Restoration Potential: Generally beavers easily can be reestablished in natural areas from which they have been extirpated. Simple protection from intensive trapping and, if necessary, translocation of animals trapped elsewhere are all that are needed if food supplies are adequate. Plans to reestablish beaver in an area should take into consideration the ecological and economic costs and benefits that may occur once the beavers begin building dams, cutting trees, and moving into adjacent available habitat as the population increases.
The recovery potential of sites damaged by flooding caused by beaver depends on the circumstances. In many instances (e.g., a flooded road or agricultural field) all that is needed is dewatering of the flooded area. However, little is known about the response and recovery potential of bogs and fens that have been flooded by beaver.
Management Requirements: Many areas inhabited by beaver, especially those also inhabited by humans, require active beaver management. The nature of the management depends on the particular conditions at the site and on the resource priorities established for the site (Allen 1987). In some situations beavers are desirable, whereas in other circumstances their presence may be detrimental to management goals. Often beavers present both positive and negative influences and a manager must weigh the benefits of beaver presence against the costs. The procedures discussed below cover most of the commonly encountered situations faced by those managing beavers.
The New York Department of Environmental Conservation (1991) pointed out that effective resolution of beaver-human conflicts is critical to maintaining public tolerance for beaver presence and that this tolerance is vital if the benefits of beaver presence are to be attained.
Protection of Individual Trees; Repellents
Individual trees can be protected by enclosing the bottom 1 m with heavy wire mesh, hardware cloth, or galvanized metal. The ends of the mesh should be wired together, rather than nailed to the tree. Leave enough space (about 15-30 cm) between the protective metal and the trunk to allow for tree growth; this also will prevent beavers from gnawing the bark through the mesh. Other coverings that sometimes have been successful include tar paper, Clark's tree wrap, and wood preservatives (e.g., Penta or creosote).
Commercial deer repellents (e.g., Thiram, Magic Circle, 10% solution of trinitrobenzene-aniline [TNB-A]) may deter beavers but may have an unpleasant odor (de Almeida 1987). The following results were reported in The American Tree Farmer (October 1983): full strength use of deer repellent (Magic Circle, $52 per 5 gallons in the late 1970s) was successful in repelling beavers every time. Beavers did not return when diluted repellent (1:50) was painted on the base of damaged trees along the shoreline. In a single pond in which the dam was blasted, a cord was run across the opening in the dam about 60 cm above the top of the remaining dam; rags were hung from the cord with the ends touching the dam and about 4 fluid ounces of repellent were poured on the rags; beavers did not return to the site for at least three years. In a 4-ha area occupied by beavers for 20 years, all dams were broken to a depth of 30 cm and rags were hung as described above; when diluted repellent was used, beavers rebuilt each dam within three months; with undiluted repellent, beavers did not return (frequency of reapplication was not stated). However, not all researchers have found deer repellents to be consistently effective when used as just described (Owen et al. 1984, cited by de Almeida 1987).
Another approach to protecting individual trees involves artificial feeding (leaving tree cuttings near beaver dens). This technique reduced damage to valued trees in Denver, Colorado, but obviously is not a long-term solution (Gray 1990).
Regulated Harvest Trapping and Nuisance Trapping
IMPORTANT NOTE: TRAPPING REGULATIONS VARY FROM STATE TO STATE; SOME OF THE PROCEDURES DISCUSSED IN THE FOLLOWING SECTIONS ARE ILLEGAL IN SOME STATES.
Beavers are most likely to become a management or stewardship problem when their population density is high. Regulated harvest during the furbearer trapping season is an obvious means of lowering population density and thus reducing the probability of a nuisance situation, while at the same time encouraging utilization of beaver as an economic resource. Moreover, this management approach can be employed at no cost to the landowner (trapper does the work for free in exchange for the benefits derived from the pelts obtained).
The level of harvest that is appropriate for a given area depends, of course, on management goals for that area. Annually removing about one-third of a beaver population (1.5-3 beaver per family, Ermer 1988) generally will prevent a population increase and will allow a sustained harvest (Hill 1982, Novak 1987a).
