WHOPPER: Carl Frauenstein with 27 pound brown trout, caught in the Ohau Canal in the McKenzie Country.

South Islanders are telling tales of giant fish in their canals. Rachel Young investigates what's making them grow so large.

Most people scoff when fishermen tell tales of giant catches, but in the central South Island the fairytales are becoming reality.

Fish & Game central South Island officer Graeme Hughes, who is the organisation's longest serving employee, has witnessed first- hand the "phenomenon of the canal fishery".

Hughes said as salmon farms developed along the central South Island canals, the number of fish, mainly brown and rainbow trout, had increased in numbers and size as they dined out on excess food leached into the water. The fish farms try to get "thousands" of salmon up to maximum size quickly so feed them high protein quality pellets specifically manufactured for commercial fish farms to help them put on weight as fast as possible. The food not eaten by the caged salmon drifted in to the wild.

"The trout, and escapee salmon, have learned that there are free meals under the salmon cages on a regular basis . . . those wild fish and escapee salmon will continue to grow if they are not caught." As the farms developed, news of the "large fish phenomenon" had spread. "There are huge fish to be caught there and a lot of them."

However, it had also increased the work of the fisheries officers as they spent more time checking licences and the numbers caught.

Hughes knew of two "canal monsters" caught recently, including a 36-pound (16.32kg) rainbow trout and a 39.7-pound (18kg) brown trout, which he suspects broke the New Zealand record.

South Canterbury Anglers Club president Gary Harrison said the farmed salmon waste, which also floated into the canals, was high in protein and nutrients which helped the "free" fish grow ridiculously quickly as they ate it.

Harrison, who preferred not to fish in the canals, said those who did had reported increased fish sizes during the past years.

He questioned whether the taste of the fish was up to the same standard of those in the rivers.

However, The Complete Angler owner Malcolm Bell, said there was nothing wrong with canal fishing. "They look like fish. They taste like fish and you still have to have skill to get them on to the bank."

He said back country fishermen considered a 10-pound (4.5kg) trout a trophy, but the canal fishing was blowing the parameters out of the water.

Bell, who has more than 30 years working in the fishing industry, went to Ohau C canal recently.

He said they were easily catching upwards of 35-pound (15kg) fish. However, he conceded he preferred fishing in the back country.

Recreational fisherman Peter Buchanan said he went to the canals because it was easier with his grandchildren. "They don't have to walk miles . . . and they can park up with their rods."

Christchurch Fishing and Casting Club president David de Montalk agreed it was a great place to take young children as they did not get wet and could cast off the side of the canal easily.

"It's very very popular. A lot of people go down there for a day."

Fish & Game central South Island officer Graeme Hughes said as summer approached they expected people to be parking up in their hundreds along the canals to get a piece of the "phenomenon".