MUSKRATS....  Management Options

    Muskrat populations may be managed at a level at which damage is tolerable and sensitive areas are protected, through a combination of lethal control (trapping and baiting) and habitat modification.  Local populations may be removed by baiting with anticoagulants or by trapping.  Removal should be systematic and aim at reducing the population by at least 75% to have any long-term effect.  Because muskrats have 2 or more large litters per year and can disperse long distances to establish a new home, treatments must be repeated as new muskrats are detected.  Monitoring of populations to determine the effectiveness of controls used, and to determine when an area is reinvaded are key components of a management strategy.    

Legal considerations
     Although muskrats are classified as furbearers (California Fish and Game Code Section 4000), if they are injuring property, they may be taken at any time and in any manner in accordance with the California Fish and Game Code (California Fish and Game Code Section 4180).  If a trap is used to remove muskrats for damage control, the fur of the animals may not be sold (California Fish and Game Code Section 4005).  In accordance with Section 4004(d), traps set to remove a depredating animal should bear an “identifying mark in a manner specified by the department” (ie Fish and Game).  Traps must be visited and all animals removed at least once daily.  Inspection and removal must be done by the person setting the trap, the owner of the land where the trap is set, or an agent of either.  Leg-hold traps are banned in California (California Fish and Game Code Section 4004).

Habitat modification
    In some areas it may be possible to modify the banks to make them less favorable to muskrats.  Muskrats can be prevented from burrowing into banks by reducing the slope or covering banks with rip-rap.  Muskrats need at least a 10-degree slope and low sand content in the soil to establish permanent burrows.  For bank and dam alteration, a slope no steeper than 18 degrees (approximately 1-ft height to each 3-ft length) may be sufficient to prevent burrowing.  Rip-Rap should consist of coarse gravel between 1- and 2-inches diameter, placed 3- to 6-inches thick and 1- to 2-feet above the water line to 4-feet below.  Removal of emergent and bank vegetation such as rushes and cattails will also discourage muskrats but may increase erosion and negatively affect bird, fish, and invertebrate habitats.

Physical Exclusion
    Wire netting or chain link fencing placed along banks may prevent muskrats from burrowing into the bank and thus be used to protect sensitive areas.  This may be especially appropriate near housing.  The netting must be of 2-inch mesh or less and placed so that the bottom is at least 4 feet below the water line and the top several feet above.

Trapping
    Muskrats are probably the easiest aquatic furbearer to trap. Although labor-intensive, if undertaken systematically, trapping may be effective in removing local populations.  Trapping often fails to control muskrat populations because trappers move to a different location as soon as trap success begins to decline. 

    Conibear® No. 110:  This is probably one of the most effective and commonly used kill trap for muskrats.  The trap is set in a run, house or den entrance, or under a feeding house.  The most effective sets are those placed in “runs” or trails where the muskrat’s hind feet have scoured out a path into the bottom from repeated trips into and out of the den. The trap should be placed as close to the den entrance as possible without restricting trap movement.

    “Stovepipe trap”:  This trap is very effective in farm ponds, rice fields, and marshes.  It requires more time and effort to set than the Conibear®, but can be very effective if the correct size is used. The trap is not available commercially, but is inexpensive and easy to make.  When properly set in a well-used den entrance, it will make multiple catches. The trap must be staked down properly and set right up against the den entrance to be most effective. As the name implies, the traps can easily be made from stovepipe, but other materials may be substituted.  An example is a sheet metal, 6- x 6-inch rectangular box, 30- to 36-inches (76 to 91 cm) long with heavy-gauge hardware cloth or welded wire doors. The doors are hinged at the top to allow easy entry from either end, but no escape. Death from drowning occurs in a short time.

    Cage trap:  The most effective of these is the double-door colony trap. This trap has a door at each end and placed in muskrat runs both above and below water. It may also be set on a floating platform. This trap can make multiple catches before it needs to be reset. 

Shooting
        Where it is legal and can be done safely, shooting may remove small numbers of muskrats. Concentrated efforts must be made at dusk and during the first hours of light in the early morning.

Baiting strategies
         The County Agricultural Commissioner should be contacted to determine any restrictions and regulations pertaining to baiting muskrats.  Baiting may be most successful during winter months when natural food supplies are at there lowest.  Anticoagulant baits (diphacinone and chlorophacinone) are registered for muskrat control.  These baits decrease the clotting ability of blood so that an animal receiving a lethal dose dies as a result of an internal hemorrhage.  To be effective, muskrats must consume several doses of bait over a 7-d period.  Because a bait residue is stored in the liver and other tissues of the animal consuming the bait, there is a risk for secondary poisoning of predators or scavengers.  If the bait is used in accordance to label directions, and carcasses of poisoned muskrats are collected and disposed of, risks may be minimized.  Dogs are especially sensitive to diphacinone so where there is a risk of a dog consuming a poisoned muskrat, chlorophacinone should be used instead.  Vitamin K is used as an antidote to treat any cases of accidental poisoning of a child, pet or other nontarget species.

        Bait may be placed as blocks secured to the riverbank or in floating bait stations near banks.  Both methods require placing the bait near signs of muskrat activity, and can be used simultaneously.  Bait blocks are made of paraffin and treated oat grain. These may be secured to the bank near burrow entrances, feeding sites, runways, and around other signs of muskrat activity.  Their use by muskrats should be monitored daily and blocks replenished as needed until signs of muskrat activity have declined.

        There are several designs for the floating bait station.  Generally these consist of an enclosed wooden box (at least 12- by 18-inches) with one to four entrances approximately 4-inches diameter.  This bait station is mounted onto a floating platform.  The platform is made by attaching ¼-inch plywood to a styrofoam pontoon.  One to 4-lb bait (grain or palletized bait) should be placed inside the box in a container.  Effectiveness of bait stations may be improved by camouflaging the station with vegetation such as cattails and bulrushes.

Landowners must be able to frequently monitor bait station use.  Because muskrats have small home ranges, in areas of high muskrat activity it may be necessary to place a bait station every 60 ft along the river.  Stations should be placed within 2 to 10 feet from the bank and adjacent to burrow entrances, feeding sites, or other muskrat signs.  Stations should be checked daily at first and supplied with sufficient fresh bait until signs of use dramatically decline.  Muskrats may take vegetation into bait boxes, and this must be removed to prevent bait becoming moldy.  A continuous supply of fresh bait must be provided.  Even if all the bait is consumed, a single day of heavy consumption will not effectively reduce population numbers.

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