Game fish
North American river otters consume an extensive assortment of fish species ranging in size from that impart sufficient caloric intake for a minute amount of energy expenditure. North American river otters generally feed on prey that is in larger supply and easier to catch. As a result, slow-swimming fish are consumed more often than game fishes when both are equally available. Slow-moving species include suckers (Catostomidae), catfish, sunfish and bass (Centrarchidae), daces, carp, and shiners (Cyprinidae). For instance, Catostomidae are the primary dietary component of North American river otters in Colorado's Upper Colorado River Basin. Likewise, the common carp ("Cyprinus carpio") is a preferred fish species for the North American river otter in other regions of Colorado. Fish species frequently found in the diets of the North American river otters include: Catostomidae, which consists of suckers ("Catostomus" spp.) and redhorses ("Moxostoma" spp.); Cyprinidae, made up of carp ("Cyprinus" spp.), chubs ("Semotilus" spp.), daces ("Rhinichthys" spp.), shiners ("Notropis" and "Richardsonius" spp.), and squawfishes ("Ptychocheilus" spp.); and Ictaluridae, which consists of bullheads and catfish ("Ictalurus" spp.). Other fish an integral part of the North American river otters' diets are those that are often plentiful and found in large schools: sunfish ("Lepomis" spp.); darters ("Etheostoma" spp.); and perches ("Perca" spp.). Bottom-dwelling species, which have the tendency to remain immobile until a predator is very close, are susceptible to North American river otters. These include mudminnows ("Umbra limi") and sculpins ("Cottus" spp.). Game fish, such as trout (Salmonidae) and pike (Esocidae), are not a significant component of their diets. They are less likely to be prey for the North American river otters since they are fast-swimming and can find good escape cover. However, river otters will prey on trout, pike, walleye ("Sander vitreus vitreus"), salmon ("Oncorhynchus" spp.), and other game fish during spawning. Otters have been found to consume invasive Asian carp.