The traps usually used for beaver are no. 330 or 220 Conibear traps (kill traps), or no. 3 or no. 4 leghold traps (Hill 1982). Conibear traps are very effective and portable; time to death or unconsciousness is 1.5-9.25 minutes (Novak 1987b). Sets made with Conibear traps usually are easier to construct and can be used in a greater variety of situations than those made with leghold traps (Baker and Dwyer 1987); most professional trappers and fur trappers who are trapping principally for beaver use the no. 330 trap (Miller 1983). Two or three Conibear traps per family, set for about two weeks in winter during each of two years, effectively eliminated beaver in four Alabama watersheds (Hill 1976). Robertson (in Novak 1987b) captured 20 beavers per 100 trap nights using 330 Conibear traps in British Columbia. See Baker and Dwyer (1987) for further details on the use of Conibear traps for beaver. Novak (1987b) discusses the humane use of Conibear traps. He also describes some safety devices effective in preventing injury to the trapper. [Note: many states require first-time trappers to complete a trapper education course prior to obtaining a trapping permit. These courses demonstrate how to safely and effectively use traps.]
Leghold traps of size No. 3 or larger also are effective when properly used. To avoid escapes, these traps should be set to ensure drowning (Miller 1983). See Baker and Dwyer (1987) for further details on the use of leghold traps. These authors also describe the use of underice snare sets for capturing and killing beavers. In summary, underwater Conibear sets and drown sets employing leghold traps are recognized as legal, quick, and efficient methods for catching beavers (Tom Decker, Massachusetts Division of Fisheries and Wildlife, pers. comm.).
According to Hutchison et al. (1986), one professional trapper using water-based explosives and traps can wipe out an entire population by a single concentrated effort, with only occasional follow-up visits. These authors reported that professional trappers believe that the only way to protect timber from beaver is to remove beaver from the watershed; the trappers said that water control devices (see below) don't work, excluders (see below) are ineffective, and removing part of a population does not necessarily result in a lower water level. The trappers noted that a perpetual effort is needed to control a population if any beaver are allowed to remain in the watershed.
Miller (1983) stated that a week of trapping, especially if trapping is allowed in lodges and bank dens (prohibited by some states), usually is sufficient to remove all beavers from a single pond (cf. Novak [1987b], who reported that trappers rarely can catch all the beavers in a house). Miller noted that the removed beavers will be quickly replaced by others if beavers are common in surrounding areas. In Newfoundland, Bergerud and Miller (1977) found that one area from which beaver were removed was recolonized and occupied at former abundance levels within a few years. Miller described various effective trapping techniques and noted that it is not difficult for anyone with reasonable strength and some outdoor experience to become an effective beaver trapper.
Despite the difficulty that is sometimes encountered in totally removing beaver from an area, their confined ecological limits and low reproductive rate, together with the ease with which they may be trapped (usually), make beaver vulnerable to overharvest (Hill 1982).
Hutchison et al. (1986) pointed out a potential problem with removing beavers from an area--lowered water levels may expose the land to invasion by exotic plants. They did not provide any documentation for this point.
A final important note on kill trapping: beware of the possibility of capturing nontarget species--do not use exposed baits, which may attract raptors to sets (Bortolotti, cited in Novak 1987b); do not leave the remains of skinned animals near trap sets (may attract raptors or carnivores); properly adjust the pan tensioning devices on leghold traps (Novak 1987b).
Shooting
Hill (1982) stated that shooting can be effective in areas where boats and spotlights can be used (large rivers, lakes). According to Miller (1983), shooting rarely is effective for complete control and one's time would be better spent on trapping.
Poisoning
Hill (1976) found that strychnine alkaloid baits (sweetgum limbs painted with storax containing the poison) were consumed apparently without hesitation by both captive and wild beaver; most beavers died within 10 hours. He cautioned that additional work is needed to determine the hazards to other wildlife when the poison is used in this way. He also determined that the approximate minimal acute lethal dose of sodium monofluoroacetate (Compound 1080) was approximately 0.202 mg/kg. Hill warned that neither Compound 1080 nor strychnine are registered for use as a beaver control agent; their use for this purpose is illegal. Though initial research indicates that poisoning has some promise as a beaver control technique, political and practical considerations make it doubtful that this method ever will be an acceptable means of beaver control.
Live trapping
Sometimes a beaver problem can be alleviated by capturing and moving nuisance beavers to areas where their presence is not regarded as undesirable. It is a labor intensive, expensive method that generally is viable only when a dedicated group of volunteers is available to do the trapping and relocation. Usually Bailey or Hancock traps are used for live trapping beavers. The Hancock trap is preferable because of its greater catch efficiency and versatility (Novak 1987b). Apples and corn are effective baits (de Almeida 1987). Muller-Schwarze et al. (no date) noted that initial live trapping can be quite successful (one catch per 2-3 trap-nights). Later the beaver are more likely to avoid traps.
