Green Crab Monitoring

2020 Salish Sea Green Crab Update

November 23, 2020

Trapping for European green crab has mostly concluded for the 2020 season, enabling us to take stock of all that was accomplished this year and provide some status updates on green crab. The obstacles this year were considerable, but volunteers and managers were more than equal to the task, masking, attesting, and distancing, not to mention riding out wildfire smoke, to trap for green crabs. The next few posts will provide updates on what was accomplished and what we’ve learned about green crabs this year, and this first post will focus on an overview of sites within the Salish Sea.

This year, managers and volunteers collectively peppered inland shorelines with more than 11,000 traps, more than any previous year. In addition to WSG Crab Team early detection efforts, trapping was conducted by four new Washington Department of Fish and Wildlife (WDFW) technicians, as well as partners with the Lummi*, Jamestown S’Klallam and Makah Tribes, Padilla Bay National Estuarine Research Reserve (NERR), and US Fish and Wildlife Service (USFWS). Here are updates on key findings.

Crab Team Monitoring Network

Crab Team volunteers, wearing masks, sample crabs while socially distant.

Crab Team volunteers, masked and distant, on Bainbridge Island navigating both the pandemic and the wildfire smoke to sample Blakely Harbor. Steve and Jane Hannuksela measure a large red rock crab while Cathy Bohlke records their data from afar. Photo: Amy Linhart/WSG

Crab Team volunteers and partners monitored 55 sites as part of the early detection network this year. While the scale of monitoring was reduced or delayed in some cases, to ensure safety of participants, together, the incredible teams completed more than 90% of regular monitoring efforts with many sites completing all six monthly efforts. This is an astounding success which was only possible with the adaptability, generosity, and commitment of the 250 volunteers and partner staff that participate. The best news of all: there were no new detections at Crab Team sites.

Whatcom County

  • Drayton Harbor: The dedicated collaborative removal effort in Drayton Harbor managed to cover a lot of ground in 2020, in the first year of a local removal effort. We will provide more detail in a future post, but removal trapping combined with the ongoing trapping by Crab Team monitors captured a total of 255 green crabs in 3,258 trap sets. This means, on average, for every 100 traps set, 8 green crabs were captured.
  • Lummi Bay*: Staff with the Lummi Natural Resources Department (LNR) have been working to trap and remove green crab all year, particularly in the Lummi Sea Pond, a human-constructed impoundment that supports the salmon and shellfish hatcheries. WSG Crab Team has been collaborating closely to support efforts, including participating in a couple of expanded efforts with LNR and WDFW. Altogether, trapping removed more than 2,500 green crabs in more than 3,000 trap sets this year, including a large number of young of the year that began to arrive in traps as of August and September. Based on the size of crabs captured, the first green crabs did not get washed into Lummi Bay until quite recently, 2018 at the earliest, well after other Salish Sea locations like Padilla Bay, and Sooke basin, B.C.

Skagit County

  • Chuckanut Bay: The eagle eyes of Crab Team volunteers picked up two molted green crab shells in Chuckanut Bay in the first part of the trapping season. However, no green crabs were detected as part of regular monthly monitoring (36 trap sets). Neither were any green crabs captured as part of a larger assessment effort by WDFW in October. In July 2019, three green crabs were captured as part of a WDFW assessment trapping effort.
  • WDFW staff conduct an assessment trapping at Samish Bay. Photo: Emily Grason/WSG

    Samish Bay: The site of a handful of opportunistic captures in 2019, effort and concern about Samish Bay has grown in 2020. Early this year, growers with Taylor Shellfish Farms opportunistically captured a few green crabs by hand when working on Taylor’s beds in east Samish Bay. In the cold months, this is the most likely way to capture green crabs when populations are still small. Bill Dewey, Senior Director of Public Affairs for Taylor, also farms his own shellfish grounds nearby. He and his sons began setting traps in spring to remove as many crabs as they could, as the waters warmed and green crabs come to baited traps more frequently. Given the constraints of their other tide-dependent work, the Deweys were setting traps by boat, and using a style of trap better suited to slightly deeper water. They noted a strong uptick in capture rates in mid-late spring and ebbing again in fall, which echoes seasonal patterns we’ve observed at other sites. In total they have trapped and removed more than 80 green crabs this year. In addition, WDFW and staff at the Padilla Bay NERR broadened the geographic scope conducted a couple of assessment efforts, to trap more of the shorelines of Samish Bay. These events captured 12 additional crabs around Taylor’s beds, as well as four green crabs in the southern portion, called Alice Bay.

