Tribes test innovative trap in battle against invasive crab

As invasive European green crab populations have spread in Washington waters, the bays and sloughs around the Tulalip Tribes’ homelands seem to have been sheltered from the invasion so far. But as the green crab has taken hold in areas to the north and south, it’s just a matter of time before the invader arrives along these shores, too.

“It’s really no longer a matter of if, it’s a matter of when,” said Todd Gray, environmental protection ecologist with the tribes’ Natural Resources Department.

In preparation for the inevitable, Tulalip has conducted trap-based monitoring for several years and has recruited help from the tribes’ Washington Conservation Corps (WCC) team to prepare, set and retrieve the traps.

This year, the crew is dispatching a collection of alternative traps alongside the traditional two varieties: cylindrical minnow traps and box-like Fukui traps. Those traditional traps are made of metal and vinyl mesh and set with bait—chopped up herring or mackerel—to lure hungry crab inside.

Tulalip WCC members slather a crab slab in sediment, hoping that if invasive European green crab arrive in the area, some will get caught inside.

The prototype trap is a wooden box Gray calls a “crab slab.” The invention doesn’t use bait. Instead, it is designed to mimic the type of habitat that European green crab—particularly gravid females preparing to have young— seek out beneath rocks on the shore.

“I hope it’s another tool for us to use in addition to these baited traps,” Gray said.

To camouflage the crab slabs as shoreline habitat, the interior is slathered in beach sediment before the top is slid into place and also covered in muck, leaving a cave-like opening along one edge of the box. The sediment also provides insulation to keep the interior cool during summer and adds weight, helping to anchor the trap in place on the shoreline.

The idea is that while baited traps are primarily used in spring and summer when the European green crab is most active in the water, these traps can be used year-round in areas where the crab takes cover on shore.

This concept is not only beneficial for extending the trapping season, but also eliminates concern about stranding non-invasive crab and fish that may be lured to baited traps in intertidal areas. And using the crab slabs during winter will increase the odds of removing reproductive females and their hundreds of thousands of potential young from the ecosystem.

Another key difference is that unlike baited traps that must be placed and retrieved within a 24-hour period, crab slabs can be left where they are placed on the landscape and checked intermittently. When checking them, a piece of wood is used to enclose the cave-like entrance to the trap. The lid is then removed to see what’s inside.

Gray deployed two crab slabs as a test in 2023. When he returned to them, he found native critters, mostly hairy shore crab, congregating inside and was relieved no European green crab are present. 

The invasive species is a concern because it can outcompete native species like Dungeness crab for habitat and food sources, eats other shellfish including native clams, and damages habitat for a variety of native shellfish—impacting treaty resources and the ecosystem. 

With the help of the WCC team this year, nine crab slabs were installed on the landscape as of the end of May, with additional installations planned.

Lummi Nation and the Makah Tribe, both of which are battling established populations of European green crab in their homelands, have asked Tulalip for their own sets of crab slabs to deploy in the fight, Gray said. He plans to deliver and help install some of the traps for both tribes this year.

Above: Todd Gray, environmental protection ecologist for the Tulalip Tribes, demonstrates to WCC members how to cover an experimental “crab slab” trapping device in beach sediment to help it blend in with the environment, secure it in place and provide insulation.

Below (left to right): Tulalip WCC member Katherine Kuehn, left, prepares a minnow trap to be set as part of the European green crab trapping effort. Tulalip WCC members Alex Casamassima, right, and Derek Bryant, left, anchor a Fukui trap at a European green crab monitoring site. A stack of crab slabs is ready for deployment. Photos and story by Kimberly Cauvel.