A 1080 drop at Maruia killed 79% of rats — not the 95% the Department of Conservation wanted.
The rat plague was so bad a few months ago that visitors reported them climbing over tents, and nesting in tractors.
Doc operations manager Shane Hall said on Thursday it monitored rats using tracking tunnels before and after the Te Maruia aerial drop in the Lewis Pass Scenic Reserve, which was completed on November 3.
Before the operation rats were recorded in 94% of tracking tunnels — the highest level at any predator control site this year.
"Following the operation rats were recorded in 45% of tracking tunnels, which is higher than was hoped for.
"Modelling shows this equates to 79% of the rats killed by the aerial 1080 operation, taking into account the ‘saturation effect' where tracking tunnels become saturated with footprints and cease to show further rat population growth.
"We were aiming for a kill of at least 95% of rats to best protect vulnerable species at this site, so this is a disappointing result."
Mr Hall said they thought the sheer number of rats and amount of beech and tussock seed probably contributed.
