Food Habits and Fish Prey Size Selection of a Newly Colonizing Population of River Otters (Lontra Canadensis) in Eastern North Dakota

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ABSTRACT.-The food habits of river otters (Lontra canadensis) on three rivers in the Red River of the North drainage of eastern North Dakota were evaluated using an analysis of 569 scats collected between 4 Oct. 2006 and 26 Nov. 2007. Fish and crayfish were the primary prey items, occurring in 83.0% and 51.1% of scats, respectively. Other prey included insects (26.7%), birds (7.9%), amphibians (6.7%), mammals (6.0%) and freshwater mussels (0.2%). Fish of Cyprinidae (carp and minnows) were the most prominent fish in the diet, occurring in 64.7% of scats. Other relatively common fish in the diet included Ictaluridae (catfish, 17.4% frequency of occurrence), Catostomidae (suckers, 13.0%), and Centrarchidae (sunfish, 11.2%). The diet of river otters changed seasonally, including a decline in the frequency of fish in the summer diet, and a corresponding increase in the occurrence of crayfish. Consumed fish ranged from 3.5 to 71.0 cm total length, with a mean of 20.7 cm (SE ± 0.5, n = 658). Fish 10.1-20.0 cm were the most frequently consumed size class (36.5% relative frequency), with the majority of other consumed fish being ≤ 10.0 cm (24.6%), 20.1-30.0 cm (14.1%), 30.1-40.0 cm (14.0%), or 40.1-50.0 cm (8.2%). The size of consumed fish changed seasonally, with spring having the largest mean prey size.

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Historically, the nearctic river otter (Lontra canadensis) occurred on most rivers in North Dakota, and was relatively common into the 1890s (Bailey, 1926; Adams, 1961). River otters still occurred in the 1920s along the major rivers and some lakes but had become rare by the 1960s, and were considered extirpated soon after (Bailey, 1926; Adams, 1961). However, in recent years reports of river otters have increased, with most coming from the Red River of the North (hereafter referred to as Red River) drainage, and Lake Sakakawea in the Missouri river drainage (Hagen et al, 2005).

River otters are opportunistic aquatic predators. Although the diet is diverse, most dietary analyses have shown fish to be the primary prey (e.g., Greer, 1955; Melquist and Hornocker, 1983; Serfass et al, 1990). River otters are presumed to select fish in proportion to their abundance and in inverse proportion to swimming speed and agility (Ryder, 1955). Therefore, the most abundant and slowest swimming fishes tend to be taken most often. Catostomidae (suckers) , Centrarchidae (sunfish and bass) , Cyprinidae (carp and minnows) and Ictaluridae (catfish) are usually among the most frequently occurring fish families detected in river otter diet studies (e.g., Wilson, 1954; Greer, 1955; Hamilton, 1961; Griess, 1987; Serfass et al, 1990; Noordhuis, 2002; Giordano, 2005). When available, crayfish are usually the second most important prey item and in a few studies have occurred most frequently in the diet (Grenfell, 1974; Griess, 1987; Noordhuis, 2002). Other organisms consumed by river otters include amphibians, insects and other invertebrates, birds, mammals and reptiles (Ryder, 1955; Melquist and Hornocker, 1983; Serfass et al, 1990).

Despite many previous food studies on river otters, rarely have studies been conducted to assess the size of their fish prey. Previous studies have made general inferences about prey size, indicating that fish prey ranges from 2-80 cm and that most fish consumed are probably 10-30 cm in length (Lagler and Ostenson, 1942; Greer, 1955; Ryder, 1955; Hamilton, 1961; Toweill, 1974; Melquist and Hornocker, 1983; Stenson et al, 1984; Griess, 1987; Tumlison and Karnes, 1987; Noordhuis, 2002; Giordano, 2005). However, these studies typically did not indicate the methods used in their assessments, or establish predictive relationships between anatomical structures (that are recoverable from the digestive tracts or scats, such as bones and scales) and fish length. Also, inferences have been limited to one or a few species (occasionally only a few individuals) and only provided information on the size range (maximum and minimum) or common prey sizes. …