The report by fisheries officer Jayde Couper will be presented to Otago Fish and Game councillors at a meeting in Roxburgh on Thursday.
Fisheries officers studied Lake Onslow during this year’s brown trout spawning run and found at the proposed new lake level the spawning area would only be about 0.38% of what it is currently.
"The survey found low gravel quantity and quality in the tributaries above the proposed lake level. A considerable number of barriers to upstream trout migration were also identified, both within and above the proposed lake footprint," Mr Couper said.
The Government proposed the Lake Onslow hydro-battery scheme as a energy storage solution during dry years.
Water from the Clutha River would be stored in reservoirs in the Teviot Valley before being pumped through a tunnel to Lake Onslow and raising the lake level by about 76m.
The Ministry of Business, Innovation and Employment began feasibility studies in April and a report is expected in December.
Mr Couper said even if the lake was raised 20m (instead of 76m), available spawning gravel would reduce by about 84%.
At the new level, the lake would be six times larger in area and the available gravel would be of poorer quality than the gravel there now.
Staff also noted excellent rearing habitats for juvenile trout through the tributories, which would be "decimated" by a higher lake.
The reduction in spawning and rearing areas meant it was unlikely to produce enough juveniles to support the fishery in a much larger lake.
The lake was about 3m below full when Otago Fish and Game did its survey.
Sedimentation when the lake was low would affect spawning, Mr Couper said.
"Without mitigation, the proposed lake’s trout fishery will be severely limited by the reduction of spawning and rearing habitat. Mitigating this will be difficult. The introduction of spawning gravel to the tributories is likely to be unsuccessful," he said.
Building a spawning race had some potential to mitigate but would need significant areas of gravel and rearing habitat and would require regular grooming to prevent didymo buildup.
Analysing the effects of the scheme on the whole of Lake Onslow and the Clutha River sports fishery would require significant work, Mr Couper said.
Lake Onslow by numbers
1890: Lake Onslow formed after the Teviot River was dammed
1982: Lake raised 5m
1100ha: Lake surface when full (now)
6900ha: Lake surface when raised (future)
684m: Above sea level (now)
760m: Above sea level (future)
