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Virginia Institute of Marine Science

Blue Crab Winter Dredge Survey

  • Callinectes sapidus
    Callinectes sapidus
    The Bay-wide Winter Dredge Survey determines the status of the blue crab population in Chesapeake Bay.
  • 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 Bay Eagle at a site in the York River.
  • 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.
  • Dredge Retrieval
    Dredge Retrieval
    Captain Durand Ward and Kathleen Knick empty the crab dredge onto the deck of the R/V Bay Eagle for processing as Mike Seebo watches for crabs.
  • 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 2008-2009 Blue Crab Winter Dredge Survey.
  • 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.
  • 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.
  • Recording Data
    Recording Data
    Kathleen Knick records survey data aboard the R/V Bay Eagle during a dredge tow.
  • 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.
  • 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. Each dredge tow lasts 1 minute. 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.

Survey data guide management actions

Blue crabs are known as feisty creatures, eager to pinch anyone who brings a finger within reach of their powerful claws. But on this day, the crabs on the deck of the VIMS research vessel Bay Eagle are as docile as lambs.

The reason? These crabs have just been dredged from the frigid bottom of Chesapeake Bay, where they lie dormant through the winter in water temperatures that approach freezing.

Sampling of these crabs is part of the Blue Crab Winter Dredge Survey, a collaborative venture between scientists at VIMS and the Maryland Department of Natural Resources that began in 1990.

Each winter since then, dredge-survey teams have sampled more than 1,500 sites throughout the Bay and its tributaries, providing a fishery-independent measure of blue crab abundance in the nation’s largest estuary. VIMS covers the entire Virginia portion of Chesapeake bay, sampling at more than 750 stations between November and March.

The purpose of the survey is to understand the dynamics of the blue crab population. Sampling in the winter, when crabs are stationary in the bottom, provides a snapshot of the population that is free from the biases otherwise introduced by the seasonal movements of these peripatetic creatures.

Blue crabs exhibit a complex life cycle in which females migrate to salty waters near the Bay mouth in the fall, and release their larvae to springtime currents that carry the larvae into the coastal ocean. There, the larvae develop until currents carry them back into the Bay and its tributaries to grow, mate, and complete their life cycle.

VIMS scientist Mike Seebo, who has led field portion of the winter dredge survey since 1990, says “the winter survey allows us to estimate year-to-year changes and is directly correlated with commercial landings. It also helps us understand the effects of over-wintering mortality, fishery-induced mortality, and seasonal migrations by means of our crab-tagging program.”

Professor Rom Lipcius, who heads up the VIMS component of the survey, adds, “the winter dredge survey is a unique, long-term, and reliable data set that is consistently cited as a vital and indispensable tool for blue crab fishery management.”