Population Status of River Otter in Western Oregon

Abstract

Population status of the river otter (Lutra canadensis) in western Oregon was evaluated by using structural population modeling. Survival and recruitment rates for the female segment of the population were estimated from the age structure of a sample of 113 female otters obtained from trappers during the 1970-71 and 1971-72 trapping seasons. Age was determined from cementum annuli in canine teeth. Annual survival rates were estimated to be 68, 46, and 73 percent for age-classes 0, 1, and 2-11, respectively. Recruitment was 1.14 female pups per adult female at the beginning of trapping season. Comparison of estimated survival and recruitment rates, by the modeling approach, revealed that the population was stationary. J. WILDL. MANAGE. 41(4):692-699 The reported harvest of river otters by fur trappers in Oregon declined 39 percent from the 1966-67 to the 1969-70 trapping seasons (Oregon State Game Commission 1967, 1968, 1969, 1970), indicating a possible decline in otter numbers. The reduced harvest may not have reflected population change because factors other than density may have influenced significantly the size of the otter catch. Preliminary investigation of the relation between fur prices and number of otters harvested in Oregon showed a positive correlation. Also, trappers who were interviewed stated that adverse weather conditions during the trapping seasons (primarily flooding and ice) reduced trapping effort and success. This paper reports a study we began in fall of 1970 to evaluate the status of the otter population in western Oregon, using a method other than the index based on harvest. A method for estimating population trends by evaluating the balance between population recruitment and survival through the use of a structural population model (Henny et al. 1970) was chosen for evaluating the status of the otter population examined in this study. This method appeared well suited to this population for which no census methods existed and available indices were not reliable, but for which a sample of harvested animals was readily available. MATERIALS AND METHODS Carcasses of 113 female otters harvested by trappers in Oregon during the 1970-71 a d 1971-72 trapping seasons (15 Novemb r-15 February) provided material for study. All otters in the sample were harvested in the western half of the state (Fig. 1). Determination of Age Layering in the cementum of an upper canine tooth was used to determine age of otters. Root portions of canines were decalcified in either buffered formic acid (Luna 1960) or 5 percent nitric acid, sectioned with a cryostat, stained in Papanicolaou Hemotoxylin Stain Solution (Fisher Scientific Company), and examined under a microscope (Tabor 1974). Thin, darkstained bands in the cementum could be 1 Study conducted under the auspices of the Oregon Cooperative Wildlife Research Unit: Oregon Department of Fish and Wildlife, Oregon State University, U.S. Fish and Wildlife Service and Wildlife Management Institute cooperating. Oregon State University Agricultural Experiment Station Technical Paper No. 4210. 2 Deceased 7 July 1975. 692 J. Wildl. Manage. 41(4):1977 This content downloaded from 207.46.13.33 on Sun, 20 Nov 2016 04:21:46 UTC All use subject to http://about.jstor.org/terms OREGON RIVER OTTER * Tabor and Wight 693