2024 Blue Crab Winter Dredge Survey

INTRODUCTION:

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. In 1989 the Virginia Institute of Marine Science joined the survey, and the two states continue to sample each winter from December through March.​​

WHERE WE SAMPLE:

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.

HOW WE SAMPLE:

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 the 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.


HOW AND WHY WE DETERMINE THE AGE OF CRABS:

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.​​

CALCULATING THE NUMBER OF CRABS:​

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 meters 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 2024.

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 increased slightly in 2024 (Figure 1) while the density of crabs bigger than 2.4 inches decreased slightly in 2024 (Figure 2).
  •  ​ Winter Dredge Figure 1
  •  
  • Winter Dredge Figure 2

  • The density of male crabs greater than 5 inches in 2024 was similar to 2023 (Figure 3).
  • Winter Dredge Figure 3

  • The density of spawning females decreased slightly in 2024 (Figure 4).
  • Winter Dredge Figure 4

  • Overall, total crab density in 2024 was similar to 2023 ​ (Figure 5).
  • Winter Dredge Figure 5

HOW MANY CRABS ARE ESTIMATED TO BE LIVING IN THE BAY?

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.

Juvenile abundance increased slightly in 2024 but the overall, total crab abundance in 2024 showed little change from 2023.


Winter Dredge Total Juvenile

Winter Dredge Total Estimate

In 2017 an update to the blue crab stock assessment recommended a female-specific target of 196 million spawning age female crabs and a threshold of 72.5 million spawning age female crabs that were adopted by Maryland, Virginia, and the Potomac River Fisheries Commission. In 2024 the abundance of spawning age females decreased to 133 million crabs, which is above the recommended threshold of 72.5 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 2002
315
 2014
 297
 1991 828 2003
 334
 2015
411
 1992 367 2004
 270
 2016
 553
 1993 852 2005
400 2017
 455
 1994 487 2006
313
 2018
371
 1995 487 2007
251
 2019
 594
 1996 661 2008
 293​ 2020
405​
 1997 680 2009
396
 20​21
​ 282
 1998 353​ 2010
663
 2022
227
 1999308 2011
452
 2023
 323
 2000281
 2012 765
 2024
317
​2001
​254
​2013
​300


The estimated number of adult female 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 117
 2002
55
 2014
 69
 1991 227
 2003
 84
 2015
101
 1992 167
 2004
 82
 2016
 194
 1993 177
 2005
110
 2017
 254
 1994 102
 2006
85
 2018
147
 1995 80
 2007
89
 2019
 191
 1996 108
 2008
91
 2020
141
 1997 93
 2009
162
 20​21
​ 158
 1998 106
 2010
246
 2022
97
 199953
 2011
191
 2023
 152
 200093
 2012 95
 2024
133
​2001
​61
​2013
​147

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The estimated number of juvenile 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 463
 2002
194
 2014
 199
 1991 356
 2003
 172
 2015
269
 1992 105
 2004
 143
 2016
 271
 1993 503
 2005
242
 2017
 125
 1994 295
 2006
196
 2018
167
 1995 300
 2007
112
 2019
 324
 1996 476
 2008
166
 2020
185
 1997 512
 2009
170
 20​21
​ 86
 1998 166
 2010
348
 2022
101
 1999223
 2011
204
 2023
 116
 2000135
 2012 581
 2024
138
​2001
​156
​2013
111

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