The brown trout catch rate in Lake Wanaka sank from 0.13 fish per hour in 2020-21 to 0.06 last season.
For rainbow trout, the rate fell from 0.09 to 0.07.
Fish & Game study projects include underwater acoustic surveys, a monitoring workshop, angler interviews and catch monitoring.
The acoustic surveys will study trout and salmon populations in Lakes Wanaka, Wakatipu and Hawea.
Fish & Game has run angler surveys and weighed catches from Lake Wanaka for the past three fishing seasons.
The survey interviewed 211 anglers, who put in a total of 234 hours of fishing. Almost 190 caught no fish.
Only one salmon was recorded during surveys. However, Fish & Game received many reports of salmon caught during the season.