From Kill Trapping Records

Over the long term, controlled collections made with kill traps can give a lot of information useful to population studies. This method is much less laborious than live trapping, so can be conducted over a wide area, but it can only be used under certain conditions. Kill trapping can affect the density, social relationships, and dynamics of the target population, and, therefore, is inappropriate for many studies, especially those of native species of weasels that are protected. On the other hand, for populations subject to trapping for fur or for control to protect prey populations, calculation of a density index from kill-trapping data corrected for effort can provide important data impossible to obtain any other way.

This method requires that the traps be set out evenly, baited and checked daily, and operated in the same way and for the same number of days per session regularly all the year round. The theory is that, if the trapping operation has been absolutely consistent, or at least if the data are corrected for variation in trapping effort (McDonald & Harris 1999), then changes in the number of animals killed probably reflect real changes in the numbers of animals available to be killed. Likewise, if the living weasels of all ages and both sexes are caught at the same rate (or at least, that differences between them are constant), then changes in the proportions of animals of each age and sex caught should reflect real changes in the structure of the population observed.

It is important to spell out these rather obvious prerequisites, because much depends on them, and they are not always as true as one might expect. For example, if traps are baited, it could be argued that when more weasels are caught it is because they are more hungry, not because there are more of them. If that happens, then changes in the actual numbers of weasels become confused with changes in their willingness to enter traps ("trapability"). Capture-per-unit-effort indices cannot distinguish changes in numbers from changes in detect-ability (Anderson 2001).

We take seriously the potential errors introduced into any index of captures-per-unit-effort by the expected changes in behavior of weasels in response to bait and natural food supplies, but these errors are not necessarily fatal. On the one hand, although baited traps catch stoats more often than unbaited ones, both do show the seasonal variation in capture rate, which reflects a real variation in density (King & Edgar 1977). On the other hand, Teplov (1952) showed that when voles were scarce in a Russian game reserve, the local population of stoats fell, but the extent of tracks recorded in snow, and the numbers of ermine pelts harvested, both increased. It stands to reason that capture rate should vary with prey density in severe climates: Stoats must avoid chilling whenever possible, and can do so readily when food is abundant by staying in their dens and feeding from a cache. Then stoats will seldom encounter a trap. In mild climates, the risk of thermal stress is less restricting but stoats do still maintain smaller home ranges when food is abundant than when it is scarce (Chapter 8), so variation in capture rate with prey density is still likely (Alterio et al. 1999; King & White 2004).

Kill trapping disrupts a population if the trap sites are too close (Sullivan et al. 2003), so there is much to be said for avoiding removal sampling if alternative methods are available. On the other hand, appropriately designed indexing provides a practical method of monitoring populations that cannot be observed any other way (Caughley & Sinclair 1994). Weasels are among the species that present formidable obstacles to anyone attempting to make conventional population estimates, whereas low-intensity trap-night indices are possible and are consistent with other changes in the population that are also correlated with density. For example, after a productive breeding season, summer density indices and the proportion of juveniles collected do vary together, as expected (King & McMillan 1982). With all their faults, density indices are still useful for handling the readily available data on weasels collected in kill traps, so we will stick with them for the moment.

A simple relative density index for weasels caught on standardized lines is to calculate the number of captures per 100 trap nights, or C per 100 TN (one trap night equals one trap set for 24 hours), allowing for unavailable traps. For example, take a line of 150 traps, checked daily for 3 days. At the end of the session, the results could be tabulated as in Table 10.1. The real density cannot be worked out from the trap-night index without calibrating the index against known densities. Nonetheless, the correlation between them is fairly good, at least until the index exceeds about 20 C per 100 TN (Caughley 1977).

The most extensive data collected this way are for stoats in New Zealand, where the index for 3 months of trapping seldom exceeds 7C per 100 TN (King 1983b), although over shorter periods of a few days in early summer of a mouse peak year it can reach more than 30C per 100 TN (King & McMillan 1982). Jgdrzejwski (1995) demonstrated a straight-line correlation between a live-trap index and absolute density of common weasels in Bialowieza (Figure 10.1), and Erlinge (1983) did the same for live stoats in Sweden, but so far the relative density indices available from kill trapping stoats have not been converted into numbers of stoats on the ground. Although the indices are consistent with how we expect density to vary, both in time and by habitat, they cannot yet

Table 10.1 Calculation of a Density Index from a Hypothetical Set of Controlled Kill-Trapping Results

a

b

c

d

e

f

g

Traps

Traps

Stoats

Rats, etc.,

sprung,

untouched

caught

caught

empty

(b+c+d)/2

150 - e

C/100 TN

Day 1

144

2

4

0

3

147

2

139

5

4

2

5.5

144.5

3

144

3

2

1

3

147

Totals

10

438.5

2.28

Although 150 traps were set for three nights, the total number of trap nights is not 3 X 150 (= 450), because every trap that is set off, by a stoat or by any other animal, cannot catch again until it is reset. Assuming that, on average, each sprung trap is out of commission for half a night, half a trap night is subtracted from the total for every trap sprung, for whatever reason (Nelson & Clark 1973). The corrected total number of trap-nights is the sum of column f, that is, 438.5; the total number of stoats caught is 10; the density index is 10/438.5 X 100 = 2.28.

Although 150 traps were set for three nights, the total number of trap nights is not 3 X 150 (= 450), because every trap that is set off, by a stoat or by any other animal, cannot catch again until it is reset. Assuming that, on average, each sprung trap is out of commission for half a night, half a trap night is subtracted from the total for every trap sprung, for whatever reason (Nelson & Clark 1973). The corrected total number of trap-nights is the sum of column f, that is, 438.5; the total number of stoats caught is 10; the density index is 10/438.5 X 100 = 2.28.

be corrected for the ways that the activity, immigration, and trappability of stoats presumably change with food supplies.

A more complex method is available, derived from fisheries management and applied to mustelids by Fryxell et al. (2001). Fryxell and his team were concerned about setting the annual harvest quotas for American martens, an important fur-bearing species in Ontario, as a constant proportion of the available population. They did not have density indices, but martens are similar to their smaller relatives the weasels in that the local population density from year to year is strongly influenced by the success of the previous breeding season, which can be deduced from the age structure of the catch. They knew the age structure of the captured martens and, therefore, could calculate backward to estimate the minimum number of martens that must have been alive at any previous time. Fur harvest managers have used this technique successfully to regulate the marten harvest at close to the maximum sustainable, around 35% pelts harvested each year. The same technique could be applied to estimating the abundance of weasel populations, although no one has tried it, so far as we know.

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