The Swinomish Tribe is monitoring the early life stages of Dungeness crab in the Strait of Juan de Fuca and Whidbey basin, in hopes of learning more about their survival to adulthood.

“Dungeness crab are culturally significant to the Swinomish Tribe, and it’s also our most economically valuable fishery,” said fisheries manager Lorraine Loomis. “With crab harvest increasing across the region, we need to know more about juvenile crab survival so we can continue to manage the resource sustainably.”

Since May, Swinomish Department of Fisheries staff have been collecting crab larvae daily using light traps at three locations, with help from Skagit Marine Resources Committee Salish Sea Stewards volunteers. They also are surveying juvenile crab at six different intertidal sites on Fidalgo and Whidbey islands.

Light traps allow researchers to sample species that are drawn to sources of light. Swinomish used blacked-out water cooler jugs based on a model Lummi Natural Resources began using last year, as well as other published designs. A timer inside the submerged trap turns on a light overnight, attracting crab larvae to swim inside.

Swinomish fisheries technician Claire Cook demonstrates a light trap for volunteers helping to survey crab. A light inside attracts the larvae, which swim through translucent white tubes to get to it.

“At first, when we were researching what a juvenile crab study would look like, the intertidal work is what we had in mind,” said Sarah Grossman, Swinomish environmental specialist. “As we researched the literature, we realized we first need to know when and where the pulses of larval crab are arriving in our area.”

For the intertidal survey, Grossman and fisheries technician Claire Cook excavate the top layer of substrate in randomly selected plots, and count the juvenile Dungeness crab they find there.

“The substrate survey looks at the juvenile crab after they settle, after three to four months in the water column,” Cook said. “We will compare the juvenile data to the larval data and determine what patterns exist between the two life history phases in important crabbing areas for Swinomish.”

The tribe is following a model developed by Alan Shanks at the University of Oregon that found a link between the number of crab larvae collected in light traps and adult populations on the Oregon coast. Marine research scientist Paul Dinnel found that the Central Salish Sea is a much more complex system, with Dungeness crab originating from local populations as well as from Hood Canal and the Pacific coast.

In decades to come, the data could be used to forecast adult populations.

Above: Swinomish fisheries technician Claire Cook excavates the top layer of substrate to sample juvenile crab in the intertidal area.