Distribution and abundance of river otter in Voyageurs National Park

Route, William; Peterson, Rolf

River otter (Lutra canadensis) distribution and abundance in relation to man-caused water fluctuations was studied from April 1985 to March 1987 in Voyageurs National Park (VNP), Minnesota. Baseline data were collected on the otter population and comparisons made between study areas with less-than-natural (Rainy Lake) and greater-than-natural (Namakan Reservoir) water fluctuations. Thirty-nine otter were live-trapped with a combined success rate of 41.1 trap-nights/otter (spring X = 13.9 ; fall X = 72.6). Nineteen of these were radio-monitored for a total of 1,098 locations. From direct observations a minimum of 270 otters (1 otter/3.6 km of shoreline) inhabited the large lakes in VNP during the summer 1986. Mean productivity was estimated at 2.6 young per breeding female. Two live-trapped otters were known to have had naturally occurring urinary infections and one suffered from adrenal failure. One natural death was documented when a female otter was killed by wolves. Home range area varied from 0.64 km2 for a yearling female in winter to 29.13 km2 for an adult male in summer. Adult males had larger summer home range areas (X = 21.38 km2) and more total shoreline available (X = 50.41 km) than did adult females (area X = 3.46 km2; shoreline X = 12.78 km) but during winter there was no difference between sexes (male area X = 7.58 km2; male shoreline X = 12.78 km; female area X = 5.75 km2; female shoreline X = 15.21 km ). Area and total shoreline available within summer home ranges of adult males were significantly larger than their respective winter home ranges but the same was not true for adult females. Otters moved freely between large lakes and adjacent inland lakes and ponds in summer. While most radioed otters stayed on the large lakes in winter, 16-35% moved to inland waters and made little or no use of the large lakes. Fish and crayfish were the most important prey of otters, occurring in 72% and 71% respectively of 796 scats analyzed; however, the relative importance of the two prey varied with season and water regime. Use of fish remained high on Rainy Lake during both summer and winter but crayfish became more important in the summer diet of otters in Namakan Reservoir. A method based on fresh sign and known movements of radioed otters was developed for comparing relative abundance. Using this method, no difference was detected in otter abundance between Rainy Lake and Namakan Reservoir. Home range size was not significantly different between adult female otters using the two study areas and insufficient data existed to test between males; however, the large winter drawdown on Namakan Reservoir likely contributed to seasonal shifts in female home ranges. It is postulated that water fluctuations contribute to the detritus load of Namakan Reservoir Lakes, thereby causing a rapid increase in crayfish abundance and accounting for the observed differences in otter feeding habit between the two water regimes. (Abstract by Author)