The rat trap pictured in the article is interesting. Small CO2 cylinder releases gas into a capture chamber.
From my basic understanding of mammal physiological responses to gases: wouldn't nitrogen gas (as an oxygen displacer) be less stressful than CO2 (as an oxygen displacer)?
The CO2 cylinder you see there is not to release CO2 into the rat's breathing atmosphere. It is more like the use of CO2 in an air pistol.
The depicted product is a Goodnature A24 self-reloading rat trap. According to the manual:
The A24 is powered by a CO2 gas canister and will last for 24 kills. The trigger is placed directly below the lure, so that when the rodent moves the trigger, the trap fires a piston directly into the back to the rodent’s head, killing the rodent swiftly and humanely.
All mammal species in New Zealand except the native ground bats are introduced species, many of them invasive. They wreak havoc on the local ecosystems and have brought quite a few unique bird species near the brink of extinction. I think its absolutely understandable that the New Zealanders want to preserve some of their endemic birds.