Female blue crabs probably don't have to worry about not having enough males to go around.
After reviewing available data, scientists believe the ratio of males to females in the Bay is adequate to sustain a healthy blue crab stock, at least for now.
The issue stems from efforts that fishery managers took five years ago to bolster the blue crab population by slashing the harvest of female crabs by a third, thereby allowing more to survive and reproduce.
But that raised questions among scientists as to whether shifting harvest pressure to males could result in a sperm shortage that could limit female crabs' ability to produce young.
A female crab mates only once, and the sperm she gets at that time has to fertilize the multiple broods of eggs she may produce the rest of her life. If the female does not receive enough sperm because too few males are trying to mate with too many females — exhausting their supply of sperm — she may not be able to produce as many broods as would otherwise be possible.
A report from a workshop conducted last fall by the Chesapeake Bay Stock Assessment Committee concluded that available evidence does not indicate sperm limitation is affecting the blue crab stock.
At the same time, it said that male blue crab harvests and populations should be carefully monitored. If male harvest rates or the ratio of males to females ever exceeds levels observed since 1990 — the year the Baywide Winter Dredge Survey began — fishery managers should consider whether actions to lower male crab harvests are warranted.
"At the end of the day, the best information we could give to the managers was to make sure [the male population was] not getting out of the historical bounds," said Lynn Fegley, assistant fisheries director of the Maryland Department of Natural Resources, and a CBSAC member. "With the data we have, we can't see any sort of relationship between the male-to-female ratios that we've measured and subsequent reproduction."
At the same time, the report noted that research shows that a significant proportion of females in the lower Bay spawning stock have decreased levels of sperm. But for a female to run out of sperm she would typically have to produce several broods — which means she would have to produce broods in multiple years.
Although regulations have resulted in more females surviving to spawn, the CBSAC report said, it is not clear that the number that live to spawn into a second year has risen. Crabs don't spawn until they are about a year old, so a female surviving into a second spawning year would need to survive for more than two years.
One of the ultimate goals of protecting female blue crabs is to have more live longer and produce multiple broods over several years. That could help maintain larger, and more stable, crab population — if the number of females produced one year is low, it would be bolstered by older crabs from earlier years.
"As we are successful in managing fishing pressure and survivorship of mature females, the hope is that they will carry over into a second spawning season," said Anson "Tuck" Hines, director of the Smithsonian Environmental Research Center and a longtime Chesapeake blue crab researcher. "We need to make sure that they have enough sperm for the second season as well as the first season."
Work by Hines has shown that in some tributaries, the sex ratio can be more skewed toward females than is the case Baywide. In those situations, males may mate with multiple female crabs, but they may not always be capable of providing females with enough sperm to produce multiple broods.
Hines, one of the scientists who participated in the workshop, said he agreed with the overall recommendation that male blue crab harvests be kept within historic bounds while research on the issue continues.
During the history of the winter dredge survey, which has become the primary tool for assessing the Bay blue crab population, the Baywide ratio of male to female blue crabs has never dropped below 0.8 to 1. The male blue crab exploitation rate — the portion of adult males caught in the fishery — has never exceeded 65 percent.
Joe Grist, deputy chief of fisheries with the Virginia Marine Resources Commission and the CBSAC chair, said that exceeding those levels would send a signal to managers that they may want to consider additional management action regarding male crabs.
"We don't want to manage females in a vacuum," Grist said. "We want to make sure we are monitoring the males closely as well."
CBSAC consists of fisheries scientists from universities, state agencies and the National Oceanic and Atmospheric Administration.