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The Department of Conservation (DOC) plans to release another 200 rare snails back into the wild at Stockton open cast mine next week.

The snails have spent several months in DOC cool stores in Hokitika.

The release would test the logistics of releasing large numbers prior to freeing another 1000 snails, said DOC conservancy advisory scientist Ingrid Gruner.

DOC began releasing snails last month, when 20 were returned to the wild to test their reaction to release.

They have been compared to another 20 snails moved from the ridgeline to a new site and a 20-strong control group which remains in its natural habitat on the ridgeline.

All are equipped with transponders so DOC can monitor them. Dr Gruner said only one had died -- one of the control group was eaten by a predator.

Solid Energy received ministerial permission in April last year to move a population of Powelliphanta augustus snails from the Stockton ridgeline so it could mine up to $400 million worth of coal.

So far 2220 snails have been caught. DOC predicted the total would reach 5500, Dr Gruner said.

Fifty had died in captivity, but only 24 of the deaths were unexplained. The others had been damaged during harvesting. The mortality figure was of no concern, she said.

Overall, the snails in captivity, and those which had been released, had done well.

DOC had increased its cool store capacity as more snails came in. It had four cool stores at present, each holding 600. Each snail had its own two-litre plastic container, apart from six now in an environmental chamber.

Dr Gruner said the snails were hermaphrodite (an animal with both male and female sex organs) but DOC did not know if they could fertilise themselves.

"They can store sperm. They have laid eggs in captivity, but the eggs take about a year to develop. We can't say at the moment whether they are viable eggs or just dummies but we keep them. In a year's time we'll know."

DOC had received 500 eggs, including 76 laid in captivity. The remainder had come from Stockton, and 14 of them had hatched in captivity. DOC planned to make environmental chambers, to imitate the natural environment, and store the remaining eggs.

"Hopefully, that will provide the triggers for the eggs to hatch, but that's all experimental at the moment."

The eggs were 5mm-6mm in diameter and looked like a miniature chicken egg, with a calcium shell, she said.

"When they're freshly laid they're quite glossy and pinkish looking. After a day or two they take on the tannin of the soil around them and they turn quite brown."

The snails usually laid one or two eggs at a time, she said. The snails, which eat earthworms in the wild, are fed a worm a fortnight from three different worm species.

They showed no preference, Dr Gruner said.

"They suck them in -- it must be similar to eating spaghetti."

The 20 snails released in December had moved from their release site in a patch of forest to scrub about 15m away. The scrub was more like their natural environment.

"Some of them have moved 10m in two nights, so they can move that distance quite happily."

Some of those to be released next week would also be equipped with transponders.

"They'll all go into the same broad area. There are several patches of habitat we think are suitable."