Fishery managers have helped to increase blue crab numbers through regulations that protect females, but some scientists warn that efforts to fully protect the population must heed a fundamental biological principle: It takes two to tango.
Their concern is that crab regulations in recent years may have increased harvest pressure on males, which could mean a sperm shortage for females. A female crab mates only once, and the sperm she gets then has to fertilize all of the broods of eggs she may produce the rest of her life.
"The question is whether those females actually receive enough sperm to go ahead and fertilize the full potential of eggs that they could produce," said Anson "Tuck" Hines, director of the Smithsonian Environmental Research Center and a longtime Chesapeake blue crab researcher.
The concern stems in part from actions Maryland and Virginia took in 2008 when, after a string of years when crab populations hovered near historic lows, they issued regulations aimed at sharply reducing the female crab harvest. The intent was to give female crabs a greater chance to survive and reproduce.
A blue crab stock assessment completed last year credited those actions with contributing to a recent rebound for blue crabs in the Bay. But the assessment also noted that harvest changes were altering the ratio of males to females which, it cautioned, could result in fewer than expected crabs in the future if that resulted in "sperm limitation." The assessment said it lacked information about the point at which an altered sex ratio would affect the blue crab numbers, but said it should be a research priority.
Hines and colleagues have been funded by the National Oceanic and Atmospheric Administration's Chesapeake Bay Office to explore whether a lack of sperm is limiting the female crabs' reproduction potential.
When crabs mate, the female should receive about 900 million sperm from the male, Hines said. Each brood she produces can contain 3 million eggs and it takes, on average, about 80 sperm to successfully fertilize an individual egg, Hines said. That means it takes about 240 million sperm to successfully fertilize a single brood.
In the laboratory, females can produce three or more broods a year, and continue to produce broods for two or three years.
Unlike females which mate just once, males continue to try to mate with females. But it take males about eight to 10 days to replenish a full supply of sperm after mating, and if they mate before then, the female can receive a smaller amount of sperm.
Hines said it is not just the overall male-to-female ratio that is important, but the "operational sex ratio" that exists when and where mating is taking place. Mating typically takes place in tributaries, which are often subject to localized fishing pressure in late summer, when much of the mating also occurs. As that fishing pressure is targeting males, it can disproportionately skew sex ratios in local populations even if the overall Bay population is healthy.
"Sperm limitation is possible if females live longer than the first season and they were, in fact, getting lower amounts of sperm in these tributaries where there was intense fishing pressure," Hines said.
To keep that from happening, Hines said the "operational sex ratio" in mating areas — the number of females and males ready to mate — should be about eight males for each mating female. But in many tributaries that Hines and colleagues have sampled, the ratio is only about three to one. "That ratio is being biased by the intensity of the fishery in those habitats," Hines said.
Some of the work by Hines and his colleagues indicates that female crabs in heavily fished areas are getting only about half the normal amount of sperm. Crabs in some parts of the Lower Bay only had a quarter of the normal amount. Their research is trying to determine whether that situation is common in other heavily fished tributaries.
If females aren't getting enough sperm, the management goal of promoting female survival, partly in the hope that they will produce multiple broods over time, could be short-circuited. It's not an issue that threatens the health of the population so much as it could hinder efforts to rebuild and maintain the large populations seen in the past.
"The population is going to persist, but if you are trying to enhance productivity in the system, you want to maximize reproductive output," Hines said. "You are working against yourself at some level."
The Chesapeake Bay Stock Assessment Committee, a panel of scientists and fishery managers that offers advice on blue crab management, is expected to make a recommendation about male blue crab management this fall, although it's uncertain what that will be.
Lynn Fegley, assistant fisheries director of the Maryland Department of Natural Resources and chair of the stock assessment committee, said it is important to make sure that the ratio of males to females stays within the range that has been observed in the past to prevent future problems, and CBSAC would likely offer guidance on that issue.
But Fegley was skeptical about making recommendations aimed at achieving specific sex ratios within tributaries during the spawning season, which would require complex — and likely difficult to enforce — regulations. She said research would need to clearly show that sperm limitation is a problem for blue crabs, noting that crab recruitment, the amount of young crabs entering the system, has been generally increasing since regulations were implemented.
"I think it's a fascinating issue," she said, "and Tuck Hines' research is going to help us learn a lot about the dynamics of this on a local level. But I'm not sure exactly how we would incorporate this into management, especially when we have no evidence of population-level impacts."