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Research Lends New Perspective to Blue Crab Population Decline

Read the research findings of SERC Assistant Director Anson Hines and his colleagues in the article "Evidence for sperm limitation in female blue crabs (Callinectes sapidus)" from the Bulletin of Marine Science 72:287-310.

Protecting female blue crabs while fishing for big males in Chesapeake Bay may be doing more harm than good for the declining crab population, according to a recent article published in the Bulletin of Marine Science. A new report by Anson Hines of the Smithsonian Environmental Research Center and his colleagues, presents evidence that the limited availability of sperm from mature males limits female reproduction in Chesapeake Bay. Although nearly all female crabs find mates, the quantity of sperm that males give to females is often greatly reduced, leading to potential limitation of fertility in Chesapeake blue crabs.

In the last dozen years, blue crab population in the Chesapeake Bay has declined by 80%, and there have been concerns that a fishery worth tens of millions is greatly over-fished. Many management efforts to help the population recover have been focused on limiting female crab harvest and increasing size limitations for male crab catch.

The recently released study by Hines, suggests that the fishing pressure on the largest males may actually limit the mating success of females.

"Although management of crab fisheries often focuses on protection of females, our analysis of blue crabs indicates that removal of males may have significant impacts on reproductive success," says Hines, "The Chesapeake blue crab stock may suffer from excessive removal of large males, which can affect population reproductive success in complex and profound ways."

Blue crabs in the Chesapeake Bay have only one or two peak mating periods per year-one in late summer throughout the bay, and a spring "peeler run'' in the lower bay. The mating season coincides with the most intense period of fishing pressure, when primarily large males are removed.

According the work of Hines and his colleagues, fishing pressure on large males leaves smaller, younger, less virile males in the mating pool. Not only do these males produce less sperm than their larger counterparts, but with the competition reduced, the remaining males mate much more frequently than they ordinarily would, leaving them little time between encounters to recharge their sperm count.

According to the report, "reproduction becomes sperm limited when the number or quality of sperm received by females is not sufficient to fertilize the total potential egg production." Because females typically mate only once in their lifetime, the quality and quantity of sperm during mating is very important for reproductive success. In Chesapeake Bay, near the northern limit of blue crab range, females that mate in the summer must store sperm for 7-11 months over the winter before they fertilize broods the following summer.

Whatever sperm they receive during a single mating must last a female crab through her reproductive lifetime. For crabs in the Chesapeake Bay, that could be as many as six or seven broods or as few as one o two "sponges" - each about three million eggs.

The study examined six types of evidence over many years and compared crabs from the upper and lower Bay and Indian River Lagoon in Florida where fishing pressure is not as heavy, the reproductive season is longer and females may produce as many as 18 broods in their lifetime.

Among the factors the researchers investigated were sperm receptacle weight, DNA amount and counts stored in mated females, estimates of sperm to egg ratios for the number of eggs a female produces per brood, per season and per lifetime, and the effect of male mating history on brood production and hatching success of females.

In addition to demonstrating the potential for sperm limitation to be a significant factor in the Chesapeake Bay, their data also indicated that blue crab populations at lower latitudes such as Florida could be even more severely impacted by sperm limitation should fishing pressure rise. Because of the longer brooding season, female crabs produce many more broods in lower latitudes than previously thought and have the potential to run out of sperm before the end of their reproductive lifetime.

Co-authors on the report were Paul R. Jivof and Paul J. Bushman from Smithsonian Environmental Research Center, Jacques van Montfrans from Virginia Institute of Marine Science, Sherry A. Reed from Smithsonian Marine Station, Donna L. Wolcott and Thomas G. Wolcott from North Carolina State University

To learn more about the work of Dr. Anson Hines visit the Fish and Invertebrate Lab home page

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