The mystery of the missing blue crabs
Scientists unable to completely explain how an extra ordinary abundance of juveniles resulted in a disappointing harvest.
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A year and a half ago, there seemed to be no shortage of good news about blue crabs. The annual winter dredge survey estimated 764 million blue crabs were in the Bay, the most in 19 years and a huge jump from the previous year.
The governors of Maryland and Virginia issued a joint news release talking about the “extraordinary explosion in juvenile blue crab abundance.” The 581 million juvenile blue crabs in the survey were the most ever seen, and were expected to result in increased catches as they reached legal harvest size last fall and this year.
Instead, harvests dropped last year, and fell more this year. Crab prices hit record highs, and seafood processors haven’t been able to get enough crabs to fill their orders at any price.
“I don’t know how many hundreds of bushels of orders we turned down this year,” said Jack Brooks, of the J.M. Clayton Company, one of the largest seafood processors on Maryland’s Eastern Shore. Much of the summer, he said, “we were turning down orders every day.”
“It’s disappointing obviously, when we had between 700 (million) and 800 million crabs out there in the winter dredge survey of 2011–12, and then they disappeared in 2012–13,” Brooks said. “What happened?”
That’s a question being asked by everyone, from watermen who catch crabs, to processors like Brooks who buy them, to fishery managers who regulate catches, to scientists who study the populations.
It is particularly frustrating because, as far as fisheries are concerned, the blue crab population is one of the best understood in the Bay. And the winter dredge survey that monitors the population is considered a model for its ability to estimate the overall blue crab population.
The survey has been conducted each year since 1990 by scientists in Maryland and Virginia who collect crabs at 1,500 sites across the Bay from December through March when they are buried in mud and stationary. Historically, the survey has provided an accurate snapshot of the crab abundance, and is the primary tool for assessing the health of the crab stock.
So when the 2012 survey showed a nearly threefold increase in juvenile blue crabs, everyone from watermen to fishery managers expected increased catches. Instead, the Baywide commercial harvest was 56 million pounds last year, an 11 million pound decrease from 2011. This year, everyone expects the harvest total to be even lower.
Part of this year’s problem is that the 2012 record juvenile production was followed by the second lowest juvenile number in the history of the survey last winter, 111 million. Poor production has been reported all along the East Coast this year, and likely relates to some coastwide climatic condition — crab larvae spend a period of their life off the coast and are particularly vulnerable to tidal currents, winds and weather. But the mystery remains of what happened to 2012’s massive juvenile production, which was never manifested in increased harvests.
Brooks pins the blame on too many predators and too little habitat such as underwater grass beds where young crabs can hide. “If they don’t do something about this fish population and restoration of this habitat, I don’t see where crabs are going to have a chance,” he said. “Basically, the bottom is bald, and you can count these crabs in the bellies of the fish.”
Those sentiments are echoed by many others. Scientists who study blue crabs agree those could be contributing factors, but suggest other issues are likely in play as well.
“We really don’t know what happened,” said Tom Miller, director of the University of Maryland Center for Environmental Science Chesapeake Bay Biological Laboratory, and a scientist who has overseen the last three blue crab stock assessments.
Miller, like other scientists, praises the winter dredge survey, but said it only provides information about the status of the stock — not all of the factors affecting it. Further, some parts of the survey, including its juvenile estimates, could use refinement.
“It is a reality check to remind us that we do not have the precision that perhaps people thought we did have,” Miller said. “And it should remind scientists, managers and stakeholders that there are still many unknowns in managing crabs.”
While no one can be certain what happened to the young 2012 crabs, Miller and other scientists offer a number of possibilities:
≈ The number of baby blue crabs found in the 2012 survey may not have been as spectacular as it appeared. The survey’s juvenile crabs figure is generally considered to be an underestimate of the actual number of juveniles — those less than 2.4 inches — in the Bay. That’s because the survey cannot sample in the shallowest waters where young crabs are thought to be more abundant.
But it’s possible that the proportion of the crab population found in monitored waters, versus unmonitored shallow water, varies from year to year, which could skew interpretation of the data. For instance, if the number of juveniles were the same for two consecutive years, but in one year a greater proportion of them overwintered in deeper water, then more would turn up in the survey. In such a situation, the actual abundance of juvenile crabs would be the same, but to the survey, it would look like there were a lot more.
“We are much more confident of the number of adults in the winter dredge survey,” Miller said. “The estimate of juveniles is a much less certain number.
“That bumper crop two years ago, when we all thought the following summer was just going to be the best crabbing summer ever — that expectation might have been wrong.”
≈ Predation. Probably nothing has received more blame for the apparent disappearance of so many juvenile crabs than predation, whether by red drum, striped bass or even blue catfish.
Anecdotal reports suggest that a lot of blue crabs were seen in the stomachs of fish. But, Miller noted, there were so many young blue crabs in the Bay that increased predation would be expected. The survey estimated that the number of young blue crabs in the Bay in 2012 increased almost threefold from 2011. That means predators would have had to eat three times as many blue crabs as they did the year before just to keep pace with the population expansion.
“To cause the decline that we see, the predators would have to have eaten a disproportionate amount of crabs,” Miller said.
Many people have blamed a recent surge of red drum, whose numbers have skyrocketed in the past couple of years, for taking a bite out of blue crab numbers. An abnormally large population of juvenile red drum (also known as puppy drum) was documented, especially in Virginia’s portion of the Bay, and they can eat large numbers of crabs.
