Adult female blue crabs far outnumber males in the Chesapeake Bay. The large imbalance is a recent development caused by humans, according to Dr. Tuck Hines, Director of the Smithsonian Environmetnal Research Center.
Ever since Maryland and Virginia imposed restrictions on catching females three years ago to stop overharvesting, the number of blue crabs in the Bay has grown by more than 60 percent. But a side-effect of this good news is that adult females now outnumber males by a three to one ratio, Hines said.
Hines theorizes that this imbalance could potentially pinch crab reproduction, although a leading expert at the Maryland Department of Natural Resources disputes this notion.
“If it’s three to one females to males –- then there may not be enough males to provide enough sperm to fertilize all of the eggs that the females can produce,” Hines said.
Hines, an authority on blue crabs, is studying the issue, and whether the Bay states should do more to restrict the catch of males, especially in shallow tributaries, where they pair up with females in the summer.
This “save the males” argument is greeted skeptically by Lynn Fegley, another leading crab expert and deputy director of the fisheries service at the Maryland Department of Natural Resources.
“There is no evidence now that this ratio we are seeing is a problem,” Fegley said.
It is true, she said, that a dredge survey last winter found adult female blue crabs outnumbering males. Data from a December 2010 to March 2011 sampling estimated a 190 million adult females in the Bay compared to 60 million males, according to the 2011 Chesapeake Bay Blue Crab Advisory Report.
But Fegley noted that each male can mate with numerous females, which produce millions of eggs. She said there has been a lopsided ratio of females to males in the past in the Bay. In the winter of 2009, for example, there were many more females than males -- and the next spring, there was a near record numbers of juvenile crabs produced, she said.
“There are crustacean fisheries around the world that allow no female harvest whatsoever, that are exclusively male-only fisheries, and in fact our regulations were modeled on these fisheries,” Fegley said. “It would be premature to say that we have mismanaged in favor of females.”
Hines, however, believes the shortage of males is potentially worrisome, because of the complex and time-consuming nature of the crab mating ritual.
Each female crab mates only once in her life -- in a narrow window of time in the summer, just before she molts, Hines said. If a male is not in the right place at the right time -– perhaps because he’s been caught in the intensive summer crab harvesting season –- the female crab can't find a mate.
When crabs do mate, they remain locked together in an embrace for up to four days, as the male protects the female from predators and other males until her new shell hardens, Hines said. After this, the male needs 10 days to recharge before he mates again.
Because of these timing issues, Hines said, it would be better for crab reproduction to have more males than females, which is the opposite of the way it is now.
“We need more males in the system to accommodate the females, and right now it appears we are out of balance,” Hines said. “And it also appears that the females in the lower Chesapeake Bay…may not be receiving enough sperm to fertilize the maximum number of eggs.”
The increasingly pink hue of the blue crab population is, in one way, a sign of success –- the result of regulations that brought an iconic Bay species back from record low populations. But too much success can breed stress.
And so now it is Jimmy’s turn to be studied, to see how he’s holding up with all these females in his life.
By Tom Pelton
Chesapeake Bay Foundation
(Photo at top from Maryland Department of Natural Resources)