Each summer for nearly four decades, scientists at the Smithsonian Environmental Research Center have tethered some baby blue crabs along a Chesapeake Bay tributary.
Then they watched, hoping to learn more about which marine creatures prey on crabs. Turns out, their biggest predators were their own kind.
Adult crabs accounted for about 97% of juvenile crab deaths at the study site in Maryland’s Rhode River, just east of Washington, D.C.
Scientists have long known that blue crabs are cannibals. But the new findings shed light on the extent of cannibalism and help identify which areas of the bay serve as refuge for young crabs.
“It's understanding the complexities of what's regulating a population of one of the most iconic species of the bay, and one of the greatest fisheries,” said Tuck Hines, emeritus director and research scientist at the Smithsonian Environmental Research Center.
“Everybody appreciates blue crabs as important food but also they're really important as major predators in the system themselves, so they're helping to regulate the ecology and the food web of the bay.”
Hines has helped lead the research since it started in 1989. Blue crabs, which are migratory, are complicated to manage as a fishery and somewhat difficult to understand ecologically, he said.
Larvae ride currents offshore into the Atlantic Ocean, then migrate back into the bay and settle into southern seagrasses to grow. Once they reach about an inch in size, many baby crabs move again into the northern reaches of the bay, which are less salty.
“What this says is that these juvenile crabs are using the mid-salinity zone as a very important refuge habitat from fish predation in the lower bay,” Hines said.
The Smithsonian researchers ran an initial experiment by attaching a fishing leader with a small harness to the back of a crab and staking it out in the water to see how long the crab survives.
The tethering technique turned out to be successful, Hines said. Based on lab experiments, researchers could tell which critters were eating the crabs by the marks left on the tether.
More than half of the tethered crabs over the 37-year period survived and were released back into the bay. Of those that were eaten, adult crabs were always the culprit and the smallest crabs usually the victim.
“We expected that we would see a mix of fish and cannibalistic blue crabs as the sources of mortality for these crabs,” Hines said. “We were surprised that we found no instances of fish predation.”
That’s a stark difference from saltier parts of the bay, such as Hampton Roads, where blue crabs are often consumed by fish such as red drum and striped bass.
In the Rhode River study, baby crabs fared much better in shallower waters. That indicates those areas are a refuge from adult crabs, within the larger mid-salinity zone that protects them from other predators.
Hines said the information can help conservationists and fishery managers understand which spots to prioritize as critical habitat.
The bay ecosystem is changing quickly, such as from shifting climate conditions.
During the wintertime, blue crabs burrow into sediment and wait for temperatures to rise, which prolongs their life cycle compared to natives of the warmer Gulf Coast, for instance, Hines said.
Warmer waters with shorter winters could be changing the species’ population dynamics.
Invasive blue catfish are another threat – especially as they spread to mid-salinity and freshwater areas of the bay.
Protecting shallow water habitat for baby crabs will also mean changing how we manage shorelines, Hines said.
Communities facing sea level rise and erosion often reinforce shorelines with rip rap or seawalls, which shrink the marine habitat. Living shorelines, which mimic natural ecosystems to protect the shore, are a good alternative when possible.
The new research comes as scientists are working on updating the way officials estimate the health and population of blue crabs in the bay.
Population numbers are highly variable because of weather conditions and the creatures’ short life cycle, Hines said.
But they appear to be on the decline. The Virginia Institute of Marine Science’s annual winter dredge survey last year counted the second-lowest number of crabs since it began in 1990.
The Smithsonian center will continue its annual tethering research. But Hines said it was a good time to publish their findings so far.
“With these data, it's a remarkably clear story.”
