Maryland has Crabs

Just not as many as it used to. Bay Journal,  Chesapeake blue crab population drops 50%. The question is: 

The Chesapeake Bay’s blue crab population has declined by about 50% since 2010, a new study estimates, and while invasive blue catfish are responsible for some of that worrisome decline, the main cause or causes remain a mystery.

That’s the bottom line of a comprehensive stock assessment of the Bay’s crab population, which was launched in 2023 to figure out why numbers of the crustaceans have swooned in recent years, threatening the Chesapeake’s most valuable fishery.

Scientists from five different research institutions and fishery managers for Maryland, Virginia and the Potomac River pored over decades of harvest figures, survey data and dozens of reports and studies. They looked for changes in water quality or climate, among other things, and examined whether an influx of crab-eating fish could be responsible for depleting the population.

The basic problem, they agreed, is poor “recruitment,” meaning there aren’t enough young crabs surviving to produce the next generation. Their 281-page draft report concludes, though, that despite identifying some potential causes, they lacked the data to fully explain it.

“We suspect multiple causes, because the Chesapeake Bay is complex and the blue crab life cycle is complicated,” said Mike Wilberg, a fisheries scientist with the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science. He led the computer-driven assessment.

“There’s no smoking gun,” agreed Rom Lipcius a fisheries scientist with the Virginia Institute of Marine Science. 

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 This assessment finds little evidence that crabs are declining because of poor water quality, specifically the warm-weather condition known as hypoxia. Hypoxia occurs when nutrient pollution leads to oxygen-starved “dead zones” where fish struggle to survive. The scientists said there were no reports of mass crab die-offs in such conditions, and their analysis of water-quality data did not suggest it was a major factor.

They did find that blue catfish are contributing to the decline, as watermen have long contended — but the assessment ruled out the invasive fish as the leading culprit.

Introduced in a couple of Virginia rivers in the 1970s, blue catfish have spread to rivers all around the Bay region. As their population increased, the voracious finfish have consumed millions of juvenile crabs, according to the assessment. In 2023, the study estimated, blue catfish consumed roughly 8% of the little crabs that year.

“They’re not the primary cause,” Ogburn said. “There’s something five times bigger than that, and we’re not sure what it is.”