[Chesapeake Quarterly masthead]
Volume 3, Number 2
Table of Contents
Download pdf

Scientists Weigh in on Blue Crabs - Not Worse Is Not Enough. By Erica Goldman

How are crabs doing in the Chesapeake? It's a question Delegate John Wood hears often. A Maryland legislator and long-time co-chair of the now disbanded Bi-State Blue Crab Advisory Committee (BBCAC), Wood is regularly approached by his southern Maryland constituents on the sidewalk and in the grocery store.

He and Ann Swanson, executive director of the Chesapeake Bay Commission, put this same question to the Technical Work Group (TWG), a bi-state team of crab experts that advises the Commission, at the group's June 22 meeting in Annapolis, Maryland. The answer from crab biologists and resource managers, whether from Maryland or Virginia, was essentially the same:

"Not worse."

They chose this answer carefully. The truth is, they said, that while a years-long slide in population has leveled off and perhaps even turned the corner slightly, the population is still well below the long-term average, and some troubling signs remain.

According to the annual Chesapeake Bay Blue Crab Advisory Report, prepared by the Chesapeake Bay Stock Assessment Committee (CBSAC) and released on June 2, blue crab abundance has improved compared to the near historically low levels of the previous four years. At the same time, watermen reported strong harvests as the season began. But while the "not worse" diagnosis may be a sign that recent declines in the crab population are slowing, it is at best a reason for "cautious optimism," says Swanson.

"It will take at least three to four years of heading in the right direction before we can really say anything," says George Abbe of the Academy of Natural Sciences Estuarine Research Center in St. Leonard, Maryland, who has conducted a crab survey off Calvert Cliffs in southern Maryland for the past 37 years.

Estimates of crab abundance, fishing mortality and size of the female spawning stock reflect combined data from four different annual surveys. These surveys vary in terms of the region of the Bay they cover and the methods they use. One of them, the Virginia Institute of Marine Science (VIMS) trawl survey, focuses on the crab spawning sanctuary at the higher-salinity southern end of the Bay, where female crabs go after mating. According to the VIMS data, reproductive-aged female crabs are not faring as well as the population as a whole.

Fewer Females?

Of the four crab surveys used, the VIMS trawl survey is the only one that counts females in the southern Bay sanctuary, where they have escaped being caught in a given season and successfully returned to their spawning ground, explains VIMS fishery biologist Rom Lipcius. While data from the other surveys show slight increases in the spawning stock, the VIMS survey reflects no obvious improvement - in fact, a preliminary analysis shows that the female spawning stock may have continued to decline for the tenth consecutive year. "We are certainly not on a doubling path," Lipcius says, referring to a goal stated in BBCAC's 2001 Action Plan to increase the blue crab spawning stock two-fold.

The precarious plight of female crabs was also the sobering subject of a presentation by VIMS fisheries population biologist John Hoenig at the TWG meeting. Hoenig's data suggested that only 2 percent of female crabs that made it to the spawning ground in 2002 survived until the next season. The remaining 98 percent of mature females either died of natural causes or were removed by the fishery. Supporting evidence comes from research by Alexei Sharov, a survey design analyst at Maryland Department of Natural Resources who has worked extensively on the Winter Dredge Survey, the only survey that comprehensively covers the whole Bay. While not as dire, his analysis also suggests very low survival rates of mature females for the past four years, averaging around 15 percent.

If these estimates of low female survivorship hold true, the crab population could depend almost exclusively on successful reproduction and recruitment of each year's juveniles, not on crabs that live and spawn for multiple seasons, he explained.

A population that replaces itself each year through reproduction may not pose a problem in itself, says fisheries economist Doug Lipton, director of the Maryland Sea Grant Extension Program. Shrimp are an "annual crop" like this, he explains. This reproductive high-wire act does, however, make the crab population more vulnerable if recruitment of juveniles fails due to disease, unusual weather or some other factor, because there will simply be fewer mature crabs around to maintain the stock's reproductive capacity.

Meanwhile, other efforts are underway to enhance recruitment of juvenile crabs to the population to ward off the specter of a future recruitment failure. Estuarine ecologist Anson Hines, from the Smithsonian Environmental Research Center (SERC) in Edgewater, Maryland presented preliminary results from collaborative work with physiologist Yonathan Zohar and others at the UM Center of Marine Biotechnology (COMB) in Baltimore. The research team is experimenting with enhancing crab populations on a local basis by releasing hatchery-raised juveniles in different tributaries. Since the project began in 2002, COMB scientists have reared a total of 81,000 juvenile crabs to a size of 20 millimeters - about as big as a grown man's thumbnail. So far, 45,000 crabs have already been released and another 30,000 will be released over the course of this summer. While it's too soon to evaluate whether these efforts will help bolster the population, Hines reports that some crabs tagged upon release have been found successfully migrating southward from the upper Bay in the fall through the deep channel - along with the wild crabs.