  • Padilla Bay: This year, staff with the Padilla Bay (NERR) made the first detections of green crabs in Padilla Bay since 2017. The five Crab Team early detection sites in Padilla Bay are monitored by staff at the NERR as well as a group of volunteers at the Swinomish Casino marsh. While conducting additional exploratory trapping at a non-regular monitoring site in August, the NERR staff captured a single male crab. Three more were captured during follow up assessment trapping in September, and a Skagit County Noxious Weed crew member provided a confirmed report of two more crabs in October, for a total of 6 green crabs. This brings the average capture rate to about 1 crab for every 100 traps set. Interestingly, all of the crabs captured were young of the year, from the newest cohort.

New Detections

The only new detection site recorded during 2020 for inland shorelines was Birch Bay. Situated between Drayton Harbor and Lummi Bay, this has been a site of interest since green crabs were discovered in both adjacent water bodies last year. In addition, a community member reported finding a green crab carcass being predated by a gull in Birch Bay last year. Though it’s difficult to know for sure where a gull may have captured and brought a green crab from, observing this suggested that more could be nearby. In October, WDFW was able to conduct an assessment in Terrell Creek, considered the most suitable habitat for green crabs available in Birch Bay, and captured one young of the year crab in 50 traps (average capture rate of two crabs per 100 trap sets).

Dungeness Spit

A graph showing number of green crabs captured declining annually since 2017. The graph also shows a line tracking average capture rate over the same period, which does not show substantial decline until the most recent year.

Number of green crabs captured, and average capture rate (in crabs per 100 trap sets) in Dungeness Bay since 2017, when green crabs were first detected by USFWS staff at the wildlife refuge. Click to enlarge.

A bright spot in the management of green crabs in the Salish Sea can be found in Dungeness Bay. When green crabs were found for the first time in Dungeness Bay, in 2017, it was the first “hot spot” where more than just a handful of crabs were trapped during assessments. Since that time, USFWS staff and volunteers with the refuge have been conducting intensive removal trapping. All spring, summer, and into fall, they set 50-150 traps for several nights a week. Though they’ve found a smaller number of green crabs each successive year since, the average catch rate has stayed relatively constant. This year, for the first time, that number plummeted – only 3 crabs were captured in 1,883 traps set, an average 0.15 crabs for every 100 traps set. That is, you’d have to set nearly 1,000 traps to capture even 1 crab!

This is the result of dedicated effort on the part of USFWS, as well as a little help from nature. In addition to removing crabs that were present, the region also may not have been subjected to influx of more larvae from coastal or other sources. Both are ingredients that are necessary to achieve such a substantial reduction.

Putting the pieces together

Map of sites along inland Washington shorelines trapped during 2020. The map does not include survey data from populations in British Columbia, e.g., Sooke Basin. Data from Lummi Bay provided by Lummi Natural Resources*. Click to enlarge.

Stepping back, what do we make of all that we have seen this year? It is a relief that there was minimal evidence of new detection sites this year. Yet the emergence of sites in Whatcom and Skagit counties as hotspots is currently the greatest concern. This year, these locations saw greater capture rates than any sites inland, and any prior years. In addition, the young of the year cohort, which started to arrive in traps in late summer, was more numerous at these sites than in previous years. A strong young of the year cohort indicates some of the population will remain over winter, and underscores the critical need to continue robust removal trapping efforts next year in these locations, and keep a close eye on adjacent shorelines for any further spread.

On the other hand, trapping in Dungeness Bay has shown that a dedicated effort, combined with favorable environmental conditions, can result in effective local population control. This year, green crab trappers may get some additional help from nature–forecasts are predicting a La Niña winter. Cold, wet La Niña conditions are celebrated perhaps only by two groups of people: skiers, because they bring good snow, and green crab managers, because they are less favorable to survival and recruitment of green crabs. Next year, our goal will be to capitalize off any opportunity this window affords.

Emily Grason

*All data from Lummi Bay shared courtesy of the Lummi Natural Resources Department, please contact Nick Jefferson for more information: NicholasJ@lummi-nsn.gov

We want to thank the generous volunteers and partners that enable sampling as part of the Crab Team early detection network, including: Lower Elwha Klallam Tribe, Jamestown S’Klallam Tribe, Port Gamble S’Klallam Tribe, Suquamish Tribe, Stillaguamish Tribe, Samish Indian Nation, Swinomish Indian Tribal Community, US Fish and Wildlife Service, Padilla Bay NERR, WA Dept. of Natural Resources Aquatic Reserves, the more than 200 volunteers, and landowners that provide access to monitoring sites.

Header photo: Amy Linhart and WDFW technician April Fleming spotted a juvenile European green crab in Lummi Bay in September. Credit: Emily Grason/WSG

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