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Oamaru Sports and Outdoors co-owner Richard Hill said he believed "100 per cent" in the story of a 24.9kg brown trout being caught.
But he said the proof remained elusive and could completely disappear if the uproar over the catch didn't die down.
"The guy who caught it, he isn't a recluse. He's just a guy who wants to be left alone. He's a shy guy.
"He says, 'when I've got it mounted, I'll put it out for the public to see. If they keep going, I'll put it in my wardrobe and no one will see it'.
"If he keeps getting hounded, he will clam and we won't see it."
The legendary size of the fish, caught in canals in Mackenzie Country, has sent fishing circles into a spin.
The fish would be more than 5kg heavier than the current International Game Fishing Association record of 19.1kg.
Fish & Game NZ has called for the angler to come forward with the fish, which was apparently caught about two weeks before Christmas after an epic two and a half hour battle.
The catch was witnessed by someone known to Hill, who said he believed the eyewitness account.
"I was dumbfounded when I was told. I thought, that's pretty unreal. The man who told me is not an idiot, not a liar."
The man who caught it apparently wanted no photographs taken, wrapping the fish in his shirt with the intent of having it mounted.
Until then, he intended to keep the fish secret; "If it was me, I couldn't wait to show it," Hill said.
Hill also believed he knew the identity of the angler, saying it was someone who had held a fishing licence for more than 50 years.
Fishing guide Michael Vetters, who is based out of Gore, was astonished to hear of the size of the fish.
However, he also said the extraordinary conditions in which the fish grew meant there were anglers who were less inclined to applaud such a weighty catch.
The fish was pulled from the Pūkaki-Ōhau A canal near Mt Cook. It's a catchment which benefits from a nearby salmon farm which has food pellets in the waterway.
The easy supply of rich food leads to massive growth in trout with out-size brown and rainbow trout being caught.
Vetters said the difference between the canal brown trout and those caught in rivers was significant.
"Eighteen pound (about 8kg) would be a real big bugger that would grow naturally. For something to get up to 55lbs (24.9kg) would be really impressive."
While he acknowledged catching the big fish required skill and technique, the fishing community was divided over the pellet-fed behemoths.
"To me, they're not a real fish. I like the wild trout."
In Twizel, the nearest town to the canals, Razza Bar & Bistro owner Mike Darling is waiting on the angler to claim the $1000 bounty and free taxidermy he has offered for a fish topping 50lb (22.7kg).
Darling had believed it only a matter of time before a trout so large would emerge from the canal.
He had a photograph on display of a trout with a rabbit in its jaws - a stunning illustration of the size of canal-grown fish.
The offer would be a considerable saving to an angler with taxidermy on such a big fish likely to cost thousands of dollars. Darling made it on condition the mounted fish - with the angler's name - would be displayed in his bar.