Where did the Puget Sound green crabs come from? We’re still not sure
Genetic testing shows that invasive European green crabs in Puget Sound likely did not come from the Sooke Basin in British Columbia as previously thought. New findings on the crab's origins were presented at the 2018 Salish Sea Ecosystem Conference in Seattle.
It is hard to look away from the European green crab invasion in the Salish Sea. When these infamous invaders were first observed in the Sooke Basin, British Columbia in 2012, scientists worried that it was only a matter of time before they entered Puget Sound. Crabs were found off of San Juan Island and in Padilla Bay in 2016, and in Dungeness Spit, Sequim and Whidbey Island the following year. The outer coast was not spared either — the crabs were found in Makah Bay in 2017.
To manage the invasion, volunteer efforts led by Washington Sea Grant’s Crab Team have put in over 3,000 days searching for and trapping these crabs. Still, one question remains: where did the Puget Sound crabs come from?
Many point their fingers at the Sooke Basin population. Sooke is just across the Strait of Juan de Fuca from Dungeness Spit, which is home to one of the larger green crab populations in Puget Sound. It’s an easy explanation, but is it the right one? To figure out where the crabs really came from, scientists need to know two things: all possible locations with crab source populations, and whether or not the Puget Sound populations match any of the British Columbia or coastal populations.
Elizabeth Brasseale, an Oceanography Ph.D student at the University of Washington, uses modeling tools to determine which green crab populations are possible sources for those in Puget Sound. According to Brasseale, modeling is the “most complete recreation of the ocean” available, and adding in biological information about the crabs themselves makes modeling realistic. Working with the WSG Crab Team, Brasseale created a simulation of crab spawning events in Sooke, Barkley Sound, British Columbia, Willapa Bay, Washington, and Coos Bay, Oregon. She then tracked the simulated crab larvae to see if they entered Puget Sound.
She found that flow reversals — temporary changes in the ocean’s circulation — helped crab larvae from the Willapa Bay and Coos Bay enter Puget Sound in 2014 and 2016.
“When the wind blows from the south, the Columbia River plume gets sucked into the Puget Sound, and that’s how crabs get in,” Brasseale said. “Crab spawning lines up with these flow reversals [in 2014 and 2016], which means our shields were down.” The changes in ocean dynamics could have also prevented the Sooke and Barkley Sound populations from entering the sound, potentially eliminating that easy explanation for where these crabs came from. But because flow reversals do not happen every year, state management does not have to constantly watch for crabs entering Puget Sound from the coast.
“We do really have to worry about every [source population], but we do not have to worry about every [source population] all the time,” Brasseale said.
Should green crab management efforts be concentrated on the coast? That’s where genetic tools come into play. Dr. Carolyn Tepolt at Woods Hole Oceanographic Institute uses population genetics tools to see which green crab populations are similar to each other — the same tools that allow forensic scientists to identify murderers from evidence at a crime scene. While more traditional genetics tools are “blunt instruments” that can distinguish between crab populations established in the past, the tools Tepolt uses allow her to pick up fine-scale differences between recently established crab populations. It’s the difference between choosing a tool that will allow you to exactly pinpoint the killer’s identity instead of just being able to tell if they are a man or woman.
Using green crab samples from British Columbia, Washington, Oregon, and California, Tepolt’s first objective is to see if there is any population structure for West Coast green crab populations. If there is structure, she will be able to pinpoint the source of Puget Sound green crabs. If there is not any structure, it is likely that Puget Sound crabs came from a variety of sources. She hopes that her analyses, in conjunction with Brasseale’s model results, will help inform green crab management in the region.
“You can use modeling to predict where [green crabs] come from, and use genetics to test those predictions,” Tepolt said. “If they back each other up, you can be more confident to use your model in the future.”
[Editor's note: According to Washington Sea Grant, Tepolt's findings now confirm that the invasive crabs "did not come from Sooke." Sea Grant reports that the crabs "did come from one of the coastal populations (CA, OR, WA, or BC) – as opposed to somewhere else in the world – but it’s not possible to tell which."]