The first records of aquatic weeds in the Wet Tropics show that they began to arrive in the 1800s. Most of the ones we are dealing with today were introduced through the aquarium trade, as culinary plants or released for agriculture. The internet has made it worse by causing an explosion in food plants being imported from other parts of the world, especially south-east Asia.

In other parts of Australia, weeds die back during the dry months but because we have so much water all year round, weeds continue to spread. They can choke and smother our rivers, alter their flow and change the water chemistry, which create the perfect conditions for pest species like tilapia to thrive. Weeds also have a negative impact on the productivity of our farms.

Pest fish compete with native fish for food, resources and habitat and, in some cases, they even prey on them. Some of the pest species that have made it into our rivers include tilapia, gambusia, platies, guppies and swordtails.

Tilapia is a particularly noxious species because it can reproduce so prolifically that it displaces whole fish communities. Tilapia was first reported in the Wet Tropics in the 1970s in the Barron River and there are now two species that have become naturalised in this catchment – the Mozambique Tilapia and the Spotted Tilapia. Tilapia have now been recorded in most of our Wet Tropics catchments.

What has been done so far?

Unfortunately, aquatic weeds are almost impossible to eradicate. The best we can do is control infestations as they crop up by removing weeds manually or with the use of herbicides. Community groups, rangers and parks services are among those involved in weed control programs. We can all help by making sure we don’t inadvertently transfer weeds to other areas on machinery and boats.

Some of the species that are affecting our waterways currently are water hyacinth, water lettuce, limnocharis, hymenachne and paragrass.

Recent surveys show that the Murray and Herbert catchments are most impacted by invasive weeds in the Wet Tropics. Further north, the Barron River recently had an infestation of floating macrophyte Amazon frogbit (an aquarium plant), which has spread through several tributaries within the basin. On the upside, the Mossman River is looking better after two weed species, Salvinia and water hyacinth, were removed from known locations.