Jared Walker Smith - 2005 Environmental Studies Senior Seminar Research Project
|How to Trap|
Most small mammals will never be easily observed in the wild. They may be nocturnal, crepuscular (active at dawn or dusk), or simply too well camouflaged to notice. What I am providing here is a guide describing the methods used to live-trap small mammals. There are many ways to capture or trap mammals - snap traps, live traps, pitfall traps, poison, shooting - but live-trapping provides the best way to survey a population without harming them. This is especially important for species that are rare or present in only small numbers.
This guide is designed to tell how trapping is done, rather than to encourage it due to the risks associated with contact with small mammals. It is recommended that only professionals attempt small mammal trapping. Trapping should only be conducted with care for both the trapper and the trapped animals, and direct handling of any wild animal should be avoided. If there is reason to believe a threatened or endangered species is present, a permit may be needed from the Minnesota Department of Natural Resources. For information regarding your area, DNR contact information is listed here.
One of the simplest and most effective traps is the Sherman live trap. The traps range in size, but for most small mammals (up to the size of a small 13-lined ground squirrel) a 3 x 3.5 x 9" trap works the best. Folding traps allow for easier storage and cleaning than non-folding traps do. Rubber gloves should be worn at all times when handling used traps.
Most small mammals of greatest agricultural abundance or significance are herbivores or omnivores. A simple mixture of rolled oats and peanut butter (chunky or smooth) is the most effective at attracting these species. Mix enough oats in with the peanut butter to form somewhat sticky balls. This bait can be either spread directly on the treadle (spring-plate inside trap), or wrapped in perforated wax paper. Cut wax paper into roughly 2 x 2" squares, place a teaspoon-sized amount of bait on paper, and wrap into a ball. Punch several small holes in each ball. This method is preferred as it minimizes mess inside of trap, and appears to minimize capture failures.
For attracting carnivores and insectivores, specifically shrews, better quality bait is needed. As recommended by the Canadian Ministry of Environment, Lands and Parks trapping protocols; walnuts, bits of earthworms, or insect larvae may be more attractive and provide better nourishment for shrews. Additional food should be included for shrews to prevent trap deaths.
The Survey Line
The next step is setting up a survey line or grid in the area of interest. The size of the surveyed area will depend on the number of traps available. Generally one or more trap lines should be set out, containing an equal number of traps. Traps should be spaced roughly 10 meters apart in each line, and the lines should be at least 10 meters apart. Traps should be placed to maximize chance of capture. Place as close to ground as possible, keeping in mind natural contours or cover. Flags should be used to mark traps and trap lines. For a general survey, traps will be set for a total of three nights in each trapping location.
Setting and Baiting the Traps
Traps should be set at dusk, shortly before the sunsets. To avoid accidental trap deaths, traps should never be set during rain, cold spells, or in exposed areas during the day (day trapping in shaded areas may be done, but should still be avoided in extreme heat). To set traps, drop 2-3 pieces of cotton in the back of the trap, and either spread bait on the treadle, or, if waxed paper balls are used, drop 1 ball into the back of the trap. Check to see that the trap can be sprung easily. Snap the trap's door into open position. Place the trap back into position in the line.
Traps should be checked immediately at dawn, shortly after the sun rises. Waiting any longer can expose animals to unnecessary stress that may result in serious injury or death. Again, rubber gloves should be worn at all times when handling traps. Avoid unnecessary shaking of the trap and never breath in trap dust. Respirators may even be necessary.
To check a closed trap, carefully lift the trap so that the front door is facing upwards. To avoid inhalation of scat, keep the trap downwind of your body. Slowly push open the trap door with a gloved finger or a ruler to see if an animal is inside. Some larger animals, such as 13-lined ground squirrels and short-tailed weasels, can be easily identified inside the trap. If a trap contains a large animal, identify it, carefully set the trap on the ground (upside down), and snap open the trap door with a ruler or stick to avoid being bitten. Back away and wait for animal to leave the trap, and then close it.
If the trap contains a smaller mammal, a quart or larger sized, sealable plastic bag with air holes can be used to aid in identification. Place the mouth of the plastic bag over the trap opening, and slowly tilt trap until animal slides into bag. Seal the bag, and identify the animal. Place the plastic bag close to the ground, open it, and wait for the animal to leave. Close the trap. Bags should be thrown away after checking all traps.
All traps should be closed after checking them in the morning.
Picking Up and Cleaning Traps
When a trapping session is completed, traps should be picked up and placed into a large plastic garbage bag. Care should be taken to not breath in trap particles stirred up by handling. Folding traps can be opened up for easy cleaning, and can be sprayed off with a hose. Traps should then be soaked in and scrubbed out with diluted bleach for sanitation. Traps should then be rinsed thoroughly with water, to avoid damage by residual bleach and to remove as much of the bleach scent as possible. Gloves should be cleaned and disinfected after each time traps are handled.
Special care is always needed when working around wild animals, both for their sake and our own. Rodents can carry many diseases, some of which are transmittable to humans. While the cold weather climate of Minnesota helps negate some viral risks, trapping should never be conducted without knowledge of the risks involved. The two diseases I will note here are rabies, and the Hantavirus pulmonary syndrome (HPS).
Rabies is generally easy to avoid - never let any wild mammal bite or scratch you simply by avoiding all direct contact with them. The Center for Disease Control (CDC) outlines the risks, symptoms, and prevention methods for rabies.
The Hantavirus pulmonary syndrome is by far the more serious threat when trapping small mammals. It is a deadly airborne virus that is transmitted through the urine, droppings, and saliva of four rodent species, two of which occur in the Cannon River Watershed; the deer mouse and the white-footed mouse. This map provided by the CDC shows the distribution of the deer mouse and the location of HPS cases.