In some areas live trapping is not a feasible or allowable control method if the objective is to relocate the beavers. For example, the Connecticut Wildlife Bureau does not permit relocation of beavers because all suitable habitats are deemed to be saturated. As beaver populations and nuisance problems have increased, many states will not now accept beavers for relocation. Release of translocated beavers may not be appropriate in regions of high human density or where agriculture or road systems occupy low lying areas vulnerable to flooding.
Behavior and Maintenance of Live-trapped Beavers
Captured beavers can lacerate human skin with their claws and, as might be expected, can give a severe bite (Hill 1982). Some beavers may die of shock if restrained sufficiently long to attach ear tags (Hill 1982). Adults are too strong to handle without anaesthesia. Kits often can be handled without anaesthesia.
If beavers are to be penned for a few days prior to release, they should be kept wet and cool, and provided with water that they can enter for defecation (Novak 1987). Food (e.g. aspen twigs and leaves, apples) should
Novak (1987a) reviewed the literature on movements of transplanted beavers. The general pattern, based on several studies in various regions, is that a transplanted beaver may or may not move a great distance from the release site; many or most individuals can be expected to move at least a few miles, averaging perhaps 5-10 miles.
See Novak (1987) for a review of methods for tagging beavers. The most common method involves ear-tagging with No. 3 or No. 4 monel tags. Other methods employ colored or reflective markers, neck collars with lights, tattoos, tail notching, freeze-branding, fur bleaching, tetracycline, and radio collars (see Novak 1987a for references).
Immobilization
Seal and Kreeger (1987) recommended using ketamine (10 mg/kg) with acepromazine (0.2 mg/kg) in yearlings and adults; Lancia et al. (1978, in Hill 1982) reported that 10-13 mg/kg of ketamine hydrochloride plus 2.5 mg of acepromazine maleate was suitable for yearlings and adults. Ketamine alone is satisfactory for kits. The combination of ketamine and acepromazine produces sedation in 3-6 minutes with recovery in 1.5-7 hours. See Seal and Kreeger (1987) for additional information.
Biological Control There is no evidence that any nonhuman predator can be effective in controlling beaver populations.
Artificial Scent Mounds
Muller-Schwarze et al. (n.d.) found that artificial scent mounds were effective at deterring transient beaver from using existing but uninhabited lodges. Svendsen and Huntsman (1988) reported that scent mounds do not necessarily exclude nonresident beavers.
Chemosterilents
Chemosterilents have been field tested successfully on both males and females at Noxubee National Wildlife Refuge, Mississippi (Gordon and Arner 1976). The orally administered chemosterilents (17 alpha-ethynylestradiol-3-cyclopentyl ether and SC-24674 of Serle Laboratories; 1-2 mg/kg body weight, mixed with ethanol and placed in an apple), suppressed spermatogenesis and reduced ovulation and pregnancy. However effective application of this technique will have to await the development of efficient methods for treating wild beavers (Hill 1982).
Surgical Sterilization
Beavers should respond well to reproductive control because they are long-lived, apparently monogamous, and form discrete family units in which older progeny normally do not reproduce unless one or both parents are removed (Brooks et al. 1980; see also Muller-Schwarze et al., no date). Though surgical sterilization has limited application as a practical management tool, in. Denver, Colorado, where beavers are regarded as a nuisance along the Platte River Greenway and kill trapping is prohibited, the state wildlife commission funded a three-year control plan (in progress as of 1990) that involves surgical sterilization. Beavers are live-trapped by volunteers, sterilized by a volunteer veterinarian, who at the same time implants a radio transmitter. Radio-tagged beavers are being monitored by Division of Wildlife personnel. It is anticipated that the territorial behavior of the sterile animals will prevent nonsterile individuals from immigrating to the area and that population size will decrease due to lack of reproduction and immigration.
Exclusion Fencing
Fencing small critical areas such as culverts, drains, and small ponds or lakes sometimes can prevent damage, but often results in the beaver using the fence as construction material for a dam (Miller 1983).
Modification of Culverts and Beaver Dams
Important note: In some states, wetlands laws require that a permit be obtained before removing or destroying a beaver dam; this may entail a lengthy application and review process.
A common problem in beaver management is that of maintaining the water level lower than that sought by the beavers. Simply removing or breaching dams, or unplugging culverts, usually is ineffective because the beavers normally repair the damage within 24 hours. Daily destruction of dams or lodges sometimes may cause a family to relocate, but this is a labor intensive approach that is not practical in many situations. A better method of lowering the water level is to insert a drain pipe through the dam or culvert at the desired water level. In addition to decreasing the water level, an effective drain occasionally will cause the beavers to move to another site (Miller 1983). One method of drain installation, originally described by Carl Frentress, is included in Hammerson (1994) (note: most designs employ drain pipes of larger diameter than that described in this article).
