The declining size and quality of rainbow trout in Lake Taupo and its tributaries is a cyclical problem caused by nature, says the Department of Conservation.
Recreational fishers, fishing guides and charter boat operators say the problem is threatening Taupo's claim to be the trout fishing capital of the world.
The Department of Conservation held a public meeting on the issue in Taupo on Thursday night.
Glenn McLean from the department says spawning has happened later during the past few yearswhich means when the season arrives the fish in the lake are younger and smaller than they used to be.
The second factor, he says, is a shortage of smelt on which the trout feed due to climatic conditions in 2005 now having an effect.
Scientist Dr Michel Dedual says the poor fish quality will slowly improve, if conditions are right.