Two Canterbury lakes are closed to the public after trout were found with a mysterious rash.
Concerns were raised when an angler found mysterious spots and red rashes on both brown and rainbow trout at Lake Sarah and Lake Grasmere.
Until experts know what caused the rash, they're warning Canterbury anglers to remain on high alert.
Fish and Game CEO Martin Taylor says they are clueless as to what's affected the fish.
"There's no idea what's causing the red rash at this stage. The samples we've taken have been sent to MPI."
The samples were gathered earlier this week and have left experts baffled about what the rash is or where it comes from.
"We've done a little bit of research, and haven't found any other situation like this that has occurred in New Zealand before," says Mr Taylor.
The picturesque lakes near Arthur's Pass are popular with anglers. It is unknown how long they will remain shut to the public, or if the mystery symptoms have spread beyond the lakes.
"It's purely precautionary," says Mr Taylor of the lake closures. "It's a biosecurity issue and we're taking the most cautious approach we can."
Until there are more answers, Fish and Game is warning Canterbury fishermen to be vigilant and clean any equipment thoroughly.
Anglers are being asked to exercise the patience they are famous for until experts can work out what is infecting the local fish.