Investigating the impacts of a native mammal predator on key native and invasive species in marine and estuarine ecosystems of Northwest Washington
The goal of this project was to document the diet of an aquatic mammal predator, the river otter (Lontra canadensis), in coastal estuaries of the Makah Reservation in Northwest Washington. We sought to understand the potential role of these predators in bio-mitigation of a recent infestation of the invasive European green crab (Carcinus maenas) in Makah estuaries, as well as their consumption of important fish and shellfish species. This funding supported the first phase of the project: sample collection and processing.
The Makah Tribe first became aware of green crab on the Makah Reservation in 2017 and an intensive trapping effort in 2018 yielded more than 1,000 crabs caught in two coastal estuaries, the Wa’atch and Tsoo-Yess rivers. Green crab are a serious concern to the Makah Tribe due to their potential threat to native fish and shellfish as well as to the natural ecosystem function. Green crab are responsible for declines in commercially fished clams and mussels, outcompete juvenile Dungeness crabs for habitat and food resources, and can also impact important shoreline habitat by disrupting eelgrass beds and causing erosion to salt marsh habitat.
After locating river otter latrine sites along the lower Wa’atch River in close proximity to locations where green crab were captured, the Makah Tribe became interested in investigating whether or not river otters may predate on the crabs and thus aid in removal and control of this invasive species. Further, the Tribe was interested in the overall diet of the otters due to concern about their levels of predation on native fauna, particularly salmonids and Dungeness crab (Metacarcinus magister). We collected river otter scats from latrine sites along the Wa’atch and Tsoo-Yess rivers in proximity to locations of green crab captures. Our goal was to collect 50 scats per river per month between April and September 2019 to coincide with the timing of Tribe’s green crab trapping efforts. Over the duration of the project, we collected a total of 538 river otter scats, 313 scats from latrine sites on the Wa’atch River and 225 scats from latrines on the Tsoo-Yess River. Individual scats were collected in Ziploc bags and frozen for later processing. Scats were later thawed and washed through 500 µm and 1 mm mesh sieves to remove prey remains. Prey remains (e.g., fish bones, crustacean shells) were washed to remove fecal material, dried, and then sorted into separate vials for fish and invertebrate remains for later identification.
Prey identification will be conducted under subsequent funding, however preliminary findings during scat cleaning and sorting revealed several scats that contained green crab remains, as well as many that contained Dungeness crab and salmonid bones. The results of the prey identification will be analyzed to describe river otter diet in the coastal estuaries of the Makah Reservation and will be utilized in a Master’s thesis (Western Washington University) and subsequent peer-reviewed publication.