The council undertook a study because of concerns ecology work done for the Lake Onslow battery project did not adequately recognise the significance of koura (freshwater crayfish) and cicadas in sustaining the fishery.
Additionally, there had been reports of low water levels, low trout numbers and trout in poor condition.
The first attempt to carry out the survey was foiled as the road to the lake was in poor condition.
Otago Fish & Game Council chief executive Ian Hadland said he had been involved in a gillnetting survey in November 2005.
However, the latest study was not intended to replicate that as the methods were not consistent, but comparisons were made where possible.
The median length of fish caught last month was 300mm and only three exceeded 350mm.
The median weight of the fish was 299g, ranging from 101g to 943g.
There was a drop in both the average length and weight between the two surveys but it was not believed to be statistically significant.
Mr Hadland said the MBIE report on Lake Onslow said there were no koura or cicadas, and this study proved it wrong.
It could be the MBIE study was done at the wrong time of the year to find koura, he said.
Fish & Game councillor Rick Boyd said the freshwater crayfish stock could be reduced because of low lake levels and higher temperatures in the past couple of years.
Councillor Mike Barker said Pioneer Energy, which drew water from Lake Onslow for hydro-electric power production, had said in its consent applications that anglers would not see any difference over the season.
"And of course they have."
Mr Boyd said redevelopment of areas of red tussock into pasture might have affected the number of cicadas.
"[The area] may already be affected by climate change."
