25 February 2000
DoC Concerned about Illegal Introductions of Mosquitofish.
The Department of Conservation is concerned that people in Northland may still be releasing Gambusia to attempt control
mosquitos on their properties.
Northland conservation officer Vince Kerr said Gambusia (or Mosquitofish) were a major threat to freshwater fish
species. It is a priority in Northland that Gambusia are not released anywhere they presently do not exist.
Recent survey work done by DoC has shown Mosquitofish are seriously endangering several of our threatened native fish
Mosquitofish are used by public health agencies in a number of countries for the control of mosquito, although research
has shown that in New Zealand and several other countries like Australia they pose a significant threat to native
species such as fish, frogs and large invertebrates.
“The introduction of Mosquitofish into our waterways is parallel to the introduction of stoats into our forests and it
is a known fact that they are not more effective in mosquito control than the naturally occurring predators in our
streams,” Mr Kerr said.
The danger these introduced fishes pose to New Zealand’s freshwater ecology is recognised in two pieces of legislation;
the Freshwater Fisheries Regulations (1983) and the Conservation Act (1987). The first makes it illegal to obtain or
keep in captivity any Mosquitofish, whilst the second requires permission for the transfer of any aquatic life to a new
The fish are now being blamed world-wide for the damage they do to native fish and frog populations, out competing and
also eating the eggs and young of our native species.
Mosquitofish are small, stout and dull grey in colour with a rounded tail. The upward pointing mouth is adapted for
feeding at the surface with mature females measuring 50-60mm while males grow to only 25-35mm.
The fish are “live-bearers” with females reaching sexual maturity in only six to eight weeks. They store sperm in their
reproductive tract for up to two months and give birth to swimming offspring.
Estimates of their enormous breeding potential has shown individual populations being capable of expanding from 7000 to
120,000 in only five months.
Mr Kerr said Mosquitofish are capable of surviving in all Northland waters except fast flowing cool streams although
even these areas were under threat as their lower reaches could have an effect on the juvenile fish migrating to and
from the ocean.
“Gambusia are not any more effective than the natural predators of mosquitos in our ponds, lakes, streams and rivers. We
would encourage people to engage alternative means to control mosquito such as getting rid of stagnant water on their
Stagnate water is the main culprit as breeding places for Gambusia. Often very small collections of water near the house
are the main source of mosquitos. It is good practice to check for gutters holding water and containers lying around
perhaps in shrubs at the start of summer. If ponds and small streams have reasonable water they will support a natural
aquatic community of aquatic insects and possibly native fish that can effectively control mosquitos,” Mr Kerr said.
For more information please contact Vince Kerr on (09) 4380299