Scientific survey shows solid blue crab stock improvement

  • Callinectes sapidus
    Callinectes sapidus   The Bay-wide Winter Dredge Survey determines the status of the blue crab population in Chesapeake Bay.   David Malmquist
  • VIMS Dredge Survey team
    VIMS Dredge Survey team   Members of the VIMS Dredge Survey team in action. From L: Captain Durand Ward, Mike Seebo, and Kathleen Knick prepare to deploy the crab dredge from the R/V {em}Bay Eagle{/em} at a site in the York River.   David Malmquist
  • Cleaning the Dredge
    Cleaning the Dredge   The survey team drags the crab dredge through the water to dislodge mud before it is brought on deck for sorting.   David Malmquist
  • Dredge Retrieval
    Dredge Retreival   Captain Durand Ward and Kathleen Knick empty the crab dredge onto the deck of the R/V {em}Bay Eagle{/em} for processing as Mike Seebo watches for crabs.   David Malmquist
  • Emptying the Dredge
    Emptying the Dredge   Alison Smith and Kathleen Knick dump a dredge sample from a sandy site in the York River onto the deck during the Blue Crab Winter Dredge Survey.   David Malmquist
  • Sorting the Catch
    Sorting the Catch   Alison Smith, Mike Seebo, and Kathleen Knick carefully sort through hydroids and other dredged materials from a York River site in search of blue crabs.   David Malmquist
  • Recording Data
    Recording Data   Kathleen Knick records survey data aboard the R/V {em}Bay Eagle{/em} during a dredge tow.   David Malmquist
  • Crab Tagging
    Crab Tagging   A VIMS researcher tags a mature female blue crab collected during the Winter Dredge Survey. Recapture of tagged crabs provides data about crab movement, survival, and effectiveness of Virginia's blue-crab spawning sanctuary.   David Malmquist
  • Navigation
    Navigation   The survey uses a crab dredge to sample blue crabs at 1,500 randomly selected sites throughout Chesapeake Bay between November and March. By sampling during winter when blue crabs are buried in the mud and stationary, scientists can develop estimates of the number of crabs present in the Bay. Here's a computer map of three dredge paths near the mouth of the York River.   David Malmquist
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The Virginia Marine Resources Commission today released the results of the 2016 blue crab winter dredge survey, which shows another year of growth in the stock and forecasts an improved harvest in 2016.

Conducted annually by the Virginia Insitute of Marine Science and the Maryland Department of Natural Resources since 1990, the winter dredge survey is the primary assessment of Chesapeake Bay’s blue crab population. Survey scientists employ crab dredges to sample blue crabs at 1,500 sites throughout the Bay from December through March. By sampling during winter when blue crabs are buried in the mud and stationary, the scientists can estimate, with good precision, the number of crabs present.

In releasing VMRC's interpretation of this year's survey results, Commissioner John M.R. Bull says, "We now have back-to-back years of solid growth in this important fishery. The crab stock has been on a rollercoaster for most of the last decade. We’ve seen a few great years of reproduction followed by awful years of abundance. Two years does not make a trend, and this news inspires both wary optimism and cautious management.”

Professor Rom Lipcius, who oversees the Blue Crab Winter Dredge Survey at VIMS, notes that an integrated advisory report combining data from the winter dredge survey and several other blue crab surveys at VIMS and other scientific institutions will be released in May.

Total number of blue crabs in Chesapeake Bay in millions (all ages and both sexes). Click for larger version.2016 Dredge Survey Results

The results of the 2016 Blue Crab Winter Dredge Survey show the total population of blue crabs in Chesapeake Bay climbed 34% to a total of 533 million. This is the fourth highest level in two decades, since 1996, and builds on last year’s 38% boost in abundance.

Stock improvements were found in both female and male crabs, as well as juveniles and adults.

The spawning female stock almost doubled, from 101 million to 194 million, while the adult male stock more than doubled, from 44 million to 91 million. These are the second highest levels recorded since 1995 and bode well for a good crab harvest this year. Still, this level of spawning age female crabs remains below the scientifically recommended target of 215 million but well above the minimum safe threshold of 70 million crabs.Number of spawning age female blue crabs in Chesapeake Bay in millions. Click for larger image.

The juvenile abundance increased only slightly, from 269 million to 271 million, which is just above the average level of juveniles recorded over the past almost 30 years.

Number of juvenile blue crabs in Chesapeake Bay in millions (both sexes). Click for larger image.The bay-wide crab harvest last year increased by 42%, to 50 million pounds.

VMRC Crab Management

Increased overall abundance means commercial crabbers can expect to see more crabs in their crab pots this year than last year and to more often catch their daily bushel limits. Some adjustment of harvest limits may be warranted.

Crab spawning naturally fluctuates and can be affected by wind, currents, weather, cannibalism, and increased predation on crabs by other species. In recent years, unexpected predation events and stressful combinations of environmental factors have caused dramatic downturns in crab stock abundance. This highlights the need for fishery managers to continue to enhance resilience of the stock through adaptive management to compensate for unusual or extreme environmental conditions.

A bay-wide 10% crab harvest reduction enacted in 2014 by VMRC, Maryland, and the Potomac River Fisheries Commission to combat low overall crab abundance and to boost a dangerously depleted female spawning stock appears to have been effective.

“Two years ago, the number of adult female crabs were so low that a harvest cut was necessary to help rebuild them,’’ says Bull. “Now that the stock has improved, we need to be cautious and make prudent management decisions that further improve the stock and increase the stock’s resilience to factors outside of anyone’s control.”

In 2014, Bay fishery managers refined their management regimen to focus on conserving juvenile crabs as well as spawning age female crabs. Each year’s juveniles become the next year’s spawning stock. Conserving more juveniles when they reach market size in the fall and emerge from hibernation in the spring increases the likelihood they will survive to spawn another generation of abundant crabs in the summer.

“This recent success in management seems to have built some resilience in the stock,’’ says Lipcius. “We’re seeing a rebound in abundance and a buffer on unpredictable environmental disturbances that can cause severe declines in the stock.'

Peyton Robertson, chair of the Chesapeake Bay Program's Sustainable Fisheries Goal Implementation Team, says "The increase in blue crab numbers is a positive sign toward the commitment of the 2014 Chesapeake Bay Watershed Agreement to rebuild long term sustainability of blue crab populations."

Percentage of female blue crabs harvested in 2015. Click for larger image.The Chesapeake Bay Stock Assessment Committee (CBSAC) is reviewing the new survey results and will release their full analysis of the results in the 2016 Blue Crab Advisory Report this summer. The advisory report is used by managers as they review and update fishery regulations.

“While the spawning stock of adult females has increased to a healthy level, it is still below the established target of 215 million. The highly variable nature of blue crab reproductive success means that caution must be maintained when considering management adjustments so harvests do not adversely affect future spawning potential,” says Glenn Davis, chair of the CBSAC.

VMRC will begin discussions with its crab management advisory committee on options to adjust harvest restrictions while maintaining higher abundance. The Commission board will be briefed on the survey results at its April 25 meeting.