Camera catches mice in the act

By SIMON COLLINS

Mice have been caught feasting on whitebait eggs at the Mokau River mouth, in North Taranaki, helping to solve part of a longstanding mystery behind declining whitebait numbers.

Dwindling catches have been blamed on increasing water pollution from land converted from bush to farming.

But experts were mystified by studies showing that up to 80 per cent of eggs laid in intertidal vegetation by the most common species, inanga, were lost before hatching.

At first grazing sheep and cattle were blamed, but the eggs kept disappearing even when the river edges were fenced.

Slugs were suspected, but they were found to be taking very few of the eggs.

Now scientists from the National Institute of Water and Atmospheric Research (Niwa) have finally caught two prime culprits on film - a mud crab which ate all the eggs at one of the two sites they filmed last year, and a mouse which ate all the eggs at both sites filmed this year.

Niwa scientist Dr Cindy Baker said: "It was a surprise, because we never thought of mice as predators of whitebait eggs. Introduced ferrets, stoats and weasels have been known to prey upon bird eggs, and the field mouse, which was thought to be benign, may be a problem for whitebait eggs."

Niwa plans to film again next season as part of a project, funded by the Foundation for Research, Science and Technology, to work out ways to reverse the whitebait decline.

For another part of the project, Dr Baker visited the United States this year to work with the world's leading expert on smell-based fish navigation, Professor Peter Sorensen at the University of Minnesota.

She has found chemicals dropped in the water by another native whitebait, the banded kokopu, which appear to attract other banded kokopu back to streams where their species lives.

Eventually, she hopes to be able to use chemical substances to build up whitebait numbers by attracting the species into unused streams with suitable habitats.