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Does coastal erosion help or harm the Gulf's blue crabs?

Oct 20, 2017


Blue crab populations could see an initial boost as Louisiana wetlands break up and erode, according to an ongoing study by researchers with the Water Institute of the Gulf and the University of Southern Mississippi. But that boost could be short-lived.

As wetlands initially fragment, the marsh breaks up into smaller patches. Those small patches result in more "edge habitat," where crabs can more easily enter the marsh to find food and protect themselves from predators, said Zack Darnell, an assistant professor at the University of Southern Mississippi's Division of Coastal Sciences. But as the patches of marsh continue to break up, creating more open water and losing marsh area, the crabs are no longer able to reap those benefits, he said. "That boost is going to be at the early stages of marsh fragmentation, but eventually a tipping point will be reached where too much marsh is lost," he said. "Too much open water is created, and blue crab populations will suffer."

The lifecycle of blue crabs begins in the Gulf of Mexico. Before blue crabs mature into anything resembling a crab, they look like a cross between a mosquito and a shrimp. While the larvae are offshore, they shed their exoskeletons seven or eight times before they turn into something called a "megalopa," which looks like a tiny lobster, Darnell said.

It's in the megalopa phase that baby crabs migrate into bays and settle down in the edges of marsh, where there's an abundance of food and places to hide. One of the species they hide from is other juvenile crabs. That's because juvenile crabs are cannibalistic, said Lennah Shakeri, a master's student studying with Darnell. "They like to eat each other a lot," she said.

Shakeri is conducting research with the Water Institute of the Gulf to study the effects of coastal erosion on blue crab populations. The research team surveyed adult and juvenile crab populations in Terrebonne Bay, near Cocodrie, throughout the spring of 2016. They targeted differing levels of marsh fragmentation during their surveys to assess the suitability of the habitat for crabs.

Findings from the study could hold important implications for the Louisiana crab fishery, which was valued at more than $58 million in 2015. Crab populations have declined in many states, including Texas and Maryland. The Texas fishery for crabs peaked in the 1980s. Since that time, the amount of crabs caught in the Lone Star State have been cut in half. Louisiana's fishery has largely remained steady but there are signs the population could be declining.

Researchers finished the fieldwork in September 2016. Since thene, Shakeri's work has been conducted in the "craboratory," an aptly nicknamed research facility in Ocean Springs, Mississippi.

There, Shakeri peered through a microscope at a thumb-sized blue crab. It's hard to say how old the crab was. After the megalopa phase, crabs molt another seven or eight times to transform into their crab shape. While fish have an inner ear bone called an "otolith" with rings that can be read like a tree's, crabs shed all their hard bones as they grow, Darnell said. That makes it difficult for researchers to estimate their age. Most of what is known about how long it takes crabs to reach sexual maturity is from observations in captivity, he said.

Juvenile crabs were a key part of the study because they're what will become future marketable crabs, Shakeri said. Juvenile crabs tend to use the marsh edge habitat more than adult crabs. As the crabs grow, they have fewer predators, which allows them to move more freely about the bays.

Crabs reach sexual maturity between six months to a year. As female crabs reach sexual maturity, they shed their exoskeleton one last time. The 10-day period that the female crab spends regenerating her final exoskeleton is the only time in her life that she is able to mate, Darnell said. "That's when they're in the soft shell phase," he said. "After that, their shells are too hard and they won't mate again. They store all the sperm they need for the rest of their life and use that."

Louisiana's fishery have remained above 40 million pounds per year since 1997, with the exception of 2005, 2010 and 2013, according to a 2016 assessment authored by researchers with the Louisiana Department of Wildlife and Fisheries and Louisiana State University.

But the 2016 assessment found that crab population have fallen below the targeted range. As a result, commercial harvesting of female blue crabs will be banned for two months in 2018 and 2019 in attempts to keep the blue crab fishery sustainable. The ban begins March 1, 2018 and will last until the end of April 2018. Harvesting female crabs will be banned during the same time period in 2019.

March and April mark the time of year when females are beginning the process of moving into more saline waters to bear and spawn their eggs. Protecting females during this mating period creates a better chance for females to spawn before harvested, said Peyton Cagle, the Louisiana Department of Wildlife and Fisheries Crustacean Program Manager.

More data is needed to understand the relationship between blue crab population dynamics and environmental conditions, according to the 2016 assessment. "The relationship between wetlands losses and the continuation of fishery production within Louisiana has been discussed by numerous authors," according to the report. "Understanding this relationship as it applies to the Louisiana blue crab stock should be an ongoing priority."

The study conducted by the Water Institute of the Gulf and University of Southern Mississippi's Division of Coastal Sciences sets out to do just that. "If steps aren't taken to reduce coastal land loss, we're going to see reductions in populations of crabs as well as a number of other species including shrimp and finish," Darnell said. "In addition to the ecological impacts, this is going to hurt the economy of coastal Louisiana due to impacts on the seafood industry."