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Virginia Institute of Marine Science

Scientific survey shows modest improvement in blue crab stock

  • 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 Retreival
    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
  • Water Conditions
    Water Conditions  Alison Smith deploys a sensor to record water temperature, salinity, and dissolved oxygen readings during the Blue Crab Winter Dredge Survey. The data are compared to dredge survey results to better understand blue crab distribution, habitat use, and behavior.  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
Photo - of -

The Virginia Marine Resources Commission today released the 2015 blue crab winter dredge survey, which showed moderate improvement in the health of the stock, despite a cold winter that killed an inordinately large number of crabs.

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.

“This is a step in the right direction but we are not out of the woods,” says VMRC Commissioner John M.R. Bull. “Responsible management dictates a cautious approach as we continue to work to build a healthy crab stock and to maintain a sustainable fishery. More work needs to be done to boost us above modest abundance levels.”

Total number of blue crabs in Chesapeake Bay in millions (all ages and both sexes). Click for larger version.While juvenile and spawning age female crab numbers increased a bit and last year’s harvest remained at a safe level (under the target fishing removal rate) for the seventh consecutive year, the total abundance of crabs was only the 11th highest in the past 25 years.

The long, abnormally cold winter appears to have resulted in one of the worst overwinter mortality events since the start of the winter dredge survey, causing the death of 29% of all adult crabs in Maryland. The winter mortality rate was well above the average.

Despite that severe setback, the crab stock improved.

A bay-wide 10% crab harvest reduction enacted last year by VMRC, Maryland, and the Potomac River Fisheries Commission to combat low overall crab abundance and to boost a dangerously depleted female spawning stock apparently was effective and just in time.

VIMS professor Rom Lipcius, who oversees VIMS component of the annual survey, says “It’s likely the collective management actions since 2008 enhanced the population’s resilience despite a severe environmental disruption. Without that, the winter’s impact on the crab stock could have been much worse.”

Number of juvenile blue crabs in Chesapeake Bay in millions (both sexes). Click for larger image.This year’s overall bay-wide crab abundance increased from 297 million crabs to 411 million crabs, a 38% increase. Juvenile crabs jumped from 199 million to 269 million, a 35% increase, and now comprise the bulk of the crab stock.

Spawning-age female crab levels emerged from depleted status, increasing from 69 million to 101 million, a 46% increase, but remained well below the healthy-stock target of 215 million. Spawning-age females are the cornerstone of stock rebuilding. Spawning-age males increased from last year but their abundance of nearly 44 million remains the third lowest in the past 25 years.

Although the overall number of crabs in the bay improved, the stock remains just above pre-2008 levels, when a coordinated bay-wide stock rebuilding program kicked off.

Number of spawning age female blue crabs in Chesapeake Bay in millions. Click for larger image. “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,” says Peyton Robertson, chair of the Chesapeake Bay Program's Sustainable Fisheries Goal Implementation Team.

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, but this year’s abundance levels would seem to preclude the possibility of reopening the winter crab dredge fishery, which has been closed annually since 2008.

Crab spawning naturally fluctuates and can be affected by wind, tide, weather, and increased predation on crabs by other species. During the past two years, unexpected predation events and abnormally harsh winters have negatively impacted the crab stock and have highlighted the need for fishery managers to enhance resilience of the stock through adaptive management to compensate for unusual or extreme environmental stresses.

Percentage of female blue crabs harvested in 2014. Click for larger image.The Chesapeake Bay Stock Assessment Committee is reviewing the new survey results and will release their full analysis of the results in the 2015 Blue Crab Advisory Report this summer. The advisory report is used by managers as they review and update fishery regulations.

“The survey results show an increase in overall blue crab abundance but female abundance is still below the 215 million target. This suggests maintaining a precautionary approach to managing the population” says Joe Grist, Chair of the Chesapeake Bay Stock Assessment Committee.

VMRC will begin discussions with its crab management advisory committee today, April 27, and the Commission’s board will be briefed on the survey results tomorrow.

Here is the history of the Bay-Wide Winter Dredge Survey results (1990 through 2015). All surveys begin in December and end in March of the next year.

Winter dredge survey results (all numbers expcept first and last column in millions)
Survey
Year
Total Crabs
(all ages)
Juvenile Crabs
(both sexes)
Spawning-Age Crabs
(both sexes)
Spawning-Age
Female Crabs
Bay-wide
Commercial Harvest (Pounds)
% of Female Crabs
Harvested
1990 791 463 276 117 96 44
1991 828 356 457 227 90 34
1992 367 105 251 167 53 60
1993 852 503 347 177 107 35
1994 487 295 190 102 77 28
1995 487 300 183 80 72 32
1996 661 476 146 108 69 20
1997 680 512 165 93 77 22
1998 353 166 187 106 56 40
1999 308 223 86 53 62 37
2000 281 135 146 93 49 43
2001 254 156 101 61 47 42
2002 315 194 121 55 50 34
2003 334 172 171 84 47 33
2004 270 143 122 82 48 42
2005 400 243 156 110 54 24
2006 313 197 120 85 49 29
2007 251 112 139 89 43 35
2008 293 166 128 91 49 24
2009 396 171 220 162 54 23
2010 663 340 310 246 85 18
2011 452 204 255 191 67 25
2012 765 581 175 95 56 10
2013 300 111 180 147  37  23
2014 297 199 180 69  35*  17*
2015 411 269 180 101  TBD  TBD
* 2014 Bay-wide commercial harvest and percentage of the female crab removal rate are preliminary.