2017 Blue Crab Winter Dredge Survey


Winter DredgeThe winter dredge survey is the only bay-wide fishery independent effort to estimate the number of blue crabs living in the Chesapeake Bay. The winter dredge survey produces information that is essential for the management of the species, such as an estimate of the number of crabs over-wintering in the bay and the number of young crabs entering the population each year. Also calculated is the estimated number of females that could spawn within the year, which is an important indicator of future spawning potential. Estimating the total number of crabs living in the bay allows us to calculate the percentage of the crab population that is removed by harvest each year. A pilot version of the winter dredge survey was first conducted in 1988 with the cooperation of the University of Maryland Chesapeake Biological Lab (CBL). In 1989 the Virginia Institute of Marine Science (VIMS) joined the survey, and the two states continue to sample each winter from December through March.


Distribution of Winter Dredge Sites 1999-2000Since 1994, the survey has been conducted according to a stratified random design (Rothschild and Sharov, 1996). The bay is divided into three regions or strata: Lower Bay (the mouth of the Chesapeake to Windmill Point, VA), Middle Bay (Windmill Point, VA to Cove Point, MD) and the Upper Bay/Tributaries (Cove Point to Pooles Island and all of the Bay's tributaries). Each year, a total of 1500 sites in waters deeper than 5 feet are randomly selected. The number of sites assigned in each region is proportional to its area.


A six-foot-wide Virginia crab dredge fitted with a half inch nylon mesh liner is towed along the bottom for one minute at a speed of three knots. Latitude and longitude, measured with a Differential Global Positioning System (GPS), is recorded at the beginning and end of each tow to determine distance covered. This distance is multiplied by the dredge's width to calculate the area covered. Beginning and ending depth, water temperature, and salinity are recorded at each site.

All crabs collected at each site are measured and weighed. Crabs are measured from point to point across the top shell, or carapace. The sex of each crab is determined and maturity of females is noted.

As a final part of our survey, we do annual experiments to determine how well our dredge catches blue crabs. This is known as gear efficiency. For more information see: Volstad et al. 2000 and Sharov et al. 2001.


We divide the male and female crabs into different categories based on age, size and maturity. Crabs that are smaller than 2.4 inches across the carapace are considered to be young-of-the-year crabs. . These are the new arrivals to the bay, and will form the backbone of the recreational and commercial fisheries in the late summer or fall. Female crabs bigger than 2.4 inches across are the females that could spawn this year. The number of spawning females is related to the number of young crabs that can potentially be produced each year, and is an important indicator of the health of the stock. Combined, all crabs bigger than 2.4 inches are grouped together and considered to be the harvestable stock for the upcoming year. These crabs will support the recreational and commercial fisheries throughout the summer.


The results of the winter dredge survey are reported as density of crabs. This is the average number of crabs we find within a 1,000 meter by 1,000 meter area (1,000 meter squared or 1000m2). The density of crabs for the different age categories is calculated each year. If you are interested in that information please click on this link to see a table of different age class densities for 1990 to 2017.

The figures below show the annual density since 1990 for important age categories as well as the densities of all age categories combined.

  • The density of juvenile crabs dropped significantly in 2017 (Figure 1), while the density of crabs bigger than 2.4 inches increased for the third year in a row (Figure 2).  ​

  • Winter Dredge Figure 1
  • Winter Dredge Figure 2

  • The density of male crabs greater than 5 inches decreased in 2017 (Figure 3).
  • Winter Dredge Figure 3

  • In 2017 the density of females that could potentially spawn continued to increase (Figure 4).
  • Winter Dredge Figure 4

  • Even with increases in the adult population the overall density of all crabs combined decreased in 2017 (Figure 5).
  • Winter Dredge Figure 5


Managers estimate abundance, the number of crabs estimated to be living in the Chesapeake Bay, by multiplying the density of all crabs by the area of the bay. Each year the overwintering mortality, those crabs that die in the winter, is estimated and abundance is adjusted for that loss.

In 2017 both juvenile and total blue crab abundance declined.

Winter Dredge Total Juvenile Winter Dredge Total Estimate

In 2011 an update to the blue crab stock assessment recommended a female specific target of 215 million spawning age female crabs and threshold of 70 million spawning age female crabs that was adopted by Maryland, Virginia, and the Potomac River Fisheries Commission. In 2017 the abundance of spawning age females was the highest in survey history and above the recommended target of 215 million crabs.

Winter Dredge Female Spawn

The total estimated number of crabs living in the bay for each year of the survey is listed below:

 Year  Millions of Crabs  Year  Millions of Crabs  Year  Millions of Crabs
 1990  791  2000  281  2010  663
 1991  828  2001  254  2011  452
 1992  367  2002  315  2012  765
 1993  852  2003  334  2013  300
 1994  487  2004  270  2014  297
 1995  487  2005  400  2015  411
 1996  661  2006  313​  2016  553​
 1997  680  2007  251  2017  455
 1998  353​  2008  293
 1999 308 2009   396

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