The sockeye salmon, introduced into Lake Ohau tributaries in 1901, disappeared in the 1980s after the Lake Ruataniwha spillway was built, cutting them off from their traditional spawning streams in Lake Ohau.
They reappeared dramatically in Lake Benmore's Haldon arm in 2006, their revival a mystery.
They have also returned to Lake Ohau.
Since then, the Lake Benmore population has grown from several hundred to about 10,000, spawning in the Ohau, Tekapo and Twizel Rivers.
The size of fish has also grown.
Now they have spread into Lake Aviemore and some have even been caught in the Waitaki River below the Waitaki dam.
Central South Island Fish and Game Council officer Graeme Hughes yesterday said the spread of the salmon was probably a result of extensive spilling from Lakes Benmore, Aviemore and Waitaki by Meridian in January, when heavy rain overfilled its upper storage lakes.
Maturing sockeye salmon had now been observed migrating up the Otematata River near the State Highway 83 bridge and he had heard reports of four being caught below the Waitaki dam by anglers fishing for chinook (or quinnat) salmon.
"The spill came at the right time of the year, with sockeye salmon migrating to the top of the Benmore dam," he said.
Mr Hughes said it was possible that could result in a permanent population being established in Lake Aviemore, although it was unlikely they would become a permanent feature in the lower Waitaki River.
A watch would be kept on Aviemore to see if that occurred.
Sockeye salmon in Lake Benmore grow to about 1.5kg, providing another sports fish for anglers, but are difficult to catch.
Plankton feeders, they do not compete with brown or rainbow trout for food, but their fry are valuable food for trout, boosting numbers and weights.
About 700 sockeye were counted in the lower Ohau River in 2006.
In 2007, 9870 were counted and this year the population is expected to be even bigger.
