Submitted to: State of Louisiana Parish of Jefferson Director of the Department of Environmental and Development Control
Prepared by: Robert H. Chabreck Ph.D. and John A. Nyman Ph.D.
August 1995

EXECUTIVE SUMMARY

The need for nutria damage control and the options available to control nutria populations and damage in the drainage canal system of Jefferson Parish Louisiana were examined. Recommendations were made based on biological economic and social consequences of different control measures. In addition to overriding constraints imposed by public safety damage control efforts are constrained by animal welfare and cost effectiveness.

The most common nutria damage results from burrows which the animals construct as underground dens. The burrows normally extend 4 to 6 feet into canal banks but some may be as long as 50 to 150 feet. Burrows can undermine a levee system and cause bank cave-ins. Burrows undermine drain pipes and roadways and increase bank erosion. Comparison of nutria damage estimates with nutria abundance indices indicated that canal damage is greatest where nutria density is highest. The ultimate need of damage control activities must be measured in terms of the cost of repairing nutria damage.

Nutria lived in Jefferson Parish drainage canals for decades but canal damage was not noted until nutria populations began to grow following the decline of commercial trapping in the late 1980's. Bank damage was first noted in the early 1990's and $8 0 0 worth of damages have accumulated even though banks of only 14% of the canals were heavily damaged. In the 280 miles of drainage canals in Jefferson Parish the nutria population is estimated to be in excess of 9 200 animals and could exceed 10 0 animals by 1996. Nutria numbers and nutria damage will likely continue to increase and spread throughout the canal system unless strong control measures are implemented. The incidence of nutria-borne diseases that humans can contact are also likely to increase as the nutria density increases.

The existing damage accumulated during the last four years at an annual rate of about 3.5% and a cost of about $2 0 0 per year; and if no action is taken to control the nutria population the population will continue to increase until it reaches a high density in all susceptible canal segments. In addition to taking no action damage control options fall into two other broad categories: (1) make the canals resistant to nutria damage and (2) reduce the number of nutria in the canals. The cost of no action provides a basis for determining the cost effectiveness of all control measures.

Making the canal banks resistant to nutria damage would involve lining canal banks with concrete or other materials. Bank lining would remove the bank habitat used by nutria but would also destroy habitat used by other wildlife. Although bank lining is feasible it is impractical to implement on a wide scale because the cost is estimated to approach 2 to 8 million dollars per mile of canal. Although impractical on a wide scale basis bank lining might be useful on a limited basis if canal segments exist that are particularly sensitive to nutria activity.

Reproductive inhibitors that reduce the growth of wildlife populations have been suggested as a method of reducing the density of nutria; however no contraceptives have been tested or approved for use on nutria. Unfortunately non-lethal nutria removal techniques are unavailable because reproductive inhibitors are not available and there is no suitable unoccupied nutria habitat in which to release captured nutria. Available lethal removal techniques include trapping shooting and poisoning. In addition to public safety and effects on non-target species the practicality of the techniques is determined by the humaneness and effectiveness of the techniques.

Shooting has been an effective tool in wildlife damage control for eliminating small isolated groups of animals rather than for reducing the numbers of widespread populations. Trapping over bait is more efficient than shooting and should be more useful at moderate nutria densities but would be too time consuming and costly in areas with high nutria density. The only rational technique for removing a significant proportion of a high density population is poisoning. Zinc phosphide is a more humane poison than traditional rodenticides because (1) it requires a single dose rather than multiple doses to cause death and (2) because death generally occurs within 30 hours rather than within 3-4 days of consumption. Zinc phosphide poses a lesser threat to non-target species than traditional rodenticides because (1) it breaks down almost immediately in the environment when exposed to water and (2) it breaks down almost immediately inside animals thus reducing the chance of secondary poisoning.

Zinc phosphide is delivered to nutria on sweet potatoes that are coated with vegetable oil and placed on floating rafts at night and is used in nutria population control to prevent death of non-target species. The vegetable oil is needed to slow the breakdown of the poison caused by moisture in the air; the sweet potatoes are needed because they are a bait that attracts nutria. Using sweet potatoes insures that carnivores will not consume the bait using large cubes of bait insures that the bait is unattractive to many wildlife and cannot be swallowed by birds and small reptiles putting the bait out in the evening and picking it up in the morning insures that only nocturnal species have access to the poison and placing the bait on rafts in the middle of canals insures that the bait is accessible only to aquatic wildlife. Furthermore emetics can be added to the poison to insure that humans and most animals will immediately vomit any poison should some be consumed (rodents cannot vomit). These procedures are therefore expected to prevent death of any non-target species in the drainage canals except Norway rats.

We recommend frequent at least annually and perhaps quarterly assessment of nutria damage and nutria numbers by canal segment. The strategy to be pursued in a given canal segment during a particular time period should be based on the size of the nutria population and the extent of damage observed in that section of canal. In canal segments with low nutria numbers and slight damage we recommend no nutria removal. In canal segments with moderate numbers and moderate damage we recommend trapping over bait using drowning sets or shooting. Poisoning with zinc phosphide would be less expensive but it appears that the residents of Jefferson Parish would prefer the extra cost over the use of poison when both strategies are available. In canal segments with high nutria density however the intensity of trapping required to reduce nutria density is probably too costly and time consuming. In canal segments with high nutria density and heavy damage we therefore recommend poisoning with zinc phosphide. If some canal segments are discovered to be extremely sensitive to nutria activity such that even low density causes rapid extensive damage we recommend that bank lining be considered.