After labelling the SPCA "environmental bandits", economist Gareth Morgan has offered to pay the society a $5 bounty for every free-ranging, homeless cat destroyed by an authorised facility.
"Any cat that is free to range should be a dead cat," Dr Morgan said.
The philanthropist economist's Cats To Go campaign has sparked an international debate between cat lovers, conservationists, animal welfare groups and scientists over domestic cats' role in killing native species, especially birds.
"If a cat's not chipped, it's euthanised - Jesus, you'd think this was radical, but this is what we do with dogs," Dr Morgan said.
Under the bounty regime, all cats turned in to authorised disposal facilities would first be tested for microchips to determine ownership. Owned cats would be returned to their homes, ideally with a fine for the owner.
SPCA national chief executive Robyn Kippenberger said bounty systems were an ineffective "nonsense" that were often abused by people who disliked or had cruel intentions towards cats.
"Not every cat is microchipped - your darling moggie could be whipped up and then we become the cat disposal society. We're not a killing shop and I don't think anyone in New Zealand would expect us to be."
A $5 bounty would not be enough to humanely destroy cats - an operation that can cost the society up to $20. Dr Morgan's money would be better spent helping the society to desex animals, Ms Kippenberger said.
Dr Morgan also railed against the SPCA's trap, neuter and return policy, which returns stray cats to colonies. He called for people to stop donating to the SPCA until the policy was discarded.
Ms Kippenberger said Dr Morgan was "showboating" and that the system was the only one proven to control and reduce stray cat populations.
Wellington's Zealandia wildlife sanctuary conservation manager Raewyn Empson supported Dr Morgan's calls for robust debate around responsible cat ownership, but not his euthanasia solution.
In July Ms Empson urged cat owners not to replace their pets once they died, so as to curb the deaths of native birds reared in the suburban eco-island.
Yesterday she said euthanising free-ranging cats was "a really gnarly question". "Who's going to make that decision that a cat does not have a home? It's a very delicate and sensitive issue."
Dr Morgan has also proposed a public meeting in Karori to discuss how to enhance the "halo effect" around the sanctuary that would allow birds to fly in and out without being attacked by cats.
"Why is the council putting money [into Zealandia] when it's just the most expensive cat food factory imaginable?" he said.
Research published in the science journal Nature this week backs his claims, showing cats are the No 1 threat to wildlife in the United States and kill between 1.4 and 3.7 billion birds yearly, more than four times the previous estimates.
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