Paula Rodgers - Fish and Invertebrate Lab
University of Maryland, College Park
Blue Crab Reproductive Biology: An Intern's Experience
Maryland's blue crab fishery has declined more than 80% in the past twenty years. This has caused concern and drawn researchers to studying various aspects of blue crab biology in an attempt to better manage the fishery. The fishery is important not only economically, but also because Maryland is identified by blue crab fishing, much the same as with the lobster in Maine. Having been able to conduct and assist with research out at SERC, located directly by the Chesapeake, I have been immersed in not only blue crab research but also in Maryland's fisheries issues. The real strength of my experience in this internship was the ability for me to assist with a variety of diverse projects involving both blue crab and fish research. This not only allowed me to gain experience with various sampling techniques, but also helped provide a fuller picture of the ecology of the Chesapeake.
One of the main projects I was able to assist with this summer was a tag and release study of 20,000 crabs reared at COMB, a hatchery located in Baltimore. These crabs were tagged with both microwire and elastomer tags, and then released into three different sites (Sheep's Head Cove, Boathouse A and Boathouse B) in the Rhode River. After release, sampling was conducted at each of the sites weekly by trawling and seining. A total of 143 COMB crabs were recaptured from June 28th to July 29th with Sheep's Head Cove returning almost 50% of the recaptures. Boathouse B, stocked at a lower density than the other two sites, returned only 18% while 28% were recaptured in Boathouse A. The remaining 4-5% were recaptured in one of three control sites, one located just outside of each sample site. Further tag and release projects will be conducted at SERC throughout the fall. This will provide a picture as to the survival, growth rate and movement of these hatchery crabs released into the wild with the ultimate goal of increasing the spawning stock biomass in the Bay. In order to contribute to the spawning stock, the hatchery crabs must survive to reproduce successfully. Therefore, continuing research on mating behavior and reproductive out put of the crab are crucial to understand the population dynamics.
There are several annual samplings throughout the summer including fishing the weir, seining and benthics. All of these field-sampling methods are used to compare data from year to year in order to provide information on long term trends in the Bay. The weir, seining and benthics provide information on the composition and sizes of species moving up and down muddy creek, composition and sizes of species in mud versus sand environments, and composition of organisms located in the benthos, respectively. All of the data together supplies a larger picture of the long term trends of populations in the Chesapeake Bay.
I was also fortunate enough to work on a project during my internship. As stated above, understanding crab reproduction is imperative to managing the fishery. Little is known about brood production in female crabs as maintaining live crabs in the lab can prove a real challenge. Throughout a large portion of the summer, I was able to work on building a recirculating system to hold brooding female crabs. As females require high salinities to produce eggs, they can not be placed in a simple flow-through system. Using a design and help provided by a technician in the lab, I managed to build a recirculating system. Attached is a diagram and picture of the tank. It consists of two tanks flowing into a sump below; this water is then pumped up into a branching pipe. The pipe directs half of the water into a biofilter. The biofilter has a sediment trap located above that removes any solid waste and allows the particle-free water to trickle into the filter containing dozens of bioballs. These bioball provide surface area for bacteria that convert ammonia into less toxic forms to thrive. This water then flows back into the two tanks. The second half of the pumped up from the sump flows into a protein skimmer that is essentially a tube with a bubbler at the bottom and water flowing from the top and out the bottom. Organic materials stick to the bubbles and it bubbles off the top of the skimmer into a collection cup. The outflow of water from the skimmer is directed back into the sump.
An initial collection of 30 brooding female crabs was made at the mouth of the Bay near VIMS. These females were collected from fisherman and brought back to the lab. Unfortunately, none of the females survived for more than two weeks. However, it was most probably the collection process that killed the crabs as the additional crabs left in a high salinity flow-through system at VIMS did not survive a week. I have since collected mature females from the Upper Chesapeake and am in the process of moving them into the recirculating tank at a low salinity and slowly increasing the salinity to allow them to produce eggs.
Also, during the summer, I successfully mated six pairs of crabs. Peeler crabs, females just about to undergo their terminal molt and thus the one time in their life that they can mate, were collected from a fisherman and mated with crabs from the Upper Chesapeake. I was able to observe courtship rituals and I am currently holding onto the females and will be placing them in the recirculating tank to brood. I have had a significantly greater success rate of crab survival in this manner. It is possible that brooding female crabs are more susceptible to stress than peeler females. I will be continuing mating experiments and brooding studies throughout the rest of my education and am remaining at SERC to pursue graduate studies.
Funding provided by the Smithsonian Women's Committee