SNAMP Pub #22: An evaluation of a weaning index for wild fishers (Pekania [Martes] pennanti) in California
Article Title: An evaluation of a weaning index for wild fishers (Pekania [Martes] pennanti) in California
Authors: Sean M. Matthews , J. Mark Higley , John T. Finn , Kerry M. Rennie , Craig M. Thompson, Kathryn L. Purcell , Rick A. Sweitzer , Sandra L. Haire , Paul R. Sievert , and Todd K. Fuller
- Understanding the influence of habitat and management on reproductive success is critical to understanding their effects on wildlife population dynamics.
- Concern for the conservation of the Pacific fisher has highlighted a need to develop cost-effective methods of monitoring reproduction within fisher populations.
- The goal of our study was to determine the effectiveness of nipple size as an indicator of weaning success for wild female fishers from northwestern coastal California and the southern Sierra Nevada of California.
Fishers (Pekania [Martes] pennanti) have been the focus of recent conservation concerns in the western states because of range contractions, their association with mature forests, and assumed sensitivity to habitat alteration, particularly extensive logging. The United States Fish and Wildlife Service concluded in 2004 that listing the West Coast distinct population segment of the fisher under the Endangered Species Act was warranted but precluded by higher priority listing actions.
Estimates of fisher vital rates, including reproduction, have been very difficult and costly to obtain and may vary widely based on habitat composition and prey availability. Female fishers have their kits in Feb/March and lactate until their kits are weaned at about 10 weeks of age, usually sometime in May. Out of concern for the fisher and her kits, female fisher are not captured during the birthing and rearing periods, making reproductive success harder to determine. This is an evaluation of whether teat size could act as an alternative indicator of reproduction within a population.
Using bait stations, 91female fishers were captured on 146 occasions and maintained radiocollars so they could be tracked using radiotelemetry between 2004 and 2011, as part of three different fisher studies in California: The Hoopa Fisher Project in the northwestern part of the state, and the Kings River Fisher Project and the Sierra Nevada Adaptive Management Project in the southern Sierra National Forest.
We evaluated the effectiveness of using nipple size as a predictive index of weaning success for females with known reproductive histories. We believe the use of weaning success is more appropriate than birthing or rearing success, as achieving weaning age increases the kits chances of survival. We calculated a weaning date for each study area by adding 10 weeks to the average date of kit birth for each area. We then calculated the number of days between weaning and capture date to account for reductions in nipple size post-weaning.
Female fishers were captured and radiocollared from June through March of 2004 through 2011, the equivalent of 6 denning seasons. The width and height of all 4 nipples were measured. We then classified each female into 1 of 3 reproductive classes based on our telemetry observations during the den season prior to capture: nonbreeder - females that did not exhibit denning behavior during the previous den season; attempted breeder - females that exhibited denning behavior but failed to exhibit behavior until weaning during the previous den season; and current breeder - females that exhibited denning behavior until weaning during the previous den season.
We then generated a predictive model of female fisher reproductive class using the data. We used a generalized linear model to account for the apparent relationship between reductions in nipple size and the number of days between weaning and capture.
We calculated the observed and predicted proportions of females that were current breeders annually for each study area. This proportion could be useful to managers as a component of a long-term population-monitoring program, particularly to evaluate the impacts of landscape-level change.
Our results indicate that nipple sizes measured during a live trapping season are useful in classifying reproductive status of wild female fishers. We measured the nipples of 46 current breeder, 8 attempted breeder, and 53 nonbreeder individuals.
We detected a general relationship between nipple size and days postweaning for breeders and nonbreeders. There was strong model performance for the training data sets and most of the test data sets. The mean percent correct classification rates for the training and test data sets was 89% and 72% respectively. The mean difference between our annual observed and predicted proportions on each study area using raw vote totals was 6.6%.
Nonbreeders were most accurately classified. This is probably influenced by the group being composed mostly of younger females. Current breeders were the next most accurately classified group with attempted breeders the least accurately classified. Possible contributors to our misclassifications were: male avoidance behavior, litter size, number of times kits were moved and the age of female.
We acknowledge that this assessment was conducted with low group sample sizes and that a larger sample might help to increase the accuracy of classification rates between breeders and attempted breeders. The high misclassification rate of attempted breeders, particularly attempted breeders predicted as current breeders, could introduce bias to an analysis with low sample sizes.
Biologists interested in using our approach for their own data can download the model supplements, enter their data, and predict the breeding status of their individuals.
Nipple size of female fishers measured during a live trapping season can be used as a cost-effective index of the weaning rates of adult female fishers. This index could prove useful for managers hoping to model fisher reproduction and the influence of habitat and other covariates on weaning success, particularly in landscapes managed for timber production.
Author(s): Sean M. Matthews, J. Mark Higley , John T. Finn , Kerry M. Rennie , Craig M. Thompson, Kathryn L. Purcell , Rick A. Sweitzer , Sandra L. Haire , Paul R. Sievert , and Todd K. Fuller
Source: Journal of Mammalogy, 94(5):1161-1168. 2013.
Published By: American Society of Mammalogists
The full paper is available here.
For more information about the SNAMP project and the Fisher team, please see the: Fisher Team Website.