One of Washington’s most lucrative commercial fisheries is also one about which the least is known. Unlike numerous salmon runs in the state, data about Dungeness crabs is scarce – leaving managers with little to go on when projecting future harvest levels.

That’s changing, as a growing collaborative of researchers is using light traps to survey the crab’s larvae and track trends.

Margaret Homerding, the Nisqually Tribe’s shellfish program manager, said in 2013, weird results started showing up in test fisheries for Dungeness crab.

“For the next four years, we just saw more and more giant male crab dominating the test fishery, and fewer and fewer small crab and fewer and fewer females,” Homerding said.

They decided they had to close the fishery, to allow it to recover.

At the same time, Dungeness crab fisheries throughout South Puget Sound had declined — enough so that all the state recreational fisheries in the area and the tribal fisheries of the Squaxin, and Puyallup tribes closed too.

The Swinomish Tribe took the initiative to start surveying larvae, by attracting the pea-sized translucent baby crabs into simple light traps.

Project coordinator Emily Buckner, with the Puget Sound Restoration Fund, said these are simple, battery-powered devices that turn on at night.

“You can think of light as the bait in this case, of like a crab trap,” Buckner said. The technique was pioneered by Oregon State University professor Alan Shanks, who showed that counts of Dungeness crab larvae in Coos Bay reliably predicted population levels four years later.

The idea has now spread throughout the Salish Sea. Last year, the collective had people working the traps four days a week, at 22 sites.

“One of the reasons we’ve been able to have so many people involved is because the light trap process itself is quite simple,” Buckner said.

“It’s easy to build,” she said, adding that you can get all the parts from the hardware store.

Homerding, with the Nisqually Tribe, said they’re generating enough data now that they hope the counts will soon help inform management decisions.

“Being able to predict the number of crab that are coming four or five years down the road is great, because we would actually be able to plan for it,” she said.

“If there’s going to be an increase in population or decrease in population, [we can] figure out how to adjust our fisheries accordingly.”

She said other scientists are using the data now too — to look at things like genetics and the effects of invasive green crabs.

All of this will become increasingly important as climate change causes warmer and more acidic ocean conditions, and human population growth adds more runoff to local waterways.

The state’s average Dungeness crab harvest for the past 10 years is estimated to be worth $63 million annually, not including tribal treaty harvests.