Project Goal:

The goal of this project is to better understand the the connectivity between nursery habitat productivity and spawning blue crab stock. This could help aid in the design of a network of protected areas that produce females throughout the spawning migration.

Female Blue Crab Migration

The blue crab, Callinectes sapidus, is on of the Chesapeake Bay's most iconic species.  It is considered the most valuable fishery in the bay.  The blue crab fishery in the Chesapeake Bay has suffered extended periods of low or highly variable harvests that coincide with low or variable spawning stock abundance.  Protecting spawning female blue crabs and their habitat is the best way to promote a sustainable blue crab fishery in the Chesapeake Bay.  Relative contributions of individual nursery habitats (subestuaries) to fishery production and to spawning stock is likely to be an important consideration as we move forward.

Previous work that our lab was involved in showed that females remain near their mating area until they migrate in a short autumn period to the spawning grounds in the lower Bay.  Until then, they spend time foraging and acquiring energy as their body grows into their new shell.  Female crabs that mate in the upper Bay area have a longer distance to travel and they may not reach the lower Bay in time to spawn in the same season that they mated, or they may not reach the lower Bay at all and have to over-winter in higher-salinity areas. Delayed spawning and over-wintering in higher-salinity areas may have an effect on sperm quality, which can then have an effect on reproductive potential and brood size.

Our lab is currently conducting a study that looks at migrating individual, mature female blue crabs in hopes that we can answer the question:  Which nurseries are the most important for producing spawning female blue crabs?  To answer this question, we are using mark-recapture and shell chemistry analysis to understand where in the Bay a female mated and compare to their reproductive potential.

Mark-Recapture

Female crabs are obtained from watermen and tagged with white, plastic tags that give information about how to report the crab and tag onces its been caught.  The tags are attached to the back of the crab's shell with wire that is twisted snuggly around each spine.  Crabs are tagged and released at multiple locations throughout the Chesapeake Bay throughout the summer.

When someone catches a tagged female crab, the tag can be reported to us for a reward (either $5 or $50).  In Maryland, recreational crabbers are not allowed to keep female crabs, so the crab would have to be thrown back in the water (though can still be reported to us!). Commercial watermen, however, are allowed to keep female crabs.  IF they catch a tagged female, they can also freeze it and return it to us for an extra reward.  Having the crabs returned to us frozen is an important part of the study.

Shell Chemistry Analysis and Reproductive Potential

the frozen crabs are later dissected and analyzed for reproductive potential and shell chemistry.  The crab's shell is ground up and analyzed for chemical composition. The chemical composition of a female blue crab's shell can be linked to the location where she mated.  This is possible because of a blue crab's mating process:

Female crabs mate when they have just molted and their shells are still soft.  They mate once in their lifetime, though they lay eggs numerous times (multiple broods).  As the female's shell hardens, it takes on some of the chemical nature of the surrounding water in which she sits.  This novel bio-geochemical fingerprinting technique allows us to identify the source nursery subestuary of individual mature female blue crabs from the spawning stock.

Sperm counting techniques are used to quantify differences in the reproductive potential of individual females.  This information combined with mating location data allows us to understand which subestuaries are most productive and therefore most important for producing spawning stock.