The use of trapping records to monitor populations of stoats Mustela erminea and weasels M-nivalis: the importance of trapping effort

1. Trapping and hunting records are frequently used as an index of animal a bundance. This study demonstrates that these records can be misleading if s ampling effort is not controlled for. 2. Mean numbers of stoats Mustela erminea and weasels M. nivalis trapped by British gamekeepers have been decreasing since 1975 and 1961 respectively, giving rise to concern that populations of both species may be declining. However, trapping effort has not been quantified over this period. 3. A total of 203 gamekeepers in England were questioned about the trapping effort they expended and the number of stoats and weasels they trapped in 1997. The most significant factor affecting the number of stoats and weasel s trapped was trapping effort. 4, Gamekeepers that relied on hand-rearing game birds for shooting regarded stoats and weasels as a less serious problem, and made substantially less trapping effort, than gamekeepers that relied on wild game birds. 5. The national decline in the numbers of stoats and weasels trapped may be the result of a decline in stoat and weasel populations. However, the decl ine is equally consistent with a reduction in trapping effort, correspondin g to a national increase in reliance on hand-rearing game birds for shootin g. 6. When the effect of trapping effort was controlled for, the number of wea sels trapped by gamekeepers in 1997 was significantly lower in the south-we st than in other regions of England and was unusually low in some local are as. 7. Trapping records can be used effectively to monitor populations of stoat s and weasels, as long as gamekeepers record the number of traps set in eac h month and monthly totals of animals killed. Ideally, the sex of each anim al and whether it was trapped or shot should also be recorded. Similar modi fications should also be made to other wildlife monitoring schemes based on trapping and hunting records.