Diet of the North American river otter (Lontra canadensis) in North Carolina using 2 methods

Abstract

River otters (Lontra canadensis) are key predators in North Carolina's aquatic systems, but they are often seen as competitors by anglers and fish hatcheries. River otter diets typically consist of fish and crayfish, but also include occasional herpetofauna, mammals, and birds. While standard diet studies focus on identification of prey through manual examination of stomach contents and feces, metabarcoding DNA analysis has become more popular to determine the presence or frequency of species that are often missed, misidentified, or underestimated. We collected river otter carcasses from licensed trappers and fur dealers across North Carolina from the 2009–10 trapping season through the 2015–16 season. We conducted necropsies and analyzed the stomach contents using standard observational methods and metabarcoding DNA analysis. We manually examined 522 river otter stomachs, of which 377 contained prey items. Decapods (crustaceans) were identified in 41% of stomachs and made up similar percentages within each Furbearer Management Unit (FMU). The order Perciformes composed the majority (62%) of fish prey across all stomach samples. Coastal Plain river otters primarily consumed crustaceans (50%) and fish (40%). Piedmont and Mountain river otters consumed fish (32% and 42%, respectively) most often followed by crustaceans (62% and 50%, respectively). Prey selection was similar between the sexes. Out of 368 samples, metabarcoding DNA examination was able to reliably match 164 prey items to species, 5 classes, 18 orders, 25 families, and 42 genera. Fishes made up 33% of the identifications, particularly Perciformes (13%), Cypriniformes (7%), and Siluriformes (5%). Twelve percent of identifications was made up by Amphibia, split evenly by Anura and Urodela. No birds or reptiles were detected in the Mountain or Piedmont FMUs, and no mammals were detected in the Coastal Plain or Mountain FMU. Overall, river otters in North Carolina consume a large variety of prey that varied regionally. The manual examination provided identifications that were not provided by the DNA examination (i.e., crayfish, brown snakes), while the DNA examination provided a more accurate identification of the broad array of prey items. To understand the composition of annual river otter diets we encourage managers to expand research to evaluate river otter diets year-round and incorporate additional noninvasive methods (e.g., scat surveys) throughout the year.