The Department of Conservation says 56 hunting parties in the Landsborough Valley — from behind Bruce Bay to Haast — are affected and some have cancelled their block for next season.
DOC South Westland operations manager Wayne Costello said the beech mast (seeding) prediction for 2023 indicated that rodent numbers were unlikely to meet the threshold required for a control operation in the Landsborough in 2023-24.
However, recent rodent tracking met critical levels, suggesting that beech seeding and subsequent rat breeding was higher than models had predicted.
DOC has now triggered an urgent predator control operation from January 2024.
"This work is critical to further protect the species that have been benefiting from predator control at this site," Mr Costello said.
"We would normally try to get this type of operation done in November/December (it was last done in December 2019).
"However, the timing of the review, which showed what was actually happening with rat numbers, meant the soonest we could get a job planned and under way was January."
That meant the poison would be dropped only a month or two before the 2024 roar period, affecting ballot holders with allocated blocks in the Landsborough, Clake and MacFarlane areas.
DOC staff contacted all the block holders whose blocks were affected, and offered them different options.
There were 56 parties impacted either directly — within the footprint of the 1080 operation — or because the buffer area extended into their block.
Of those, 42 chose to retain their block allocation and go hunting; six chose to go into a special second chance draw; six chose to cancel and roll their block/period into the 2025 roar; and two cancelled and were refunded.
Mr Costello said he understood there were moderate deer numbers there.
"The roar blocks in this valley are popular. The 1080 operation is not expected to have a significant impact on trophy hunting through the roar (the animals there that will be trophies would have survived numerous other ops in the past). "However, that is not to say that deer won't be affected at all."
The Landsborough Valley was a stronghold for biodiversity due to sustained ground and aerial predator control undertaken over at least the past 25 years, Mr Costello said.
In years when beech species seed heavily, rats (and then stoats) boom too. When the supply of beech seed is exhausted or germinates, these predators target birds and nests.
DOC's team of bird experts spend several days in early summer each year doing five-minute bird counts at 174 stations in the Landsborough Valley. Together they have counted about 106,000 birds since the monitoring began.
Mr Costello said DOC staff directly contacted the Game Animal Council, New Zealand Deerstalkers Association and NZ Tahr Foundation to explain the hunting situation.
"We have been thanked for being upfront with everyone involved."