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Investigating interactions between Dungeness crabs and invasive European green crab in two estuaries on the Makah Reservation

The goal of this project was to evaluate the threat of invasive European green crab (Carcinus maenas; hereafter green crab) on subadult Dungeness crab (Metacarcinus magister) in two riverine estuaries on the Makah Reservation. We had two primary study questions:

1) What is the population and distribution of Dungeness crabs and green crabs in the lower Wa’atch and Tsoo-Yess rivers in summer?

2) Are green crab negatively impacting (or do they have the potential to negatively impact) Dungeness crab in Makah estuaries?

To answer the first research question, we conducted a mark-recapture study and estimated the population size of the crabs using the rivers. We used two methods of marking: In 2021, crabs were marked with pigment epoxy on the crab’s carapace. In 2022, crabs were marked with individually identifiable tags secured to the carapace with epoxy. In August 2021, we estimated that more than 12,600 Dungeness crabs and more than 500 green crabs were using the lower mainstem of the Tsoo-Yess River. In July 2022, we estimated that close to 10,000 Dungeness crabs and 1,000 green crabs were using the lower mainstem of both the Wa’atch and Tsoo-Yess rivers. We continued to collect resights beyond our initial mark-recapture periods and found that Dungeness crabs move between the two rivers.

To answer the second research question, we used baited remote underwater video (BRUV) to evaluate interactions between the two species. We constructed BRUVs using PVC and underwater video cameras. We recorded 35 videos in 2021 and 72 videos in 2022; analysis of 19 of the videos is on-going. Green crabs were present in about half of the videos evaluated. We conducted behavioral assays to determine green crab success at accessing the bait and evaluate aggressive interactions between green crabs and Dungeness crabs. We documented more than 600 instances of a green crab approaching the bait, in 48% of which they were successful in reaching the bait. The primary reason for an approach being unsuccessful was due to one or more Dungeness crabs preventing access to the site of the bait. Using logistic regression, we found the total number of crabs present in the video frame and the relative size of other crabs in the video frame at the start of a green crab’s approach affected approach success. When more crabs were present and were primarily larger than the green crab, the approach was less likely to be successful. We evaluated nearly 100 instances of aggressive interactions between green crabs and Dungeness crabs and documented agonistic behaviors at the bait site and away from the bait site, as well as passive behaviors. During agonistic interactions, the aggressor of either species was most likely to displace the other crab. During passive interactions, green crabs were most likely to be displaced from the bait. Following these efforts, we conducted a large-scale trapping effort to remove a large volume of green crab from the estuaries and removed more than 5,300 green crabs in four days. Together, our findings confirm that green crabs overlap in space and time with subadult Dungeness crab in the coastal estuaries on the Makah Reservation and that subadult Dungeness crabs appear competitive against green crab.