13. Action Plan for North American Otters
Paul Polechla
North America, defined here as Canada and the United States of America, has two otter species, the sea otter (Enhydra lutris, which is covered separately in Chapter 8), and the North American river otter (Lutra canadensis). Like its Eurasian counterpart, the North American river otter ranges widely in aquatic habitats, utilizing rivers, lakes, and freshwater marshes. It also occurs on rocky sea coasts, for example in Alaska, California, and British Columbia, and in coastal saltmarshes on the Atlantic seaboard. The species originally ranged from 25° to 70° N latitude and from 53° to 166° W longitude (Hall 1981) and occurred in virtually every major drainage basin and wetland habitat type north of the Rio Grande and Colorado river drainages (see Figure 1). The range contracted markedly during this century (see Figure 2) although otters have more recently been found over a wider area (see Figure 3).
Fossils of the North American river otter are known from the early Pleistocene in North America (Zyll de Jong 1972). Archeological remains, dating from 200 B.C. (Polechla 1987) to the mid-1400s (Wright 1987) indicate that the North American river otter has long been harvested for fur and meat. There is also documentation of the hunting and trapping of otters by Native Americans, immigrant Europeans, and their naturalized descendants from the 1500s to the present (Hutchins 1987; McGee 1987; Obbard 1987; Polechla 1987, 1989b; Ray 1987).
The diet of the North American river otter consists largely of fish, with crustaceans, amphibians, reptiles, birds, and insects generally of lesser importance (Mason and Macdonald 1986a; Melquist and Dronkert 1987). Slow-moving fish and prey unable to escape detection are selected preferentially. Otters are considered beneficial to game fisheries (Melquist and Dronkert 1987). Although there are numerous reports on diet, only a few studies have investigated the prey availability to the otter (e.g. Melquist and Hornocker 1983; Mack 1985).
Melquist and Dronkert (1987) reviewed several river otter behavior studies which used telemetry. Individual home ranges varied from 4–78 km in length. Adult males generally have the largest home ranges, especially in the spring breeding season, while the ranges of lactating females are the most restricted. Home ranges of individuals, both males and females, over-lapped considerably in study areas in Idaho, Louisiana, Missouri, and Alberta, but territoriality was indicated by similar studies in Texas and coastal Alaska. It has been suggested that territoriality may occur in unexploited, well-established otter populations in areas where food supplies are evenly distributed (Melquist and Dronkert 1987). Population density estimates range from 1 otter per 1.2–2.1 km of coastline in Alaska to 1 otter per 4–17 km in freshwater habitats (Melquist and Dronkert 1987).
Riparian vegetation is an essential component of otter habitat and waterside vegetation may also attract beavers, which create ponds and lodges that are later used by otters (Melquist and Hornocker 1983; Larsen 1983; Woolington 1984; Polechla 1987). River otters den and temporarily rest in a multitude of natural cavities. In Idaho, one animal used 88 different sites during a 16 month period and the local population of about 23 otters used 15 different sites 1,283 times. Of these repeatedly used sites, 32% were in bank dens of beaver, 18% in log jams, and 11% in riparian vegetation (Melquist and Hornocker 1983).
Polechla (1987) reviewed the studies concerning the reproductive biology of the North American river otter. The North American river otter reaches sexual maturity in one or two years. Mating occurs from winter to early spring. Delayed implantation occurs, with the development of the embryo arrested following fertilization. Timing of reproductive events varies according to latitude. Implantation occurs from November in Florida to March in Alaska, with subsequent birth dates varying from January in Florida to May in Alaska. Embryonic litter size varies from one to six, with an average of two or three.
In some areas North American river otters may be active during the day, but generally they are nocturnal or crepuscular. The basic social group consists of a female and her offspring with male otters playing little or no part in the family group (Melquist and Hornocker 1983). A population of otters in coastal northern California segregated into two distinct social groupings, one group consisting of a maternal female, her pups and adult female offspring, the other consisting entirely of males (Shannon 1989). Social behavior therefore appears to be flexible. Olfactory communication between members of social groups and between groups is important and involves scent-marking with feces, urine, and anal sac secretions (Melquist and Dronkert 1987). Scent-marking activity increases during the breeding season (Humphrey and Zinn 1982).
