Polluted rivers and streams are being blamed for the disappearance of whitebait.
Massey University researcher Mike Joy has found whitebait have disappeared from about 75 percent of their expected habitats in Manawatu and Horowhenua.
Dr Joy said the national group that monitored the fate of the adult whitebait (galaxiids) had reported a similar level of disappearance.
The dwindling numbers were further affected by a number of fishers who caught juvenile whitebait, and sold them for up to $150 per kg, Dr Joy said.
"Not enough galaxiids are able to return to the streams because of the whitebaiting.
"Two of those species have the same threat ranking as a kiwi yet selling whitebait is a crucial incentive to get people out there.
"In the West Coast fishing stands sell for $60,000. Clearly, it's an industry for some people."
Dr Joy said his computer modelling, which he has focused on the greater Manawatu catchment, showed him where the whitebait should be, including the upper Oroua, upper Pohangina and upper Manawatu rivers.
"But they are not there, we have searched and searched for them."
Dr Joy said a few simple measures could protect what is left of the stocks, including prohibiting the sale of whitebait in the same way trout is protected, minimising high-country erosion and cleaning up waterways from pollutants including sewerage and run-off.
He also believed better monitoring of waterways would provide a clearer picture of their state.
"If we don't do something quickly we won't have these species any more."
