Conservation, Research and Education Opportunities International

Investigating the impacts of a native mammal predator on key native and invasive species in marine and estuarine ecosystems of Northwest Washington

Makah Tribe

This project seeks to understand the potential role of river otter (Lontra canadensis) 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, such as salmonids and Dungeness crabs (Metacarcinus magister). The Makah Tribe became aware of the occurrence of European green crab on the Makah Reservation in 2017, after trapping efforts yielded 34 of the invasive crabs. In 2018, intensive trapping effort in two coastal estuaries, the Wa’atch and Tsoo-Yess Rivers, and in nearshore areas of Neah Bay yielded 1,029 green crabs in the two estuaries and six molts (discarded carapaces) inside Neah Bay.

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. Despite only discovering the crabs in 2017, the size classes of crabs caught suggest that they established in Makah waterways as early as 2015. Green crab larval development occurs in marine waters and is strongly influenced by warm ocean temperatures and conditions such as El Niño. In spite of the intensive trapping effort in 2018, by the end of the season, young-of-the-year crabs as small as 10 mm carapace width were caught, suggesting that local propagation and recruitment may be occurring.

In August 2018, coinciding with green crab trapping, we discovered a location along the lower Wa’atch River adjacent to an area of high green crab catch with several productive river otter latrines sites (i.e., shore-based locations repeatedly used by the otters to deposit scats). In August and September, we opportunistically collected 130 scats. We seek to continue scat collections in the Wa’atch River in 2019 and to determine other latrine sites in the Tsoo-Yess River and inside Neah Bay. Our goal is to determine if river otters are eating green crabs, and therefore acting as bio-mitigation for this invasive species. An opportunistic predator, river otters are known to eat many littoral crab species, including green crab in their native range. Better understanding the role of native predators in controlling the population of this invasive species will aid the Tribe in decisions about long-term management for the crabs.

The diet of river otters on the Makah Reservation is also of interest due to their impact on important native species such as salmonids and Dungeness crabs. River otters are a noted concern for the Tribe’s Fisheries Managers due to their observed predation on both out-migrating and returning salmon below the Makah National Fish Hatchery located in the Tsoo-Yess River. In the nearby Ozette River, the Makah Tribe documented significant salmon predation by otters near a salmon weir used by the Tribe to count returning fish. Near the San Juan Islands, river otters also had significant predation Dungeness crab, a species that is important for both tribal and non-tribal commercial, recreational, and subsistence harvest. By documenting the diet of river otters in Makah waterways, we can better understand their impacts on these important fisheries species. ($18,000)