Freshwater Fish - Under Threat

The freshwater fish of the Wet Tropics face several current threats as well as the possibility of some major difficulties in the future. Most species occur in the lower reaches of streams and rivers and, therefore, fall outside the borders and protection of the World Heritage Area. Many development proposals along the north Queensland coast are awaiting approval. Even protected reserves are not immune to the pressures of growth and development.

Another worry is the relocation of both native and exotic species by people, for example when live bait caught from one area is discarded in another. Many of the streams and rivers in northern Queensland are effectively independent bodies of water because of the abundance of waterfalls which limit the movement and distribution of many fish, especially the smaller species. Introduction of species with different requirements or habits, even if they are native, can have a severe impact on the resident species, such as the Lake Eacham Rainbowfish.

Common species of the aquarium trade have also made their way into our streams such as Guppies, Swordtails and Platys. Although these species are small fish, they compete for available food resources to the disadvantage of some native fish. The Mosquito Fish (Gambusia holbrookii) was introduced in the past in the hope of providing insect pest control services but it has proven itself to be a voracious controller of tadpoles, too.

Human alteration to the flow of a watercourse can have a dramatic effect on the aquatic habitat. Changes in velocity, volume and water quality has an impact on the species present, their patterns of distribution and their abundance. Seasonal changes act as cues to the life cycles of aquatic species, so it is important that these natural changes are retained. Other possible impacts of interference with water systems are not yet obvious. Researchers are concerned that increased pumping from bores could have a negative effect on the Tableland's aquifer which recharges and maintains streams in the dry season.

The region's water supply strategies have the potential to endanger or perhaps wipe out entire species of freshwater fish if not very carefully planned. If water extraction projects proceed without sufficient examination of the breeding, feeding and migration requirements of the resident fish, both immediately upstream and for a great distance downstream, the end result could be interference with breeding activity and reduced food supply. It is essential that extensive and detailed environmental studies are done before any development projects on or affecting watercourses are approved. However, such studies will require substantial funding and this is always a struggle to secure.

Barrier projects such as dams and weirs can also have an impact on marine species. Many marine species such as Mangrove Jack spend their adult lives in salt water but migrate to brackish water estuaries or fresh water to breed. Barriers in the lower reaches of rivers could stop marine species from reaching their selected breeding areas, either prohibiting their reproduction or forcing them to breed in unsuitable areas where offspring might not survive. Likewise, the same obstacle could affect freshwater species such as some gobies and gudgeons which breed in estuaries or salt water.

Recreational and commercial fishing could also have an indirect impact on smaller fish species. Barramundi is so eagerly sought after by anglers that it is cultured and restocked in watercourses to keep up with the demand. Barra grow quite large and eat any other fish smaller than itself; therefore, the periodic introduction of large numbers of carnivorous fish into a system has an effect on the status of the smaller resident species.

Other exotic and aggressive fish have found their way into northern waters and some were introduced specifically to cater to the recreational fishing interests.