A strong incoming tide on a morning earlier this week pushed a surge of brackish water up a marsh-rimmed slough connected to Galveston Bay, the tidal flow bearing on its bosom some of the fortunes and future of one of the bay systems’ keystone marine species.
Dozens of young blue crabs, some barely the size of a fingernail and few larger than a half-dollar, rode the murky current, treading water as the flow carried them deeper into the estuary that has served as nursery, larder and refuge since wind and tide deposited them there when they were barely more than microscopic larvae.
Where they were born — where these young blue crabs hatched from the one million to six million eggs their mother carried in a sponge-like mass attached to the underside of her shell — is a mystery. So is the location where their mother grew to adulthood.
They may have hatched relatively nearby, in the salty water of the lower reaches of the bay system they now inhabit. Or they might have come into being in the open Gulf of Mexico, scores of miles away.
Their mother might have been a local girl. Or she might have grown up in a bay system scores of miles away — maybe Matagorda Bay or Sabine Lake or even Lake Calcasieu over in Louisiana.
Maybe, the young crabs are a mix of both.
Answers to those questions — answers that could significantly enhance fisheries scientists’ understanding of the population dynamics of this key coastal marine species and be used to guide science-based management decisions benefiting the crabs and all connected to them — are the aim of a research project that began two years ago with the goal of tracking movement of adult female blue crabs in bays along the Gulf Coast to learning where those crabs spawn.
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Searching for answers
That information, combined with the understanding of currents, tides and other environmental factors, can give scientist insight into where the young produced by those crabs end up, as well as illuminating previously unknown connections between the crab populations of different bay systems.
“There’s a lot we don’t know about movement of spawning female crabs,” said Dr. Zack Darnell, assistant professor of coastal sciences at the University of Southern Mississippi and coordinator of what is the largest, most wide-ranging study of the movement of blue crabs on the Gulf Coast. “We know a lot of them move out of their home estuaries and into the Gulf of Mexico to spawn. Where do they go? Which direction to they travel and how far?
“How connected are blue crab populations? Does something that affects blue crabs in one system have an impact on crabs in another system?”
To help answer those questions, Darnell coordinates a project that began in 2016 and involves cooperation between commercial crabbers, academic staff and coastal fisheries in the five Gulf states to capture adult female blue crabs and release them after fitting them with a 1-inch-by-2-inch, bright orange, plastic tag, attached to the back of the crab via stainless steel wire. The tag includes a unique number tied to that individual crab and a request that anyone recapturing the crab report it through the phone number or website listed on the tag.
Over the two years since the program began, almost 17,000 adult female blue crabs have been tagged between the upper Gulf Coast region of Florida and the tip of Texas. About 2,000 of those crabs were tagged in Texas waters, almost all by Texas Parks and Wildlife Department coastal fisheries staffers.
The program focuses only on adult female crabs for a couple of reasons. Those adult females appear to be the long-range travelers of the species, driven to move in their search for environmental conditions that allow their eggs to develop and their progeny to have a chance at survival. And adult females don’t molt, a process that would result in a tagged crab shedding not only its hard shell but the tag attached to it.
Blue crabs are an estuary-dependent species requiring the wide range of salinity levels usually found in bays for different stages in their life. They do best in waters of estuaries with modest or low salinity levels, where forage is abundant and the lower salinity levels aids growth rates. Blue crabs molt as many as two dozen or more times over their lives, shedding their hard carapace, swelling absorbing water into their now-soft bodies while a new carapace grows around that larger body. The lower the salinity of water, the more the crabs can absorb. So crabs grow fastest in areas where salinities are lower — usually the upper reaches of bays and the brackish water of marshes and other estuarine habitat.
Male blue crabs continue to molt and grow larger over their entire lives. Females do not. Once a female blue crab reaches sexual maturity, she molts a final time. During this final molt, she mates with a male, an event that provides all the ingredients she needs to fertilize the tens of millions of eggs she can produce over the rest of her life.
But those eggs will develop and hatch and the larvae have a chance to survive only in water holding a salinity level of at least 20 parts per thousand. In many bay systems along the Gulf Coast, especially those fed by large rivers or other watersheds that pump lots of freshwater into the systems, water with that salinity level is hard to find.
“What you see in a lot of areas is a pattern where adult females migrate, moving out of low salinity areas of bays and often into the Gulf of Mexico to find salinities high enough to successfully spawn,” Darnell said. Timing of that movement depends on several factors and varies along the Gulf Coast. In Texas, the peak spawning movement occurs in late spring and early summer but can continue into August.
Those crabs, it turns out, can move a long way despite their seeming limited mobility. And where they end up after the 10 days or so it takes their eggs to develop and hatch determines where their larvae end up.
Larvae hatched in the open Gulf are carried by currents, tides and wind, a lucky few of them ending up transported into bay estuarine systems, where they grow to adulthood. So it is possible — probable, even — that the young crabs produced by a female that moves out of her home bay and into the Gulf will end up populating an entirely different bay system.
The crab tagging program is aimed at trying to track some of these movements of spawning-minded females. And while the study is ongoing and results preliminary and anecdotal, some of the early findings are enlightening.
Turns out, blue crabs travel amazing distances, but some are real homebodies.
As of this past month, the crab tagging program has seen about 3,000 tagged blue crabs recaptured, Darnell said. About 500 people have reported recapturing tagged crabs. Most of those recaptures were made by commercial crabbers. But about 7 percent came from recreational crabbers, he said. And at least one was reported by a scuba diver who encountered a tagged crab.
Where and in which direction did the crabs go once they leave their home bay systems?
“They’ve gone all over the place,” Darnell said. “Crabs aren’t sharks or tunas, so you don’t expect them to move hundreds of miles. But they can move a lot farther than people might think.”
Most of the crabs were recaptured less than 50 miles from where they were tagged, Darnell said. One crab was reported recaptured 180 miles from where it was tagged, he said.
Direction of movement was also all over the map, he said. Some spawning female blue crabs left bays and moved considerable distances up the coastline, while others moved down the coast. Sometimes movement tended to track the prevalent longshore. Other times it didn’t.
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All of this information is valuable, Darnell said.
“It’s important to help us understand how connected these crab populations are,” he said. “Knowing their movements, when and where they are spawning and the areas they prefer to spawn is crucial to making those connections.”
If the crabs that grow up in Galveston Bay are from eggs produced by adult crabs from Louisiana, the effects of impacts on crabs in that Louisiana bay system will affect Galveston Bay.
The goal of the tagging research program is, ultimately, to give managers the best science to use to maintain a healthy population of blue crabs, the Gulf Coast’s most abundant large crab, a crucial component of the marine ecosystem and economically and culturally vital part of the region’s recreational and commercial fishery.
Darnell said the research program is planned to continue tagging crabs through at least this autumn and hopes to fit tags to a total of 23,000 or so adult female blue crabs.
Anyone recapturing a tagged crab is asked to pass along as much information as possible about date and location of that capture, with GPS coordinates being most helpful.
More information on the Gulf-wide blue crab tagging program is available
on the project’s website at bluecrab.usm.edu.