Decline in capture rate of stoats at high mouse densities in New Zealand

We present two statistical models documenting variations in density indices of stoats and of mice in New Zealand southern beech (Nothofagus spp.) forests. They confirm previous, simpler correlations showing that the summer capture rate of stoats increases with spring mouse density index up to about 20–25 mouse captures per 100 trap-nights (C/100TN). However, at much higher mouse densities (60–80 C/100TN), observed in the Grebe and Borland Valleys in southern Fiordland in 1979/80 and again in 1999/2000, fewer stoats were caught than expected. These models quantify a serious decline in capture rate of stoats during periods of high mouse abundance over the range 25-80 C/100TN. At such times, management strategies aiming to protect threatened birds by intensive lethal trapping of stoats during the nesting seasons may be least effective just when they are most needed.