NORTH AMERICAN RIVER OTTER DIET INCLUDES INVASIVE SACRAMENTO PIKEMINNOW AND HERPETOFAUNA ON SOUTH FORK EEL RIVER, NORTHERN CALIFORNIA

Abstract We identified species and approximate size of prey items in 109 digital photographs of North American River Otters (Lontra canadensis) hunting in a 2-km stretch of South Fork Eel River, California, from January 2017 through December 2021. Over the 5-y study, 403 observations of River Otter social-group types and sizes were recorded, including singles, family groups with pups, and groups of up to 8 adults. An average of 3.0 River Otters per week (sx = 0.2, range 1 to 13 individuals) were observed in the study area. Eleven species of prey were taken by River Otters, including primarily, Sacramento Pikeminnow (Ptychocheilus grandis; 54.1% of 109), followed by crayfish spp. (24.8%), Pacific Lamprey (Entosphenus tridentatus; 10.1%), herpetofauna (6.4%), and less-frequently taken fish species (4.6%). Contingency table frequencies indicated Sacramento Pikeminnow and crayfish were taken more in months of low water flows in the river. Herpetofauna prey included 1 American Bullfrog (Lithobates catesbeinus), 2 Foothill Yellow-legged Frogs (Rana boylii), 1 Northwestern Pond Turtle (Actinemys marmorata), and 3 Rough-skinned Newts (Taricha granulosa), apparently with no ill effects to the otters. Sacramento Pikeminnows were captured by social groups of 2 and 3 or more River Otters more often than expected, and by single otters and family groups less often than expected; whereas crayfish were captured by single otters and family groups more often than expected, and by social groups of 2 and 3 or more, less often than expected. Different River Otter social-group types and sizes captured smaller (<30 cm) and larger fish (≥30 cm) at similar frequencies.