Masters Theses

Date of Award

12-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Wildlife and Fisheries Science

Major Professor

Joesph D. Clark

Committee Members

James B. Layzer, Lisa I. Muller, Frank T. van Manen

Abstract

The southeastern United States has a great diversity of freshwater mussel species, many of which are threatened with extinction. The muskrat (Ondatra zibethicus) is one of the few animals to extensively prey on freshwater mussels, and it has been implicated in retarding the recovery of, or further threatening, some endangered mussel populations. Muskrat predation on mussels may be significant at Mammoth Cave National Park (MCNP) where a river otter (Lontra canadensis) restoration has been proposed. Anecdotal observations suggest that the reintroduction of river otters dramatically reduces muskrat populations, which, in tum, may reduce the number of mussels consumed by muskrats. As a first step to determine the potential effects of river otter restoration on muskrat and mussel communities, I established baseline information on mussels, muskrats, and river otters and documented ecological relationships between muskrats and mussels on the Green and Nolin rivers in MCNP.

I used radio telemetry, stable isotope analysis, and GIS habitat modeling to characterize muskrat movements, diets, and habitat use, respectively. I captured 50 muskrats (29M: 21F) between June 2002 and August 2003 and placed radio collars on 12 adult muskrats. Average linear home range for muskrats with > 10 locations was 410 m (SD = 277). Predation by mink (Mustela vison) and owls was the most common cause of death for collared muskrats. I submitted hair samples from 49 muskrats, toe samples from 35 muskrats, and 5 muskrat food items for stable isotope analysis of carbon and nitrogen ratios. Stable isotope methodology for diet studies is based on the premise that isotopic ratios of heavy to light elements in animal tissue reflect those of their diet and V ) thus, can be used to quantify the importance of a food resource. Different tissues from the same animal can provide dietary information representing different time spans. Animal matter comprised approximately 45% of adult muskrat diets based on hair samples and 28% based on toe samples, as estimated with a multiple-source mixing model. Trophic level estimates of 2.40 and 2.19 were also consistent with an omnivorous diet. I used GIS coupled with the Mahalanobis distance statistic to predict muskrat habitat use and sites of mussel middens. There was a strong positive relationship between the 2 models (r2 = 0.68, P <0.001) suggesting that mussels are an important component of muskrat habitat use at MCNP.

I used scent stations to detect river otter presence on the Green and Nolin rivers. River otters visited scent stations on 16 occasions (2.2%) and 2 otter spraints were found along the rivers. I concluded that otters existed within MCNP, but the population was small or transient and restricted to the Nolin and the impounded part of the Green rivers.

I used spotlight surveys to monitor muskrat populations. Numbers of muskrats seen during surveys conducted from January to August 2003 reflected the expected seasonal decline and recovery of the muskrat population. Linear regression analysis revealed that 44.6% of the variation in numbers of muskrats observed was related to time or variables correlated with time (i.e., water level and water temperature), suggesting that the observed trend was due to actual changes in the muskrat population rather than responses to environmental factors. Power to detect declines in muskrat populations with spotlight surveys varied from 0.18 to 0.94 over a range of possible scenarios. Reducing the between-night variation of spotlight surveys will be important to increase power to detect potential declines in muskrat populations due to otter restoration.

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