May 212014

Original story by Dr Ben Diggles at Fishing World

ONE of the scourges of our tropical freshwater rivers, the Mozambique Tilapia (Oreochromis mossambicus, also known as the Mozambique mouth-brooder) was introduced into Australia in the 1960s as an ornamental fish.

Tilapia signage from Fisheries Qld.After being released into the wild (eventually most types of ornamental fish get released at some stage), O. mossambicus and two other species of closely related cichlids (the black mangrove cichlid Tilapia mariae and the redbelly tilapia (Tilapia zilii) have survived and established populations which have since spread into many new places, such that today they dominate many of Queenslands waterways.

These species have been listed in the top 100 of the world’s worst introduced species and they are regarded as one of the greatest threats to Australia’s native biodiversity, because they have several adaptations that combine to enable them to overwhelm native fish populations.

Firstly, tilapia females carry their eggs and larvae inside their mouths, providing protection which virtually ensures the survival of the next generation. They are also a particularly hardy fish, tolerant of poor water quality, pollution, low oxygen levels (which they tolerate by gulping air at the water surface) and a wide range of water temperatures (8- 42 °C). They can also survive in brackish water, which allows them to move between river systems after river flow events, and have rather flexible dietary requirements. When their favoured algae, animal waste and planktonic food items are scarce, they can readily adapt to alternative food sources, ranging from completely herbivorous to totally carnivorous (including cannibalism).

They are also able to exist in extremely high population densities by stunting (early maturation at less than 10cm long) that directs energy to reproduction rather than growth. In other words, they are the fishy equivalent of some sort of nasty exotic tropical weed, a “cane toad of our waterways”, if you will.

The spread of tilapia in Australia has been alarming for authorities and lovers of native fish. In Queensland, incursions into new areas increased at a steady rate from the 1970s onwards. Particularly notable was the spread of tilapia throughout the Burdekin River system. In a four year period since 2004, they had spread throughout 3000km of the waterway.

New incursions of tilapia have resulted from deliberate and accidental introduction of these fish into dams and rivers, even in instances when the fish are dead. This is because juvenile tilapia can survive for a considerable period of time in the female’s mouth after the female dies. When a dead female tilapia is thrown back into a stream or dam, some of the juveniles in her mouth can survive and colonise the new water body.

These signs urge anglers to help

These signs urge anglers to help “stop the spread” of carp and tilapia in the Logan-Albert River system in SE QLD. Let’s hope a similar situation never happens in the Murray Darling Basin.

Tilapia have caused big problems in many eastern river drainages in Queensland, which is why they are listed as a pest species so that their possession in Queensland is illegal, with fines of over $200,000 being liable for serious offences. Because of this, any tilapia captured by recreational anglers in Queensland must be immediately killed and disposed of. Disposal must be into an appropriate rubbish bin or by burial well away from the water (so that scavengers can’t drag the carcass back into the water). The most important things for recreational anglers to remember is it’s illegal to use tilapia as bait, alive or dead, and certainly not to release them back into any waterway.

The only light at the end of the tunnel at the moment is that tilapia are currently not known to be present in the Murray-Darling Basin. But if they got in there, it is thought that the combined effect of both carp and tilapia on this system would be devastating – even more detrimental than carp alone, resulting in further decreases in water quality, introduction of new diseases and reduced survival of native fish species through increased competition.

To try to raise public awareness of the need to keep things that way, a Stop the Spread project has been established with the aim to educate fishers, community groups and local governments about the threat of tilapia and how to stop them from spreading into the Murray-Darling Basin. More information on this project is available HERE and an excellent information resource is available HERE.

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