Despite the potential availability of tissues due to legal trapping of otters, few studies of bio-accumulating contaminants have been made. Metals are widely distributed in tissues at low concentrations and elevated levels of mercury have been reported from Georgia (Cumbie 1975). Wren (1985) reported the death from mercury poisoning of a wild otter in Ontario. Organochlorine pesticides and PCBs have been reported in a number of studies (see review in Mason 1989). PCBs were considered to be likely causes of declines in otter populations in Oregon (Henny et al. 1981) and New York (Foley et al. 1988).
During the late 1800s and early 1900s, the synergistic effect of wetland destruction, pollution, and overexploitation for furs was devastating to North American river otter populations. Other direct causes of otter mortality include incidental take during beaver trapping, drowning in fishing nets, and road kills. Populations were reduced to the greatest extent in regions where: 1) human populations were dense; 2) agriculture or industrial practices were intense; 3) wetlands were naturally sparse (i.e. in semi-arid grasslands of the Great Plains and arid deserts of the southwestern United States); or 4) oligotrophic waters could not support an adequate prey base (e.g. the north slope of Alaska, northern Northwest Territories, and the northern Hudson Bay region).
Conservation measures implemented at the beginning of the 20th century, namely restricted trapping and hunting seasons, preserving wetlands, and public education, undoubtedly contributed to the initial recovery of the North American river otter populations. The reintroduction and restocking of beavers from the 1920s to the 1950s also had a positive influence on otter populations. Otters benefit from a facultative commensalism with beavers (Tumlison et al. 1982; Reid 1984; Polechla 1987, 1989b). Beaver dams increase wetland area (Novak 1987a) and otters take refuge in beaver lodges (Melquist and Hornocker 1983; Reid 1984; Polechla 1987, 1989b). Furthermore, prompted by low grain prices, an Act was passed by the U.S. Congress in 1985 allowing flood-prone agricultural land to revert to wetland vegetation (Polechla 1988). This is expected to have a positive influence on otter populations. However, North American wetlands have become increasingly polluted because of the relaxation in enforcement of anti-pollution legislation, and pollution may be the most severe long-term threat to otter populations (Wren 1987).
Sources of information
Information regarding the legal status of North American river otter populations was obtained from questionnaires mailed to furbearer or endangered species biologists (or equivalent) in the 49 continental United States and the 13 Provinces, Territories, and islands of Canada. The addresses were obtained from the National Wildlife Federation's Conservation Directory (National Wildlife Federation 1989). The prospective respondents were asked to list legal status as protected, harvest permitted, or unknown. A 100% response was obtained through follow-up telephone calls when necessary (see Table 1).
Assessment
The North American river otter is designated as an extirpated species in Indiana and Prince Edward Island, although there have been recent reports of accidental deaths in both areas (S. Johnson and R. Dibblee respectively, pers. comm.). North American river otters are protected in 21 of the United States. Trapping is permitted in 27 states and 11 Canadian Provinces and Territories. Minnesota allows a harvest in the northern part of the state where populations reportedly are large and stable but not in the southwestern part where populations are low. Populations were reported to be stable or increasing in all states and provinces where trapping is allowed (see Table 1).
Sources of Information
Table 1 also summarizes the response by each state or province to the questions concerning the population status of the North American river otter. Respondents were asked to describe the population of this species in their area as extirpated, declining, stable, increasing, or unknown. These determinations were based on trend estimates, parameters, indices, and density estimates for each population. Methods used by the states and provinces included questionnaires, track and scat surveys, age structure analyses, analyses of natality and recruitment rates, and density estimates based on field surveys (i.e. radio tracking and radioisotope marking). The quality of the data and the reliability of the status determinations are unknown. Harvest data are available for all the states and provinces that allow trapping.
Assessment
The population status was unknown in Utah, although Bich (1988) recently surveyed northern Utah. Colorado reported a declining population despite an ambitious reintroduction program. There were no detailed reports on population status (per se) in the states of Arizona, Indiana, Iowa, Kansas, Kentucky, Nebraska, North Dakota, Ohio, South Dakota, West Virginia, Wyoming, and Prince Edward Island. There is considerable concern over the status of the subspecies sonora in the southwestern United States. An Arizona specimen, believed to be L. c. sonora, was collected as late as 1970, and otter tracks were found in New Mexico during a reconnaissance trip down the Rio Grande Gorge in April 1989 (P.J. Polechla, pers. observ.). Despite the persistence of records of occurrence, there have been no intensive field surveys for L. c. sonora. To confound the issue, other subspecies are being released into adjacent parts of Arizona and Colorado (Britt et al. 1984; T. Beck, pers. comm.).
