Question

Once widely bred in south-east Asia for blood sport but now often kept instead domestically or as pets, what escaped in large numbers into the wild in Australia and was identified as a possible severe threat to natives?

Answer 1

Siamese fighting fish
The fish were discovered in 2016 to have gone feral and bred in huge numbers in the Adelaide River and floodplains near Darwin. Researchers in the Northern Territory comment "We are not sure how far they have bred at this stage, nor their impacts on our ecosystem... In the wild they can be a very territorial, aggressive, tough fish " but that they are smaller than many native fish and "are only three or so centimetres — although they may be impacting our wetlands, they are also becoming a food source and this may control the situation".