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Home » Projects » Exploring biotic resistance of European green crab via river otter diet in the Wa’atch and Tsoo-Yess river estuaries

Exploring biotic resistance of European green crab via river otter diet in the Wa’atch and Tsoo-Yess river estuaries

  • Makah Tribe / Western Washington University

  • 2020

This study was conducted on the land of the Makah people with approval by the Makah Tribal Council (MOA 2019-01).

This project was a continuation of a investigating river otter (Lontra canadensis) diet on the Makah Reservation to understand the potential role of these predators in bio-mitigation of a recent infestation of the invasive European green crab (Carcinus maenas), as well as their consumption of important fish and shellfish species. This phase of the project focused on the prey identification and diet results from the samples collected under the 2019 funding.

Establishment of the European green crab on the west coast United States has led to heightened concerns regarding loss of eelgrass beds, threats to important shellfish resources, and impacts on marine communities. In the lower Wa’atch and Tsoo-Yess river estuaries, more than 4,200 green crabs have been trapped since their discovery on the Makah Indian Reservation in 2017. To begin examining whether predators can potentially buffer green crab expansion, researchers documented river otter diet from scat remains and estimated green crab abundance from removal trapping efforts since no population estimate currently exists.

From river otter scats collected in 2018 and 2019 by researchers, technicians and interns with the Makah Tribe, fish bone and crustacean shell remains were cleaned and sorted with help from undergraduate students at Western Washington University. Prey were identified and reported to the lowest possible taxon as percent frequency of occurrence (%FO). When possible, a minimum number of individuals (MNI) was determined for scat samples using quantifiable fragments (e.g., fish otoliths, crab claws) to estimate numbers of consumed fish and crustacean prey. Green crab consumption was compared to catch-per-unit-effort (CPUE) summarized from 2019 trapping data on the Wa’atch and Tsoo-Yess rivers.

River otter diet was composed of highly diverse freshwater and marine fish and crustacean species, including green crab, with 38 species identified and an additional 17 prey taxa with limited identification due to a lack of distinguishable hard remains (e.g., broken vertebrae, carapace shell, etc.). Overall, the most frequently occurring prey (>20 %FO) included saddleback gunnel (Pholis ornata), starry flounder (Platichthys stellatus), Pacific staghorn sculpin (Leptacottus armatus), prickly sculpin (Cottus asper), Dungeness crab (Metacarcinus magister), shiner perch (Cymatogaster aggregata), threespine stickleback (Gasterosteus aculeatus), and bay pipefish (Sygnathus leptorhynchus). These species commonly occur in estuarine habitats and have been previously documented in marine-estuarine river otter diet within other Pacific Northwest coastal locations.

River otters consumed green crab in the Wa’atch River, but not in the Tsoo-Yess River. Green crab occurrence in the Wa’atch River scats was low (0.7-5.2 %FO), suggesting that they were not an important prey source. Scats collected from the Tsoo-Yess River contained no green crab, which was surprising given the higher CPUE of green crab during removal trapping in the Tsoo-Yess compared to the Wa’atch River. This suggests that there is still relatively low abundance of green crab compared to other crustacean and fish prey. If green crab numbers increase, researchers hypothesize that there will be a subsequent increase in consumption by river otters. Documenting the population status of green crabs in both rivers and conducting additional predator-prey diet studies to gauge the potential for long-term biotic resistance of green crab populations are important next steps for understanding the impact of this invasive species.