Beaver dams enhance the wetland habitat, and thus the conservation of beavers has facilitated the recovery of otters in some parts of the North American interior (Polechla 1987, 1989). Although beavers provide habitat for otters, they also conflict with human interests by flooding such areas as agricultural land, commercially important forests, and roadways (Novak 1987a). Trapping is done to alleviate these problems as well as to provide beaver furs. The 330 size Conibear trap, used most often to capture beavers (Novak 1987b), also captures otters incidentally, since they frequent the same habitats (Polechla 1989). Otters are also caught in Conibear traps targeting the exotic coypu or “nutria” (Mowbray et al. 1979).
To summarize, North American river otters are most abundant in wetland areas (Fremlin 1974; Tiner 1984) of the Mississippi river delta and adjacent Gulf Coast (east Texas to Florida), the Atlantic Coast (Florida to Newfoundland excluding large metropolitan areas such as New York City, Washington, D.C., and Philadelphia), the Great Lakes region, the southern Hudson Bay region, and the Pacific Coast (northern California to Alaska). Their current distribution (Figure 3) reportedly is greater than it was in the 1970s (Figure 2).
Table 1. Legal and population status of the North American river otter (Lutra canadensis) in the United States and Canada.
The interior regions of North America (the Appalachian Mountains, Rocky Mountains, Midwest and Great Plains of the United States and Canada, and the arid southwestern United States) have remnant indigenous populations that reportedly have been increasing slowly. The North American river otter historically occurred in these regions (Hall 1981) but probably at relatively low densities due to the low density of wetlands.
Since 1977, all otter species, including the North American river otter, have been listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) (Greenwalt 1977), with the exception of those already listed on Appendix 1. Trade in otter pelts can only be sustainable under strict trapping seasons and management programmes, as described below.
CITES stipulates that harvest of Appendix II species must not be detrimental to the survival of the species or population. It is on this basis that export permits for the North American river otter are issued. Each exported pelt must be affixed with a sealed federal tag bearing a stamped code identifying the species, the state or province where it was captured, year of capture, and a unique number (Polechla 1987). This procedure allows for tallying the number of North American river otter pelts exported during an individual trapping season. Tags are issued to qualifying states or provinces prior to each harvest season. Personnel of the U.S. Fish and Wildlife Service and the Canadian Wildlife Service, game wardens of state, and provincial wildlife agencies and customs agents enforce the tagging requirements. The Endangered Species Scientific Authority (i.e. the Office of Scientific Authority, U.S. Fish and Wildlife Service) has adopted additional requirements developed by the “Working Group on Bobcat, Lynx, and River Otter” (C. Dane, pers. comm.). These include further restricting seasons or trapping methods, registering tagged pelts, and determining acceptable harvest levels annually (ESSA 1978 in Halbrook 1978). The number of CITES tags issued for the next season is based on past harvests and estimates of various population parameters. Harvests are limited to winter in all states and provinces.
The “Working Group on Bobcat, Lynx, and River Otter” set the following minimal requirements for assessment of river otter populations: information on:
population trend, the method(s) of determination to be a matter of state or provincial choice;
total harvest;
distribution of the harvest;
habitat evaluation (ESSA 1978 in Halbrook 1978).
These requirements are being met in most cases, although the quality of data in some instances is poor. For example, population status or “trend” information is of varying quality among the states and provinces that harvest otters. The regulation of the harvest in the North American river otter, including the sealing of pelts, may in itself provide little information on population trends, for harvests are influenced by fluctuations in pelt prices, supply and demand, and unpredictable weather conditions (Melquist and Dronkert 1987).
The CITES tagging system allows for enumeration of pelts that are traded internationally, thus providing a reliable estimate of total annual export. It also identifies the state or province where the animal was harvested (summarized in Melquist and Dronkert 1987). Some information on habitat usually is reported, at least in a cursory way. North American river otter pelts are traded “cased” and usually “leather out”, whereas those from other parts of the world are “open”. This means that pelts originating from North America have a totally different appearance from those from all other origins. International trade in the North American river otter can therefore be regulated without the risk of rarer species being traded under its cover.
Figure 1. Original distribution of the North American river otter (Lutra canadensis) in the United States and Canada (from Hall (1981) and Polechla (1988).