Counting Crabs

In order to gauge how the crab population is faring year-to-year and to set safe guidelines for future harvests, scientists and managers need to know more than just how many crabs are in the Bay. They need to have a handle on the rate at which crabs are removed from the population by fishing, as well as how fast they are dying by natural causes.

Think of it like a credit card account, says fisheries ecologist Thomas Miller, from the UMCES Chesapeake Biological Laboratory in Solomons, Maryland. "Crab abundance is your balance, fishing mortality is the interest rate charged on that money. When the population is overfished, it will not have the reproductive capacity to replace itself. This is the equivalent of too little money to meet your monthly expenses," he says. "If we fish the population at too high a rate, no matter what the abundance now, we will eventually go broke."

Accurately estimating fishing mortality on the crab population in the Bay is no easy feat. Over the past few years, scientists have come to a consensus that the method that they have used to measure fishing mortality has several important drawbacks, and this year CBSAC has formally adopted a new approach.

Previously, crab mortality was calculated using a "length-based" approach, similar to the method used for tracking fish populations. Basically, the length-based method assumes that if the average size of the population goes down - meaning that a greater percentage of surviving crabs are small - then more legal-sized large crabs are being removed from the population either by natural causes or by fishing, explains Miller.

But relying on size for crabs can be misleading. Unlike fish, which increase in size continuously as they get older, Miller says, crabs grow discontinuously and undergo a series of molts, reaching sexual maturity at 12-18 months. While we understand the crab's life history pretty well, the relationship between growth and age remains less certain - so knowing a crab's length may not tell us its age. This means you can't tell if you're looking at a product of this year's spawn, or last year's or the year before that.

The new method, called "direct enumeration," does not rely on crab size at all. Instead, mortality is calculated based on a direct count of the number of crabs buried in the sediment over the winter - measured by the Winter Dredge Survey - and from a measure of the number of crabs harvested during the previous season. Because this method does not rely on certain assumptions about whether the crab population is currently in a stable state of equilibrium, as does the length-based method, researchers believe it to be a more precise way to measure fishing mortality for species like crabs that grow by leaps and spurts.

According to Miller and others, the direct enumeration method is more sensitive to changes in fishing pressure, and it provides a more accurate way to estimate fishing mortality. Relying on this method will likely produce higher mortality rate estimates than indicated by the length-based approach, says Miller. And factoring in higher mortality rates when setting population targets for the future will lead to a more precautionary approach, one that the scientific community endorses.

Science and Policy

Overall, the mix of good news and bad news for blue crabs presented at the June TWG meeting sparked lively debate. But scientists were unified in urging continued financial support for high quality research and data collection, effective monitoring, and a continued cautious approach to management. It is hard to tell at this point, they said, whether recent increases in the crab population have resulted from new fishing regulations that went into effect in 2001, or whether changes are due mainly to high year-to-year population variability in the Bay. In either case, the group agreed that "holding the line" on crab regulations would be the decision most consistent with the best scientific evidence available.

Another challenge facing this group of scientists - who have contributed their expertise to evaluating and advising on the Bay's precariously poised blue crab population over the past few years - is that shifting regional priorities have pulled legislative attention away from the problem. Although the scientists in the TWG do still report to the Chesapeake Bay Commission and the legislators involved, the formal body (BBCAC), which included a representative group of stakeholders, dissolved last year from lack of financial input from the states. "What is really missing is a feeling of a bi-state investment at a coordinated level," says Swanson.

"It is clear from the meeting that there remains a strong commitment from the scientific community to remain engaged and to advise on the best science available for crab management," says Miller. But the loss of BBCAC has left its mark. "It is sad. I see a need for a formalized way for states to collaborate. The formal structure was really important," Miller says. In the meantime, the TWG scientists remain determined to place good data in the hands of decision-makers. They have agreed to meet again in the fall, and plan to produce another science-based status report on the health of the crab population and on sustainable management of the fishery.

For more on blue crabs, see last year's TWG report, Blue Crab 2003: Status of the Chesapeake Population and its Fisheries, at www.chesbay.state.va.us/crabpubs.htm.

Top of Page

[Chesapeake Quarterly]
Other Issues

[Chesapeake Quarterly Bar]
[Maryland Sea Grant][NOAA]