mosquito fish(reviewed)

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					                                 Eastern Gambusia
                                 Gambusia holbrooki
                             DESCRIPTION AND CHARACTERISTICS

Eastern Gambusia, more commonly known as Mos-         reaching    a      much  larger    size     of
quito fish, are a noxious fish species introduced     up to 6cm, compared to males, which only grow
into Australia in 1925 from North and Central         to around 3cm in length.
America. They have a dorsally flattened head,
rounded tail and a single dorsal fin. Their back is
green to brown, becoming grey with a bluish tinge
down their sides, and silver on the belly. They
also have an upturned mouth, large eyes, and a
rounded belly.

                                                      Female G.holbooki-Photo by Gunther Schmida. Sourced
                                                      from the Murray-Darling Basin Commission.

                                                      Females also possess a deeper stomach and a dis-
                                                      tinguishing black spot near the rear of their abdo-
                                                      men, which becomes more pronounced when
                                                      gravid (carrying eggs).
          Male G.holbrooki—Photo: Tarmo A. Raadik
                                                      Males lack the prominent stomach bulge present
Eastern Gambusia are sexually dimorphic (females      in females, and possess a longer anal fin which is
and males have different forms), with females         used as a breeding tool.

                                     BIOLOGY AND LIFE CYCLE
Eastern Gambusia prefer warm, still or slow flow-     diet, and are known to voraciously predate a vari-
ing water, but are found in a wide variety of habi-   ety of native aquatic organisms, including aquatic
tats around Australia. They are extremely tolerant    bugs, beetles, frog and fish eggs/larvae, ants and
of adverse water conditions. Eastern Gambusia         flies. They also display aggressive fin-nipping be-
can tolerate a wide range of temperatures, (from      haviour, biting the fins of fish and tadpoles.
<5˚C to 44˚C) and salinities (from freshwater to      Unlike many native fish, they give birth to live
estuaries). They are also able to live in oxygen      young. They can bear up to nine broods per year,
poor environments, and will gulp air from the sur-    averaging 30-50 young per brood. Young fish ma-
face to supplement their oxygen supply.               ture very quickly, reaching reproductive age at
Eastern Gambusia possess an extremely diverse         between 4 and 6 weeks.
                                     ENVIRONMENTAL IMPACTS
Eastern Gambusia are extremely aggressive preda-       and have been implicated in the decline of the
tors that compete with native fish, eat their eggs     endangered Growling Grass Frog. Known to nip the
and predate their young. Evidence suggests that        fins of native fish causing stress and often death.
they are partially responsible for the decline of      High reproduction rates and long breeding season
various frog and fish populations across Australia,    means they can rapidly dominate systems.

                                    HISTORY OF INTRODUCTION
Eastern Gambusia were introduced to Australia in       cies at controlling mosquito numbers. In some in-
1925 as a biological control agent for mosquitoes      stances, they may even exacerbate the problem
(up until 1982 the World Health Organisation rec-      due to their voracious appetite for the natural in-
ommended their use for malaria control pro-            vertebrate predators of mosquito larvae.
grams). Soon after, populations of Eastern Gam-        Unfortunately, they are now the most widespread
busia became well established and were widely          freshwater fish in Australia, occurring in all States
dispersed by military and health authorities.          and Territories!
There is a strong body of evidence that suggests
this species are no more effective than native spe-

                                              DISPERSAL
Once introduced into a waterway Eastern Gam-           Although there is anecdotal evidence suggesting
busia are usually able to disperse along its entire    that Eastern Gambusia can be transported “live”
length due to their tolerance to a diverse range of    in the crop and gullet of birds and deposited in
environmental conditions.                              new areas, it is far more likely that dispersal oc-
Unfortunately, many people have introduced them        curs during times of flood and through uninten-
to household ponds, not realising they are actually    tional relocation by humans. Eastern Gambusia can
a noxious pest. Please note, the deliberate re-        also be inadvertently transported in aquatic equip-
lease, transfer, or disposal of Eastern Gambusia       ment, such as nets, boats and fishing gear.
into local creeks, rivers, wetlands or ponds is
illegal, as it may lead to further dispersal of this
noxious fish throughout Australian waterways.
                                    LEGAL STATUS IN VICTORIA
In Victoria Gambusia holbrooki has been declared       eries Act, 1995. As a declared noxious fish, this
a Noxious Aquatic Species as stated under the Fish-    species cannot be returned to water.

                                   POSSIBLE CONTROL MEASURES
Eastern Gambusia are notoriously difficult to          native fish preferentially predates Eastern Gam-
eradicate once introduced. One control measure is      busia when alternative food sources are available.
the addition of piscivorous poisons to waterways.      By far the best control method for this species is
Unfortunately this method eradicates all fish pre-     to prevent further dispersal and introduction to
sent, including native fish.                           new waterways.
They can be removed from wetlands and other            Never replace or relocate this fish to any water
‘closed’ water bodies through draining and drying.     body within Australia - it is illegal under the Fish-
Once again, non-target species are also affected.      eries Act, 1995.
It is unlikely that this fish could be controlled      If you use equipment i.e. boats, nets etc. in wa-
through a viral, bacterial or parasitic control        ters known to contain mosquito fish, please ensure
agent, due to the non species-specific nature of       they are well cleaned and air-dried before next
fish disease and parasites.                            use.
There is also little evidence to suggest that any

				
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posted:4/17/2010
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Description: mosquito fish(reviewed)