METHODS AND MATERIAL
NOTES ON POPULATION ESTIMATION
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Using the Visible Implant Fluorescent Elastomer (VIE) Tagging System to Mark Salamanders
Christopher A. Binckley, Bodo Plesky, Katrin Werner and Sam Droege
USGS Patuxent Wildlife Research Center
12100 Beech Forest Rd., Laurel, MD 20708-4038
In March 1998, several experiments were initiated to investigate bias and variability of coverboard monitoring techniques for terrestrial salamander populations. The objective of one of these experiments was to estimate population size and variability of capture probabilities of red-backed salamanders (Plethodon cinereus). For that study we needed a means to individually identify red-backed salamander without affecting their recapture probabilities.
Due to the reports of negative influence of toe-clipping amphibians, (e.g., Clark (1972) for Bufo woodhousei fowleri), and the small size of red-backed salamanders (6.4-12.7 cm) the Visible Implant Fluorescent Elastomer (VIE) tagging system was chosen as marking method for this capture-recapture experiment. This system was developed by Northwest Marine Technology Inc. (phone (360) 468-3375, http://www.nwt-inc.com/) and provides the possibility of individual marks using several fluorescent colors and tag locations. This system was initially tested on fishes with recent tests on amphibians for such as larval Northwestern Salamander (Ambystoma gracile), Pacific Giant Salamander (Karl Mallory pers. comm.) and several species of Ranidae (Northwest Marine Technology Inc. pers. comm.).
Methods and Material
Six study plots were constructed at the Patuxent Wildlife Research Center (PWRC), located in Laurel, MD. Each plot consisted of 50, 12 X 12 X 1 oak cover boards placed 6 m apart in a 10 X 5 grid wherein was found numerous natural cover objects. Primary sampling period occurred from March, 1998 to April, 1998. Plots were checked weekly for a total of 6 times.The fluorescent elastomer system is bio-compatible and consist of two materials (color elastomer and curing agent) which were mixed at a 10:1 ratio. After mixing, the material was placed into 0.3 cc syringes and stored until use in a freezer to slow hardening, which will occur within 24 hours at room temperature. While in the field syringes were kept in a thermos bottle with ice. Four fluorescent colors (green, red, orange, yellow) were used. Injected pigments are difficult to see under ambient light but are visible in ultra-violet-light. Furthermore, colors green and yellow were difficult to distinguish in UV light thus we used either yellow or green for each plot. Colors remained visible throughout the spring with no apparent degradation of their quality. Results from fish studies indicates that these marks should be visible for the life of the salamander. In order to mark each salamander uniquely, a different color pigment was injected using a 0.3 cc syringes under the skin of salamanders in 1 to 4 body locations (behind the front legs and in front of the hind legs or , if possible, in the legs itself) (Figure 1). Injections into the legs showed up extremely well. Using three colors and 4 locations allowed 255 unique marks per plot. Before and after injection, the syringes were cleaned with an alcohol wipe to reduce the likelihood of infections. After marking, tags were checked for visibility using a hand held UV light and remarked, if necessary.
Figure 1. Marking Positions.
Salamanders were placed in small zip lock plastic bags with several drops of water to avoid dessication and for easier handling. Salamander were firmly compressed against the edge of the bag or a fold to keep them still, the bag placed on a clipboard (Figure 2) or on the workers knee, and held there with one set of fingers while injecting with the other hand. The elastomer pigment was than injected through the plastic bag into the salamander (Figure 3). No anesthesia was needed.
In Shenandoah National Park the VIE system was also used on two Desmognathus species. This technique may work equally well with these group (Sam Droege pers. comm.). In particular their larger legs make injecting into both front and rear legs more practical than most Plethodons (Droege per. Obs.).
During the Spring 1998 we marked 459 red-backed salamanders (P. cinereus) with a fluorescent elastomer at 6 plots with 50 artificial cover objects each plus numerous natural cover objects. In all, we had 135 recaptures and 103 individual recaptures. The decreasing count of captured salamanders was due to increasing temperatures during the sampling periods. The number of recaptured salamanders increased slower than the number of total marked individuals. The lower number of recaptures at the last period could have been an effect of increasing temperatures and decreasing soil moisture. Most recaptures occurred the next time period after an individual was marked with recapture probability decreasing as time since marking increased.
This graph compares the capture and recapture rates between natural (nco) and artificial cover objects (aco) during the Spring 1998. Data are from 6 separate sampling periods and 6 plots with 50 aco each. Difference in the total number of captured salamanders might be due to differences in areas covered by nco and aco. The number of total recapture events includes salamanders which have been caught several times. The recapture rate under nco is 3X (13.1%) lower than the rate under aco (39.4%). This could be due to better visibility of salamanders under aco than nco, where the salamanders are often hidden in the nco. Thus, aco have advantages for mark - recapture studies compared to nco.
This graph compares the recapture rates of salamanders (P. cinereus) with 1 to 4 marks.
A common marking method for salamanders is toe clipping. It has been described as convenient and the least expensive technique for marking salamanders (Donnelly et.al 1994). However, one disadvantage is that marks are not permanent which is necessary for numerous studies. Karl Mallory (British Columbia, Canada) used two techniques, toe clipping and the fluorescent elastomer tagging system on larval Northwestern Salamander (A. gracile) and Pacific Giant Salamander (D. tenebrosus). He reported toe clipping for these larval salamanders was not a viable marking technique since toes grew back after 3 months (Karl Mallory pers. comm.). Between June and October 1996, he caught and successful marked 421 Pacific Giant Salamander (D. tenebrosus) with the VIE tagging system. 55 of these larvae were recaptured in 1996 while 63 were recaptured in summer 1997. Continuing with this technique in 1997, he marked 471 larvae and recaptured 127 at last once (Karl Mallory pers. comm.).
Notes on Population Estimation:
The collected field data are currently being analyzed using program CAPTURE. Capture is a comprehensive computer program to compute population estimates from marked populations. Capture provides several models and combinations of models which can be summarized by 3 types of unequal capture probabilities.capture probabilities vary with time or trapping occasion (Mt) capture probabilities vary due to behavioral response (different capture probabilities for marked and unmarked animals) (Mb) capture probabilities vary due to heterogeneity (Mh)
Combinations of these 3 types are also available.
So far our preliminary results indicate that population size vary greatly among study plots. Plot 4, located in an old-growth forest, shows the highest population estimate based on Model Mh. Plot 6 and Plot 2 ,located in former farmed sites (70 previously), show low population sizes based on Model Mh. We opted for Model Mh since the field data point to capture probabilities which vary from individual to individual and in 5 of the plots the model selection procedure, included in program capture, chose Model Mh as reasonable estimator.
We would like to thank Karl Mallory for sharing his elastomer marking experiences from British Columbia.
Clark, R. D. 1972. The effect of toe clipping on survival in Fowler's toad (Bufo woodhousei fowleri). Copeia 1972: 182-185
Donnelly, M. A., C. Guyer, J.E. Juterbock, and R. A. Alford. 1994. Techniques for marking amphibians. In W.R. Heyer, M.A. Donnelly, R. W. McDiarmid, L. C. Hayek, and M. S. Foster, Measuring and Monitoring Biological Diversity. Standard Methods for Amphibians, pp. 277-284. Smithsonian Insitiution Press, Washington, D.C.