Wire mesh cylinders (up to around four feet in diameter), rectangular home-made wooden pipes, and three-log drains have been used successfully to control water level in much the same way as described above. One successful design involves the use of a 4.5-m-long cylinder of concrete reinforcing mesh wrapped closely with one-inch by two-inch mesh fencing wire. This cylinder is sized to fit snugly inside a culvert (it also could be placed in a breach in a beaver dam). For most of its length, the protruding upstream end of the cylinder is surrounded by a larger-diameter cylinder of reinforcing mesh, with a six-inch space between the two-layered core cylinder and the outer mesh. Both ends of the core cylinder are capped with reinforcing mesh. The whole unit is held in place by metal fence posts driven into the pond bottom.
The three-log drain method (Arner 1963, Teaford 1986) is basically the same as the wire mesh cylinder method just described but substitutes for the wire mesh cylinder three green or waterlogged 10-16-foot logs (fastened together lengthwise, separated by spacers, and covered by roofing tin). Another technique substitutes for the wire mesh cylinder a rectangular wooden pipe that has the bottom side covered with wire mesh (Laramie 1963, Schemnitz 1980). See de Almeida (1987) and Schemnitz (1980:381) for some additional brief descriptions of drains that have been used to control water levels in beaver ponds.
Laramie (1963) noted that without monthly maintenance, about half the drain devices will fail.
Electrified Barriers
Electrified wires can be highly successful in controlling beaver flooding at sites where the activity of beavers is otherwise acceptable (Muller-Schwarze et al., n.d.). Such systems should be checked following heavy rains because rising water levels may short out the battery. The wires should be placed such that the beavers will not drag limbs behind the wire to plug a breached dam. Electrified systems work best in areas with little public exposure; otherwise vandalism probably will be a problem. Gaylord (no date) provided a description of a procedure for installing an electrified barrier.
Manipulation of the Food Supply
In the north and in the mountains, availability of a winter food supply is the most important factor affecting beaver distribution and abundance. Hence it is theoretically possible to affect beaver populations through manipulations of the vegetation. In general, activities that favor young woody deciduous growth will benefit the beaver. For example, clearing of tall hardwoods (for pine plantations) along streams along the Gulf Coast resulted in an abundance of beaver food (new growth) and nuisance complaints (Hill 1982).
According to Slough and Sadleir (1977), management of aspen, which depends primarily on fire to open up new areas for colonization, is a powerful tool in beaver management. They suggested that beaver populations (in the northern interior of British Columbia) would benefit from the protection of aspen from overuse by beaver by rotating the beaver harvest to allow aspen regeneration. In the Adirondacks of New York, Stegeman (1954) found that the size of aspen producing the greatest amount of beaver food per acre was the one-inch (2.5 cm) diameter class, which averaged 6.6 years of age. He suggested that the most advantageous rotation period for aspen for production of beaver food would be about 10 years.
Information gathered by Dieter and McCabe (1989) along a prairie river in South Dakota indicates that restricting grazing in the riparian zone would improve and expand beaver habitat.
Food supply manipulation is not an important management strategy in the south where food is not cached and where winter foods include many plant species (Hill 1982). Beier and Barrett (1987), who studied beaver habitat in the Sierra Nevada of California and Nevada, felt that the beaver's highly opportunistic food habits made it unlikely that beaver populations could be controlled through manipulation of forage resources.
Beaver, Cattle, and Stream Rehabilitation
Beavers have been used to help return to a predisturbance condition damaged streams in Wyoming and Oregon. In areas of southwestern Wyoming, overgrazing by cattle resulted in the loss of riparian vegetation, erosion, and lowering of the water table (Apple 1983, 1985). Initially an electrified fence was used to exclude livestock from a degraded stream course. This resulted in initial vegetative recovery and some bank stabilization. Then beavers were live-trapped and released in the enclosure (before complete willow recovery was attained). Several loads of aspen (stems 10-13 cm in diameter) were placed in the release sites coincident with the release of the beavers, which used the aspen to construct dams. The beaver dams changed a gully system into a pond system with adjacent meadows that benefited from the elevated water table. Willows recovered, with regrowth averaging 1.5 m tall at the end of the second year. In another degraded stream (unfenced) that beavers were attempting to colonize (but had failed to do so in the past due to their dams being too weak and washing out in spring), truck tires were wired together and placed across washed-out beaver dams. The beavers were able to build strong dams in these areas, impounding much more water than they could previously, and the dams withstood spring runoff. Mud bars and wetted sites above the dams provided excellent sites for vegetation reestablishment. The series of dams along the stream created conditions that prevented livestock from trailing up and down the stream, which aided in riparian recovery. The habitat changes induced by the beavers appeared to result in improved conditions for various wildlife species.
