|Kristin Larson - Crab Lab
Colby College, Waterville, ME
The Reproductive Potential of Blue Crabs ( Callinectes sapidus )
in the Upper Chesapeake Bay
The blue crab population in the Chesapeake Bay has declined in recent years, and a major cause of this trend is over fishing. Watermen are legally allowed to keep crabs that are over 127mm across the carapace, and they prefer to catch larger males, as they are most desirable in the markets. This selective removal of large males not only reduces the population directly, but also indirectly because the large males are important for breeding. If the average size of males in the bay decreases, and the operational sex ratio (the ratio of males and females that are breeding) is skewed to females, then the small males that are left behind have to perform a large part of the breeding. If this is the case, then males might not have enough seminal resources to sufficiently inseminate all of his female mates. This is important because females mate only once in their life, immediately following their final molt to maturity. The only sperm that they receive comes from the male that they mate with at that time, and they store the sperm until brooding.
This summer, I looked at the reproductive potential of male and female blue crabs in the upper Chesapeake Bay. There is a lot of previous work that looks at the possibility of sperm limitation in the population, which occurs if females are not able to fertilize all of their eggs due to inadequate sperm supplies. Sperm levels have been shown to be declining as the population decreases, and I was interested in seeing if sperm levels continue to be lower in 2003 than in years when the population was larger. I collected individual mature male and female crabs in the Rhode River and Chesapeake Bay via trawl and crab pots, and I caught “doublers,” or paired crabs, at a commercial pound net. I conducted dissections of all the crabs I caught, and then performed sperm counts on the females and I weighed the vas deferens of all the males. I concluded that male blue crabs do not have depleted sperm resources in 2003, but that females continue to hold low levels of sperm (mean=6.89x10 8 ).As a 2003 graduate of Colby College, I plan to work in the environmental field or education before applying to graduate school in a couple years.
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