Pest fish that attack native fish could lead to native whitebait being wiped out in some lowland Manawatu waterways.
The pest fish have been found in high numbers in Manawatu.
Surveys were carried out more than a decade after extensive investigations by the Department of Conservation revealed high numbers of gambusia and koi carp.
Some eradication work was done and public awareness increased of how to curb the infestation.
Ecologists Natasha Petrove and Ursula Brandes were employed by DOC to see the effects of that eradication 10 years down the track.
The gambusia fish, also known as mosquito fish, is found in several Manawatu waterways.
The fish was introduced to New Zealand to control mosquito populations. However, it was discovered native fish were much better at it and gambusia were attacking native fish by nipping at their fins and eyes. They also competed with native fish for habitat and food.
Gambusia bred quickly and could quickly outnumber native fish.
“Unfortunately, we’ve now got these very aggressive, invasive little fish in lots of waterways,” Miss Petrove said.
“This could lead to our native whitebait species being excluded from lowland waterways.”
The gambusia appeared to be taking hold. Ecologists have recorded sightings in several sites in Manawatu, including Makowhai Stream near Sanson and Burke’s Drain just south of Palmerston North.
“This is a shame, as they are very difficult to control once they are established in flowing water,” Miss Petrove said.
Koi carp were less of a problem but were destructive in high numbers. They were released into waterways as ornamental fish, but deteriorated water quality and competed with native fish for their habitat.
Koi carp were only found in one of the sites surveyed by the ecologists, but were believed to be more widespread than the surveying revealed, with many people reporting sightings in other areas.
DOC said people spread these fish both accidentally and on purpose. Pest fish such as gambusia and koi carp could be inadvertently released into new waterways through transferring plants such as oxygen weed and water lilies.
Fish eggs and fry could be transported on leaves.
– Manawatu Standard