Researchers say sunlight is killing millions of whitebait eggs each year, but the species is not under threat.
The University of Canterbury, which has been studying the fish for the past six years, says a link between sun exposure and the destruction of eggs has been confirmed.
Researcher Mike Hickford says it's hard to determine whether this could be linked to opinion that stocks are decreasing, as there are no records on whitebait stock.
He says the average female whitebait lays 10,000 eggs at a time, but about 60% of them do not survive.
Dr Hickford says an hour of two of sunlight is enough to kill about 95% of the eggs.
He says vegetation cover in popular spawning areas will help protect the species.