Lake Eacham Rainbow Fish:
As with other types of animals, there is a great diversity of freshwater fish in the Wet Tropics. Seventy-eight of Australia's 190 species occur here. One of the most common families is the Rainbowfish which is found throughout the area, except around Cape Tribulation. Another widely distributed small fish is the Pacific Blue-eye. Larger species found in the coastal reaches of Wet Tropics rivers include Jungle Perch, Catfish, Sooty Grunter and Mangrove Jack - all popular among recreational anglers. The numbers of endemic fish (species that occur nowhere else) are surprisingly low. Only eight Wet Tropics endemic species have been identified so far, but this could change radically if more taxonomic work was done.
The streams of this area contain many barriers to fish migration (such as waterfalls at one end and saltwater at the other) so that many watercourses are somewhat 'independent'. Such isolation often leads to a high level of endemism, at least to the subspecies level if not the species level. Unfortunately, there haven't been many studies of northern fish apart from those with a demonstrated commercial or recreational value.
For example, the biology and ecology of barramundi, a popular recreational sport and eating fish, is very well known. On the other hand, the small Rainbowfishes and Gudgeons haven't received such focused attention. As more taxonomic and DNA studies are completed, we will probably see many more uniquely Wet Tropics subspecies and possibly species being confirmed, bringing the endemism levels for our freshwater fish into line with other types of animals.
Many fish species seen on the Barrier Reef as adults actually started their lives in streams and brackish water estuaries (mixed fresh and salt water). The reverse also takes place with some freshwater fish migrating to salt water to breed. The freshwater rivers and streams of the Wet Tropics also support a myriad of fauna in addition to fish such as platypus, crayfish, aquatic insects, frog and tadpoles, terrestrial insect larvae, shrimp and even some marine species.
Lake Eacham Rainbow Fish Story
This fish is the subject of a truly amazing story, the final chapters of which have yet to be written. Several rainbowfish are present throughout various parts of the Wet Tropics but the Lake Eacham inhabitant in particular has a strange history.
Lake Eacham is actually a volcanic crater which has filled with water and is isolated from any other watercourse (making it an enclosed catchment). How any fish arrived there to begin with is a mystery in itself, but somehow, the Lake Eacham Rainbowfish (Melanotaenia eachamensis) - which is very similar to the Eastern Rainbowfish - found its way into the volcanic lake. Unfortunately for the small species, other larger native fish were introduced into this closed system and eventually, these larger fish ate the Lake Eacham Rainbowfish into extinction - well, at least as far as the lake was concerned.
As it turned out, hobbyists had been collecting the fish from the Lake Eacham National Park (illegally) and were very successful at breeding them. It actually came to pass that these private collections became the source stock to reintroduce the fish to the lake. However, the cause of the species' demise was still living in the lake and proceeded to eat up the entire population of introduced stock. But the story doesn't end there.
Fish researchers working in the Wet Tropics rivers and streams have found the Lake Eacham Rainbowfish in the Tully, Herbert and Johnstone Rivers and Dirran Creek. But the plot thickens … some of them are the genetically pure version that used to occupy the 'closed' Lake Eacham but others are hybrids caused by interbreeding with the Eastern Rainbowfish.
It's hard to predict where the next chapter will lead. Because of the complete infestation of Lake Eacham by translocated native fish from other water systems, it is futile to continue restocking the lake with rainbowfish. Getting rid of the predatory species is a major problem and a solution which is non-destructive to the species that belong in the lake is not going to be found in a hurry. The moral of this story is that the translocation of fish, even native ones, is a really bad idea that can create irreversible problems.