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Brown trout in ''amazing condition'' have been found in the stream which supplies Lake Hayes, surprising Fish and Game Otago's staff and council.
The lake had repeated algal blooms which posed a significant and ongoing threat to the health of the trout fishery and lake ecosystem, Fish and Game officer Clare Morris said.
In 2012, the public raised concerns about declining numbers of spawning trout and the poor condition of the fish in Mill Creek, the main spawning stream for Lake Hayes.
Fish and Game councillor Dan Rae said at a council meeting this week he was ''amazed'' at the condition of the fish sampled during the spawning run from Mill Creek to Lake Hayes in May.
''I didn't believe that area could support that standard of fish.''
Ms Morris said despite the presence of healthy fish in this season's spawning run, the water quality of the lake continued to be of concern.
Fish and Game captured 55 brown trout in the sample.
A factor of 1 represented fish in an average condition and 1.4 represented optimal condition.
Those caught in May rated 1.23, compared with 1.15 in 2013.
The sample was considered to be healthy and in above average condition with ''several outstanding specimens'', she said.
Average length (508.6mm) and weight (1651g), maximum weight (2640g) and average condition factors all showed increases compared with the previous season.
The Fish and Game Council this week agreed to repeat the sampling next season.