Dead fish found in the Ahuriri River over the past few weeks were probably sockeye salmon dying after spawning, Central South Island Fish and Game officer Graeme Hughes says.
Anglers seeing dead fish in the river reported the sightings, fearing they were trout, but Mr Hughes said they were probably sockeye salmon from Lake Benmore which had spawned in the river.
Sockeye salmon were introduced into Lake Ohau tributaries in 1901 and became prolific, their young providing a valuable food source for the renowned Ohau trout fishery.
However, when Lake Ruataniwha was built in the 1980s, it cut the salmon's traditional spawning route from Lake Benmore to the Ohau tributaries and the population vanished.
For some unknown reason, they reappeared in Lake Benmore's Haldon arm in 2006 and also returned to Lake Ohau.
