.

As the dredges go down, year after year, and the reports come in, year after year, these surveys of the blue crab in winter will keep altering our classic picture of the species, often in small ways, adding details and deeper coloring, illuminating how we need to act to save the Bay's last great fishery.




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