In the past decade many states and provinces have conducted studies relating to the status of otter populations. A computer and bibliographic search on the North American river otter (Tumlison and Shalaway 1985; Polechla in litt.) revealed 165 references published in the ten years between 1976 and 1985, significantly more than the 128 reports published in 100 years from 1865 to 1965.
Altogether, 991 North American river otters have been reintroduced or restocked in 15 states and provinces in the North American interior. An additional three attempts are planned. Minnesota has captured otters from the northern part of the state where populations are high and released them into the southern part of the state where populations are low. Missouri's release program has been the most ambitious and arguably the most successful (see Chapter 7). Most of these programs have documented survival of individuals of two years or more (Polechla 1989a), although reproduction of reintroduced animals has been recorded in only a limited number of instances (Erickson et al. 1984). The ultimate test of success is documentation of sustained population increases.
Habitat destruction, including urban and agricultural development.
Pollution from a variety of causes including release of heavy metals, PCBs, and pesticides into watersheds, as well as acidification due to mining operations.
Harvesting that is not based on adequate population data.
Incidental mortality during trapping for other species, especially beavers and coypu.
The subspecies Lutra canadensis sonora, which occurs in the Rio Grande, Colorado, and Canadian river drainages of the southwestern United States and possibly Mexico (J. Pablo Gallo-Reynoso, pers. comm.) (see Chapter 12) is imperiled, largely because of over-grazing and failure to maintain year-round minimum in-stream flow (Bradley 1986; Bich 1988). Scattered reports of otters inhabiting this region persist (Britt et al. 1984; Stahlecker 1986).
Critical evaluations of current status determinations are of highest priority for this species, especially in states and provinces where harvest is permitted. Further take should be prohibited in any state or province that cannot scientifically demonstrate 1) that its otter population is stable or increasing, and 2) that the level of take is not jeopardizing the population. Consistent analytical procedures and decision criteria should be implemented by all states and provinces in which the species is harvested. A combination of methods, including the following, should be used in determining population status: density estimates, natality and recruitment estimates, age structure analyses, track and scat surveys, questionnaires, and harvest data analyses. Where possible, radio-tracking should be used to obtain population parameters and radioisotope tagging should be tested further as a means of obtaining density estimates (Shirley et al. 1988).
Extensive and intensive studies of the distribution and relative abundance of L. c. sonora in the Rio Grande, Colorado, and Canadian River drainages of the southwestern United States are urgently needed. In the interest of maintaining genetic diversity (Hummel 1987), release of otters from other areas should be curtailed until the status of this subspecies is fully assessed. Additional studies of the North American river otter in the North American interior (e.g. the Great Plains of southern Canada and midwestern United States) are needed.
Carcasses from the fur harvest and accidental deaths should be analyzed more often to increase knowledge of pollutants, reproductive biology, parasite load, diet, and taxonomy. A detailed and geographically comprehensive analysis of pollutant levels is needed.
Guidelines provided in Chapter 7 on reintroductions should be followed prior to release in order to avoid needless deaths of otters, waste of time and money, and genetic mixing.
All beaver management plans should be developed to safeguard otter populations as well. In areas where otters are endangered, the use of Conibear traps should be prohibited. Modified leg hold-traps (Shirley et al. 1983) or Hancock live traps (Northcott and Slade 1976) should be used in areas where beavers have become a nuisance and otters are endangered, taking into account any new developments in trap technology to make these more humane. Education and enforcement programs should be instituted to ensure that these guidelines are met (e.g. Anderson 1982).
Cooperative studies with aquatic biologists are needed to determine the North American river otter's habitat requirements, prey preferences, and role in the ecological community.
Cooperation is needed between neighboring states and provinces in studies of North American river otters within major drainage systems.
A concerted effort should be made to refine and publish final reports in order to disseminate the latest knowledge on the species.
A symposium on the ecology of the North American river otter is needed to synthesize current knowledge and to provide direction for future research and management. The last one was held in 1984 in Columbia, Missouri.
Figure 2. Distribution of the North American river otter in the United States and Canada, circa 1977 (from Deems and Pursley (1978)).
Figure 3. Distribution of the North American river otter in the United States and Canada, circa 1988. Stars represent locations of accidental deaths or reliable, recently reported sightings. Dots represent sites of reintroduction or restocking. Question marks represent locations of unverified sightings. Data were derived from questionnaire responses (see text).