Swallows return to Capistrano, and buzzards to Ohio. In the Chesapeake and Delaware bays, it’s the tasty and feisty blue crabs, and researchers are still trying to find out how.

“What we know points up everything we still don’t understand about what triggers what behavior,” said Elizabeth W. North, a researcher at the University of Maryland Center for Environmental Studies. “Somehow, they seem to sense the need to get back to the bays.”

Like the buzzards that return to Hinckley, OH, and the swallows that come home to the San Juan Capistrano in California every spring, blue crabs also have a yearly peregrination.

Larval crabs that are spawned in estuaries such as the Chesapeake or Delaware bays spend weeks in saltier ocean waters before returning to live out their lives. North is leading a project studying how currents, salinity and nutrients affect the movement of billions of drifting blue crab larvae.

Some researchers suspect the larvae floating along in ocean currents are attracted by less salty bay water.

“The hypothesis is that the crab larvae smell the lower salinity in the bay, then ride a high tide in from the ocean,” said Michael Roman, who directs the UMCES Horn Point lab near Cambridge, MD. “There is a change in behavior when they get close to the Bay that we need to understand.”

North’s team, comprising researchers from Maryland and Delaware, recently spent a week at sea aboard the University of Delaware’s new $19.4 million research vessel, the Hugh R. Sharp.

The team spent six days at the mouth of Chesapeake Bay and two days about 10 miles offshore from the University of Delaware’s marine studies center in Lewes, where the ship is based.

The researchers used an elaborate nylon net to catch the nearly invisible crab larvae at various depths and an underwater device resembling an airplane wing to collect data on the number of larvae at various depths.

Analyzing the samples could take a year, said University of Delaware researcher Ana Dittel, an expert in larvae ecology.

“You can’t really see much to catalog until you get the sample under a microscope,” she said.