About 35 farmers attended a field day run by the Otago Regional Council on Dean Gilbert's property in Inch Clutha, near Balclutha, last month.
They were told by Department of Conservation freshwater ranger Pete Ravenscroft how the eggs of inanga - tiny native fish which make up most of the whitebait catch - are laid above the normal water level on the moist, grassy banks of rivers and streams.
"Pete showed us that we can protect their habitats with a single [electric fence] wire," Mr Gilbert said.
He said he had fenced off 3km to exclude stock from a valuable whitebait-spawning area on the Clutha Mata-Au, which bordered his property.
"We fenced it off five years ago. We don't want cows upsetting the fish - we're fishermen too," he said.
Cows were not the only stock to upset fish, he said. One deer farmer mentioned deer often upset whitebait and other fish by wallowing in creeks and rivers.
ORC land resources officer Alice Webster said inanga needed grassy edges on which to spawn, so where there was no permanent fence, a simple electrified "hot-wire" kept out stock, protecting the eggs from being trampled or eaten by stock. The move also helped improve water quality.
Whitebait populations had been declining for many years, she said.
The field day also looked at environmentally beneficial farm practices Mr Gilbert has put in place on his property. They include cambered laneways to keep waste from running into drains, yard water diversion and effluent management.
Ms Webster said the Gilbert farm was a good example of a dairy farmer taking actions with positive environmental outcomes.
Inanga eggs, laid during the high spring tides from January through to May, take a month to hatch before washing out to sea. March is generally the peak time for inanga to spawn on the Clutha River.