Blue Crab Life Stages Complicate Predicting Population Swings
July 22, 2022 by Dennis Forney Talbot Spy
Those feisty blue crabs that snap their claws and defiantly spit streams of water when we reach for them with tongs turn out to be a whole lot more instinctively smart than many of us realize. That’s why managing them as a highly sought-after coastal resource can be tricky, complicated and – most of all – filled with uncertainty.A dredge survey this year of thousands of sites throughout the Chesapeake where blue crabs hibernate through the winter found the lowest number of crabs since the annual survey began 30 years ago. That has led to lower than usual harvests through the first half of the 2022 season.
Resource managers in Maryland and Virginia responded by shortening the 2022 season by a couple of weeks, placing bushel limits on the commercial harvest of male crabs where there had been no limits before, reducing the bushel limits on commercial harvesting of female crabs for the months of July through October, and reducing the bushel limits for recreational crabbers.But they and the scientists who guide them in their work are still scratching their heads. Why did the Chesapeake’s overall crab population decline significantly this year? Asked if there were any predominant reasons being bandied about, Gregg Bortz in the communications office for Maryland’s Department of Natural Resources said this: “No consensus as of yet. There may be more discussion on that among the scientists and managers later in the year. The immediate focus has been on management adjustments that can be made this season.”
Watermen are now reporting increasing numbers of smaller and larger crabs in their pots and on their trotlines as July races toward August. That bodes well for stronger late-season crabbing, after a couple more sheds and the consequent increase in size.But, is there any way of predicting how many baby crabs will be born during 2022 and eventually become part of next winter’s dredge survey? Female crabs in the Chesapeake, in total, produce hundreds of millions of eggs each year. How many of those eggs will hatch and survive over the course of the 18 months or so that it takes for them to go through several life stages to reach juvenile crab size and eventual market size?
That’s where the tricky, complicated and uncertain parts come in.Dr. Charles Epifanio of University of Delaware’s School of Marine Science and Policy in Lewes, along with several students and academic colleagues, has been studying blue crabs for several decades. In 2019, the Journal of Shellfish Research published a review paper by Epifanio that provided an overview of more than 100 years of science related to the early life history of the blue crab.
The review cited hundreds of papers written on the subject.Research leading to the published studies – scientists building a body of knowledge through questioning, collaboration and sharing of results – revealed an amazing picture. Epifanio’s review shows how crabs travel hundreds of miles back and forth between ocean and bay. They have to do that to find the right conditions for the successful completion of their various life stages from fertilized egg to the mature crabs so prized for their delicate and flavorful meat.
That journey takes them from the fresher, middle and upper reaches of estuaries like the Chesapeake and Delaware bays, all the way out to the much higher salinity of surface waters along the Atlantic’s inner continental shelf. It’s in those higher salinity waters that the newly hatched larvae begin to grow and shed and grow and shed before riding tides and wind currents back into the estuaries.
Eating, drinking, swimming and growing, and responding to physical and chemical cues in all those activities, evolving crabs know where to position themselves in the water column to get where they want to go. For instance, constantly swimming upwards in the ocean water column keeps tiny crab larvae near the surface where wind currents carry them back into their ultimate estuaries where they can mature into adult crabs. They’re attracted to the surface waters by sensing the fresher, less salty and lighter water that floats to the surface as well as other cues like light and turbulence..
At the other end of their journey, mature female crabs – using more of the same cues that brought them this far – catch flood tides to make their way up to mating grounds in the middle and upper reaches of the bays. There, they gather all the sperm they can from a male crab during a once-in-a-lifetime, ten-hour mating session.
After carefully packaging the sperm to be used for fertilizing up to three or four broods of millions of eggs, and a sensitive-but-firm hit-the-road Jack discussion with their mate, the females catch ebbing tides to carry them back to the saltier water of the lower bays. In their tidal journeys, crabs sink to the bottom when the flow isn’t in the right direction. Currents are weaker there.It’s in those saltier waters that the females use the stored sperm to fertilize their eggs before the next phase of their seaward migration takes them and their hatching eggs to the inner shelf waters where the larvae begin their development to start the whole process over again.
In the conclusion to his review, Epifanio wrote: “Perhaps the most important contribution of the past few decades has been the discovery that larval development in blue crabs occurs mainly in surface waters of the inner continental shelf – not in the estuary – and that larval transport on the shelf is is controlled by interactions between buoyant discharge from the estuary and wind patterns over the coastal ocean.”
Buoyant discharge relates to the fact that the freshwater of rainfall flowing out of estuaries like the Chesapeake is lighter than the saltwater of the ocean and tends to float near the surface.Studying the wind currents and rainfall patterns in any given year that can influence the movement and numbers of developing larvae back into the estuary from the inner continental shelf might give a sense of how many crabs may eventually be on their way.
“The model is pretty good at predicting the magnitude of tiny, tiny juvenile crabs that might make it into the estuary,” said Epifanio in an interview this week. “Funding agencies are willing to give money to people like me because of the possibility of predicting the catch. But to get from the variations in the numbers of the tiny, tiny crabs to the variations in the catch of adults is not straightforward. We’ve tried that kind of predicting. It didn’t work that well. These studies explain a lot of how larvae make their way into the juvenile habitat where they can grow. And they show us how chemical cues in the water guide them into the sea grass and oyster reef bottoms where they find protection.”But a lot can happen to them as they move up the bays, said Epifanio. Predators and availability of proper habitat from year to year make it hard to explain the great variations in the numbers that actually make it to the point of being counted in the winter dredge surveys.
For now at least, despite the great advances since the late 1800s in science’s insight into the mysteries of the complicated blue crab, Epifanio strikes a pragmatic tone:
“I wouldn’t bet heavy money on predicting catch based on wind and rainfall.”But he agreed the gap between the magnitude of developing larvae that make their way into the bays’ nursery areas, and the adult crab population, offers rich ground for future scientific inquiry and potentially better models for predicting potential catch.
Dennis Forney has been a publisher, journalist and columnist on the Delmarva Peninsula since 1972. He writes from his home on Grace Creek in Bozman.