Munther (1981) stated that the immediate impact of cattle on a stream is often minor if beavers are established in wide valley willow habitats prior to the introduction of the cattle. Munther pointed out that in narrow valleys in western Montana, simultaneous feeding by beavers and cattle results in a gradual decline of willow and aspen due to removal of mature growth by beavers and harvest of new growth by cattle. Such a situation eventually may lead to the elimination of beaver, with subsequent further degradation of the vegetation by cattle as dams fail and the site becomes dewatered. The stream cuts a deeper channel, leaving the water table several feet lower than it was when the system was influenced by beavers. The end result is a system less suitable for both beavers and cattle. In western Montana, Munther felt that a year of rest from grazing was inadequate for rapid shrub recovery and that plantings (protected from livestock grazing) might be appropriate. He regarded the reestablishment of beaver and management for their continued presence as part of a well-conceived management strategy for grazed riparian ecosystems. Munther (1983) commented briefly on the incorporation of beavers into forest management in Lolo National Forest, Montana.
For information on the use of beaver to rehabilitate streams in Wyoming and Oregon, contact: Bruce Smith or Larry Apple, BLM, P.O. Box 1869, Rock Springs, WY 82901, and Jim R. Sedell, USFS, Forest Sciences Laboratory, Oregon State Univ., Corvallis, OR. Other contacts: Cliff Dahm, Oregon State Univ. Dept. of Fisheries and Wildlife; Greg Munther, U.S. Forest Service, Lolo National Forest, Missoula, MT 59801.
Managing Beaver Ponds for Waterfowl
Arner and Hepp (1989) discussed the importance and management of beaver ponds for waterfowl (especially mallard and wood duck) in the southeastern United States. Three situations were addressed: (1) ponds with shallow water and little emergent vegetation, (2) shallow ponds dominated by emergent vegetation, and (3) ponds with no possibilities for drainage. Recommended management techniques include installation of drains and planting of Japanese millet (in areas lacking suitable native vegetation, Teaford 1986) (situation 1); and maintaining or introducing favorable food and cover plants, and removing undesirable vegetation through burning, water-level manipulation, mechanical disturbance, and/or herbicides (situations 2 and 3). Arner and Hepp gave specific examples of favorable and unfavorable plants and provided some details on pond drains, burning, and herbicides.
Management Research Needs: Novak (1987) concluded that the following are important research needs: (1) quantification of when and how dispersing individuals die and how new colonies are established; (2) improved techniques for attaching radio transmitters; (3) development of a multiple-capture live trap, (a) for effective control of nuisance beavers, (b) to make trapping more economical for the trapper in the time it takes to catch a beaver, and (c) to enable the trapper to harvest selectively specific individuals from a group; (4) development of markets for the meat and other by-products of trapped beavers; (5) development of techniques to enable trappers to practice aspen regeneration around beaver ponds; and (6) development of techniques that can use the vast amount of data found at fur auction house. To these can be added several others, including the development of effective and efficient techniques for preventing flooding by beavers in areas where their eradication is not possible or desireable. Clearly research also is needed on the effect of the beaver on ecosystem structure and dynamics (e.g., in relation to rare wetland plants and animals).
Comments: Those working with beavers should be aware that humans can contract tularemia by handling carcasses or wet skins of infected animals, by being bitten by a tick or deer fly that already has bitten an infected animal, or by consuming infected meat or water. Symptoms of tularemia include a small ulcer, sore, or black scab where the germ penetrated, enlarged underarm lymph nodes, fever, and other complications (see Addison et al. 1987 for further details). The disease responds to the various antibiotics. A basic precaution in handling a beaver carcass is to wear rubber gloves and disinfect any equipment that may have been in direct contact with a carcass. However, tularemia is not present in beaver populations in some areas of North America.
Beavers may transmit GIARDIA LAMBLIA, a pathogenic intestinal parasite that has caused human health problems in water supply systems (von Oettingen 1982). However, research by Woo and Paterson (1986) casts doubt on the role of beaver in contaminating water supplies with GIARDIA. Human cases of giardiasis seem to be caused primarily by drinking water from lakes and streams that have been contaminated with human feces (LaBastille 1986).
