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At least one of the 20 rare snails released back into the wild at Solid Energy's Stockton opencast mine a fortnight ago has been eaten.

Solid Energy chief executive Don Elder said the predation, probably by possums or wekas, was what the company had expected. It disproved a Department of Conservation expert's opinion that predators were not a problem.

"We found the shell, and that's what's unfortunate about this. We find hundreds of empty shells up there, and the only way you find a shell is if a snail dies of old age or they've been eaten and the shells are left in open ground, which is where we tend to find them. So predators are clearly a huge issue up there."

The Powelliphanta augustus snail was among around 2000 which had previously been moved from the Mt Augustus area to fridges in Hokitika.

The survival of the 20 freed snails is being compared with 20 snails which were moved directly from the Mt Augustus ridgeline to a new site, and a control group of 20 which remain within their natural habitat on the ridgeline. All the snails have been fitted with transponders so their movements can be monitored.

"Most of the snails that we released seem to be doing very well and moving very happily," Dr Elder said. One had slid 10m in the first day. Dr Elder said he hoped more snails would be set free soon, as the company's wildlife permit had intended.

"While it's completely up to DOC, and nothing to do with us, we're paying, and ultimately the taxpayer is paying, for the snails in the fridges.

"It would make a lot more sense to be releasing a lot more of them. I think that's probably going to start happening pretty quickly."

Solid Energy still had 40 snail collectors combing the ridgeline.

The project was costing the company about $1000 a snail, down from the original $8000 because of the number of snails found.

He said Solid Energy expected to find at least 5000 snails, but could not stop searching until it was sure it had every one.