Such a new source of predation could affect the overall population, Miller said, but he also said it’s unclear if there were enough red drum throughout the Bay, or that they ate enough. Anson “Tuck” Hines, director of the Smithsonian Environmental Research Center and a longtime Chesapeake blue crab researcher, said Maryland’s Rhode River, which he has monitored for years, had fewer juveniles than expected. Yet, he added, “we did not see a big increase in puppy drum in the Upper Bay and in the Rhode River in that time.”
“Definitely, predation probably had an impact at some level,” said Joe Grist, deputy chief of fisheries with the Virginia Marine Resources Commission and chair of the Chesapeake Bay Stock Assessment Committee, which makes Baywide recommendations for crab management. “Is that the sole contributor? No. It has got to be multiple contributions to this.”
≈ Cannibalism. Closely related to the notion that fish took a bite out of the crab population is the notion that crabs themselves caused the decline. Blue crabs, after all, are known cannibals. “You can put a couple in a tank, wait a couple days — and there will be one left,” Grist said.
Hines said that research on the Rhode River has shown “predation rates on blue crab are largely driven by cannibalism by the adult crabs, and that it tracks with changes in abundance of adult crabs.”
But in 2012, the numbers of adult crabs in the winter dredge survey was actually less than the abundance seen the previous year. So again, there is no evidence to suggest a disproportionate rate of cannibalism that would have affected the Baywide population.
“As far as them wiping out hundreds of millions of crabs themselves, I don’t buy it,” said Brooks, whose family has been processing crabs since 1890.
≈ Environmental factors. Other factors could have played a role in the apparent disappearance of juvenile blue crabs. For instance, 2012 was one of the worst recent years for underwater grass abundance in the Bay. Studies indicate that young blue crabs like to seek refuge from predators in grass beds. But blue crabs use so many habitats, it’s hard to say that change was substantial enough to have affected crab numbers. And, the last blue crab stock assessment cast doubt on underwater grasses having a major impact on the overall population.
Another possibility is that the long, cool spring of 2012 might have slowed the growth of blue crabs which, Miller said, would have made crabs vulnerable to predation for a longer period of time. “The size of crabs relative to their predators controls predation,” Miller said. “It’s an arms race, with crabs that get big enough fast enough, winning.”
Hines agreed that growth of the juvenile cohort “is directly dependent on temperature. Vulnerability to predation and cannibalism is directly related to the size of those juveniles.” As soon as they get up to 90 or 100 millimeters [3.5–4 inches], they get refuge from predation just by being larger.
But, Hines said, all suggestions about what happened to the 2012 crab numbers are pure speculation.
“I will tell you that I don’t know what happened to the cohort of crabs in 2012, and I don’t think anybody does,” he said. “There are a bunch of ideas out there, and I could conceive of how all of them in some ways could have contributed to it. But nobody actually did any experiments to determine cause and effect.”
He and others say the winter dredge survey continues to be one of the best, and most accurate surveys, for any fishery in the Bay. That said, it does not provide a full picture of everything that affects the crab populations.
“Currently, we are basing everything on the winter dredge survey, which is really good information, but it is a snapshot and then we don’t take another snapshot until the next winter,” said Bruce Vogt, of the National Oceanic and Atmospheric Administration Chesapeake Bay Office, which has funded much of the blue crab research over the years.
“In between is when most of the activity is occurring in terms of predation and movement of crabs, as well as when most of the fishing effort is in play,” Vogt said.
The Chesapeake Bay Stock Assessment Committee, a panel of scientists and fishery managers from around the region, in its annual blue crab advisory report, outlined research needs that would improve understanding of the crab stock. High on its list of recommendations was analyzing existing surveys, and possibly creating a supplemental crab survey during the spring or summer to both verify results from the winter survey and to better understand other variables that might influence crab abundance during the year, such as weather, predation or fishing.
Also among those recommendations was research to understand year-to-year variations in the number of juvenile crabs that may be in shallow areas during the winter dredge survey. In addition, the committee recommended a review of the effectiveness of equipment used in Maryland and Virginia portions of the survey — scientists in each state use different boats and equipment, which introduces uncertainties into survey results.
The research may never explain what happened to the 2012 year class of juvenile blue crabs. But it would provide more certainty to estimates in the future.
“It’s too soon to tell if we are ever going to be able to fully explain 2012,” said Grist, chair of the stock assessment committee. “But hopefully, we will take some lessons from that, look at what data we were lacking to help us explain it, and work toward filling some of those gaps. If nothing else, 2012 will serve as a year from which we can learn some lessons.”
Grist noted that the 111 million juvenile crabs estimated to be in the Bay this year was the second lowest since the winter dredge survey began in 1990. The lowest number, 105 million, was in 1992. But the numbers bounced back the next year, with a fivefold increase in juvenile crabs and the highest overall crab abundance seen in the survey — 852 million.
“That really shows the resiliency of the blue crab,” Grist said.
In fact, while the overall number of crabs declined in the Bay, the number of spawning age females increased according to the latest survey. That could set the stage for a similar comeback next year.
That would be good news for watermen and seafood processors. Much of their livelihoods depend on the year-to-year abundance of the blue crab, the Bay’s most valuable commercial species.
“When the watermen hurt, we hurt, and when they do well, we do well for the most part,” Brooks said. “And this is one of those years where we certainly hope we don’t see another one like it anytime soon.”
- Category: Fisheries
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