Stock Enhancement of Blue Crabs

Stock Enhancement is recently being considered as a possible technique, in conjunction with traditional management strategies, to restore stocks of the recruitment-limited, overexploited Callinectes sapidus blue crab in the Chesapeake Bay. Enhancement strategies with hatchery-raised animals have been long used in Asia as a method of supplementing declining wild stocks of crustaceans and fishes; however these approaches have not been evaluated adequately in North America, and have not been attempted for blue crabs. In the Chesapeake Bay, scientists at SERC in collaboration with the Center of Marine Biotechnology (COMB), Virginia Institute of Marine Science, and North Carolina State University are evaluating potential of a stock enhancement of blue crabs that have declined by 85% since 1991.

jana and midge releasing hatchery crabs
  interns and volunteers tagging crabs at SERC dock

Here at SERC, we conduct tests of a small- scale enhancement by tagging and following the success of several tens of thousands hatchery raised blue crabs released into semi-enclosed sites in the Rhode River and South River in the upper Chesapeake Bay.

hatchery crab with elastomer tag   inserting microwire tag

We collect wild inseminated mature female crabs for the scientists at COMB who, encourage brooding in the adults, and provide us with juvenile, intermolt stage C7 (approximately 18 mm carapace width), crabs raised from the wild females. At SERC, all hatchery crabs are tagged with two types of physical tags. Elastomer tags are pliable pieces of polymer that are injected, liquid form, into the leg of the crab. The brightly colored elastomer is clearly visible through the semi-transparent, moderately calcified shell. The second tag, the mircowire tag, is also injected, but into the body of the animal. An electronic "wand" detects the presence of the microwire tag. If the crab has been tagged, the wand will emit a beeping sound. Our lab extensively reviewed both tag types (and others) and these two methods were found to be the most feasible (Davis et al. 2004).

Once released into the field, sites are sampled weekly by beach seine and benthic trawl. Data collected yields information on growth, survivorship and movement in the field. The tags enable us to distinguish our hatchery crabs from wild individuals.

seining for released hatchery crabs
sled tow used to recapture hatchery crabs
tow and seine sites at Quietwaters

Results suggest that our releases resulted in levels of enhancement that ranged from 12% - 54% (the total proportion of crabs made up of hatchery crabs)(Davis et al. 2004). Differences in enhancement may have been influenced by the abundance of wild crabs in each site and physical factors ( temperature, salinity), benthic prey, and predator guild.

stock enhancement graphs

We also conduct lab experiments that compare behavioral and morphological differences between wild and hatchery crabs. Hatchery crabs are raised under artificial conditions vastly different from the natural environment they would otherwise encounter. They are fed a readily available pellet diet so they do not have to forage for natural prey. They have never been exposed to natural fish predators. They are bred in hard bottom fiberglass tanks with no sediment.

tanks at COMBWe now know that these conditions produce differences between hatchery and wild crabs (Davis et al, 2004); differences which have the potential to seriously affect the animal. For example, spine length, burying behavior or aggressive behavior can affect a hatchery crab's chances of survival once released. At SERC we assess these deficiencies and attempt to increase survivorship rates by adjusting hatchery crab characteristics (conditioning).

Conditioning enhances hatchery crab survivorship by subjecting them to various treatments that bring about changes in behavior or morphology. We have already had success with this in our lab.

two hatchery crabs before release As our ongoing assessment of stock enhancement potential we examine: growth rates, growth variation within a single cohort, aggressive behavior, foraging behavior, spine length, immigration rates, swimming behavior, color differences, response to predators, stocking densities and conditioning.

These studies are valuable in terms of addressing conditions for optimizing releases and will allow us to accurately evaluate the feasibility of using stock enhancement as a tool in increasing crab stocks.