Smithsonian highlights how to protect young blue crabs — from cannibalism

Marylanders’ voracious appetite for blue crab smothered in Old Bay may be no match for the crabs’ hunger for each other.

Blue crabs are famously cannibalistic. The No. 1 killer of juvenile blue crabs in the waters where they congregate? Other, larger blue crabs.

Marine biologist Tuck Hines, emeritus director of the Smithsonian Environmental Research Center (SERC) in Edgewater, led a 37-year field study of juvenile blue crabs in the Rhode River, a southern Anne Arundel County tributary of the Chesapeake Bay.

“It wasn’t all-consuming, but it takes persistence and coordination to make sure it gets done every June, July and August for 37 years,” Hines said.

Over that period, Hines said, he and his team did not record any fish predation on juvenile crabs in the shallow, mid-salinity waters of the Rhode River.

Fish prey on blue crabs in the high-salinity waters of the lower Chesapeake and the low-salinity waters of the upper Chesapeake. But in the bay’s “extensive” mid-salinity region, Hines said, fish predation is far less common. Crab cannibalism, however, is not.

Juvenile blue crabs take “refuge in shallow water,” Hines said.

One way to help the juvenile population reach adulthood is to protect and preserve shallow-water, mid-salinity sanctuaries where young crabs grow without fear of fish, Hines explained.

Adult blue crabs appear less likely to venture into the shallow waters where juveniles grow, Hines said, making those habitats crucial to the species’ life cycle in the Chesapeake Bay.

“These near-shore shallows, the knee-deep water, is an important depth refuge from cannibalism predation,” Hines said.

The decades-long study also shows that most crab cannibalism is focused on smaller crustaceans. As they grow, crabs become far less likely to get ripped apart by other crabs.

That understanding will help scientists as they work on a new Chesapeake Bay stock-assessment model for blue crabs, said Matt Ogburn, a SERC ecologist who works on that team.

“Knowing that juveniles become less susceptible to cannibalism as they grow is important for getting the new model right,” Ogburn said in a statement.

The researchers closely tracked how the crabs were preyed upon — or survived — by attaching juveniles to metal stakes with one-meter tethers, so they could move around.

The tethers allowed the crabs to freely burrow into sediment, where fish were unable to see them, but adult crabs could still feel them out.

Over the course of the study, about 74% of all tethered juveniles survived and were released back into the bay, SERC said in a statement. There were marks of cannibalism on about 42% of the crabs, noted by injuries on surviving crabs or the remains of killed crabs.

The study underscores the importance of the shallowest waters. In water 1.3 to 2.5 feet deep, juvenile crabs had a 60% to 80% chance of being eaten by larger crabs.

In waters of about half a foot, they had only about a 30% chance of being preyed upon. But those zones are at risk.

“That shallow water is being squeezed by the construction of hardened shorelines to protect land from rising sea level and storm surges,” Hines said.

Constructing living shorelines with native plant species, Hines said, is a way to preserve those shallow sanctuaries while also protecting against erosion and flooding.

That way, the Chesapeake blue crab population can be protected — for the health of the bay ecosystem, and for the appetite of all who enjoy them steamed or in a crab cake, including Hines.

“We have a rule that says you have to eat your research animal, so pick it wisely,” he said.