Exploring the biotic resistance of the invasive green crab (Carcinus maenas) by examining the diet of North American river otters (Lontra canadensis)

We examined the potential of North American river otters (Lontra canadensis) to buffer the expansion of the invasion by green crab (Carcinus maenas) on the West Coast of the United States, documenting the diet of otters from scat remains on the Wa'atch and Tsoo-Yess Rivers, in Washington State, in 2018 and 2019. We tallied hard remains of prey and calculated frequency of occurrence, and we compared predation of the green crab to monthly values of catch per unit of effort for this crab species. North American river otters did not consume green crab in the Tsoo-Yess River and infrequently consumed green crab in the Wa'atch River (1.66% frequency of occurrence), likely because of the lower abundance of the green crab compared to the abundance of other prey in these rivers. Although our results indicate that North American river otters were not a biotic control of green crab, future studies on the population status of the green crab and North American river otter in both rivers and the long -term predator-prey dynamics could help to better gauge the potential for biotic resistance in populations of the green crab.