Stoat population 'exploding'

Checking one of 65 new predator traps in the Landsborough Valley late last year are Fulton Hogan...
Checking one of 65 new predator traps in the Landsborough Valley late last year are Fulton Hogan’s Central Otago regional manager Anna Sinclair and New Zealand Deerstalkers Association’s Central Otago branch president Reid Gare. PHOTO: REGAN HARRIS
A recently deployed predator trapline in the Lower Landsborough Valley has killed 158 stoats in less than three months, a level of success the Department of Conservation (Doc) considers "very concerning".

The kills, tallied by the Central Otago branch of the New Zealand Deerstalkers Association (NZDA) whose members voluntarily clear the 65 new traps, were recorded over four trapline runs between November and February.

NZDA  Central Otago branch secretary Lex Coutts said the stoat population appeared to be "exploding", especially when compared with the number of rats (26) and mice (7) trapped in the same timeframe.

"We’re getting lots of younger animals."

Mr Coutts said the kill rate was so high they had brought their February trapline run forward by two weeks in order to keep up.

Doc operations manager Wayne Costello said there were generally more stoats around in summer with kits leaving their dens, but "the high numbers of stoats being caught in traps this summer was unexpected".

He said the experience of volunteers on the new trapline matched those of Doc workers, who manage an existing 280 trap boxes in the Upper Landsborough Valley.

"Stoat catches were low over last winter then rose sharply in December."

Mr Costello said the rise had been preceded by an unexpected spike in the populations of rats and mice, both of which are common sources of food for stoats.

Tracking data for rat and mice populations showed their growth had reached "critical levels" by October last year, leading Doc to trigger an "urgent" aerial 1080 drop which took place between January 29 and 30.

"The aerial 1080 operation over 33,695ha targeted rats, stoats and possums and included the lower Landsborough."

Mr Costello said post-operation monitoring would take place this month, and Doc expected to see rat and stoat numbers fall "well below critical levels".

The product of a collaboration between Fulton Hogan, NZDA’s Central Otago branch   and Doc, the 65 traps were installed along a 12.5km stretch of the Haast and Landsborough Rivers late last year.

The initiative aimed to complement existing Doc operations in the upper Landsborough Valley, which represents more than 25 years of work to both reduce predator numbers and aid the native bird population.

According to data from Doc’s annual bird counts, the Landsborough Valley has seen a steady increase in the populations of several bird species, including tui, grey warbler/riroriro and yellow-crowned parakeet/kākāriki.

The most notable development was observed in the mohua/yellowhead population, whose numbers increased from 14 in 1998 to 485 in 2022.