The Chesapeake Bay's most popular crustacean has a dark streak. Cannibalism is the No. 1 killer of juvenile blue crabs in mid-salinity waters where they are known to congregate, according to a new study from the Smithsonian Environmental Research Center (SERC) published in the journal Proceedings of the National Academy of Science. But shallow waters can offer a vital refuge.
Blue crabs lead a life on the run. After spending roughly two months as larvae in the ocean, they are swept back into the lower bay to morph into juvenile crabs. There, the juveniles rely on seagrass to provide partial refuge from predatory fish like striped bass. But after they grow to about 1 inch, many of them migrate up the bay to mid-salinities, where fish and seagrasses are scarcer. There they face another enemy: larger blue crabs.
"Blue crabs are notoriously cannibalistic," said Tuck Hines, marine biologist and director emeritus of SERC. But although cannibalism is common throughout aquatic ecosystems, he added, long-term studies attempting to quantify it are rare. And the data could make a difference for managers trying to keep the blue crab fishery sustainable.
Hines led the 37-year field study. His team sought to find out what is eating young blue crabs and what makes them less vulnerable. The study began in 1989, when Hines headed SERC's Fish and Invertebrate Ecology Lab. It took place in the Rhode River, a mid-salinity tributary of Chesapeake Bay located at SERC's campus in Edgewater, Maryland.
The team conducted the study by attaching juvenile crabs to small metallic spikes in the water, with a 1-meter tether that allowed the crabs to move around. Tethering allows juvenile crabs to spend much of their time buried in the sediment, avoiding predatory fish that hunt by sight. But buried juveniles are still vulnerable to large crabs that use chemical and tactile cues to detect buried prey. After 24 hours, the researchers returned to the sites to see how the crabs had fared. Most crabs in the study—roughly 74%—survived, and over half remained unscathed and were released back into the bay.
The biologists detected hallmarks of cannibalism on 42% of the crabs—either alive with injuries or killed with some remains left behind. They found no evidence of fish predators in the study, suggesting that mid-salinity rivers do offer a refuge from some predators.
"We were amazed to find that over our 37-year study, cannibalism accounted for all of the predation, and we found no fish predation on tethered crabs," Hines said.
"We've recorded a few of the tethering experiments with a high-resolution sonar," said Matt Ogburn, a research ecologist at SERC and coauthor on the study. "In the sonar videos, most fish didn't show any interest in the tethered crabs, and only adult crabs attacked them."
The smallest crabs were the most vulnerable, and more than twice as likely to get eaten compared to medium or large juveniles. But in shallow waters they were far safer. In the experiment, a small juvenile crab was 60–80% likely to get eaten in deeper waters (1.3 to 2.5 feet deep). But in the shallowest zones (15 centimeters or a half-foot), they were only about 30% likely to be cannibalized. This rule of safety in the shallows held true for larger juveniles as well.
Scientists are working on a new stock-assessment model for blue crabs in the Chesapeake that is expected to come out later this year. SERC ecologists Matt Ogburn and Rob Aguilar are both part of the team. The long-term data from this study can help make the new model more accurate—especially the discovery that most cannibalism is focused on smaller juvenile crabs.
"Knowing that juveniles become less susceptible to cannibalism as they grow is important for getting the new model right," said Ogburn.
Meanwhile, shallow-water sanctuaries are at risk. Seawalls, riprap (a layer of large stones to protect soil from erosion) and other shoreline-hardening projects are shrinking the nearshore territory where young blue crabs can escape. Invasions by non-native species, especially blue catfish, also may impact blue crabs and further diminish these shallow refuge areas, as highlighted in the study.
"The nearshore shallows of the mid-salinity zone provide juvenile crabs with a crucial refuge habitat from cannibalism by large crabs," Hines said. The authors point out that with cannibalism so intense and predators in every region of the bay, protecting these nearshore habitats—and restoring them where they have been lost—is critical for stabilizing blue crab numbers and protecting the blue crab fishery as a whole.
