Predicting the Blue Crab Blues

Atlantic blue crabs are a summer delicacy in this area. For people who grow up along the Delaware or Maryland shores, crab picking is practically a sport. For visitors, it’s a sign that summer is in full swing. 

But blue crabs are more than just delicious. They also are also an important link in the local food chain! Blue crabs are scavengers as well as predators, feasting on fish, clams, snails, and aquatic vegetation. The crabs themselves are a food source for many bird species, fish, and surprisingly, Diamondback terrapins!

So why am I talking about this in the dead of winter?

Populations of crabs in the Inland Bays vary from year to year. This can be influenced by a variety of factors, including the severity of winter temperatures!

During winter, blue crabs try to survive by lying dormant in the mud and sands at the bottom of their habitats – whether that’s the Inland Bays, Chesapeake Bay, or other coastal waters. Unfortunately, this is sometimes not enough. According to the Chesapeake Bay Program: “changes in water temperature can affect predator abundance, prey availability and winter mortality rates.” Extremely cold winters can cause significant blue crab mortality. This is especially true in low-salinity waters that freeze more easily.

In order to predict the effects of winter mortality on the coming summer harvest, the Virginia Institute of Marine Science conducts a winter dredge-survey. By sampling over 1,500 sites throughout the Chesapeake Bay, they receive helpful data on blue crab life cycles and current abundance. 

Although smaller, the Inland Bays experience similar winter mortality. Annual trawl surveys in our Bays indicate that blue crab populations decreased from 1986 to the mid-2000’s. They have remained low since 2011, with no obvious trend and reasons for this decline are uncertain. But we do know that harsh winters could be a factor, in addition to the lack of important grasses in our Bays, low oxygen levels due to nutrient pollution, mortality by predation, and the pressure of harvest by us humans.

Without a concrete idea of the cause of this decline, it’s difficult to pinpoint a course of action. In 2013, there was some hope that the elimination of once-through cooling water at the NRG power plant would help boost crab numbers in the Indian River. Unfortunately, data do not yet support this idea.

For now, the CIB will continue to collect data and monitor blue crab populations for our State of the Bays reports, looking for the key to helping our local blue crab populations thrive!


Further Reading

2016 State of the Inland Bays Report – Delaware Center for the Inland Bays

From the Field: Could blue crabs weather a changing climate? – Chesapeake Bay Program

Blue Crab Abundance Outcome: Factors Influencing Progress – Chesapeake Bay Program

Blue Crab Winter Dredge Survey – Virginia Institute of Marine Science

Posted Under: Staff Blog