The Department of Conservation (DoC) is concerned about a decline in the number of trout in Lake Taupo.
DoC says there was a healthy number of trout in the lake in spring, but anglers are now finding it hard to find fish.
The number of smelt - the trout's main food source - has also declined.
DoC says it is unclear what has caused the decline in fish numbers, but suspects the trout have moved to open water, away from anglers, to follow their food source.
It says it will be able to better assess the situation in the next few months as the trout spawning season begins.