The loss, deterioration and fragmentation of ecosystems and their component habitats is a crucial issue for biodiversity management. Once ecosystems and habitats have undergone significant modification, the cost of environmental rehabilitation can be very great. Sometimes, however, rehabilitation can be achieved at a relatively low cost with major gains for the environment, for example, the rehabilitation of fish stocks and habitat achieved as part of the NSW Fisheries fishways program. There are 83 species of freshwater fish in NSW, almost half of which must migrate at least once during their life cycle.
Migrations can be wholly within fresh water, or between fresh water and the sea.
Dams, weirs, road culverts and floodgates all prevent or inhibit fish migration. There are over 500 of these barriers on major streams in NSW, and an estimated 2,500 more on minor streams and tributaries. These have all seriously reduced the habitat available to fish, and caused significant declines in fish abundance and diversity above individual barriers. Forty-five fishways were built in NSW between 1913 and 1985, but unfortunately these were constructed using northern hemisphere designs for salmon. The majority of our native fish are smaller, poorer swimmers, and unable to use these structures, particularly migrating juvenile fish, which move upstream from the spawning grounds in the estuaries, and within the major inland rivers such as the Murray River.