In the past 50 years, native fish in the basin have declined significantly in both abundance and distribution. Many factors have contributed to this decline, including changes to water flow, barriers to migration, thermal pollution and increasing alien fish populations. Some impacts on fish are summarised below.
The regulation of almost all of the Basin’s rivers to provide water for irrigation and other purposes has been to the detriment of the native fish populations. Total flows have been reduced; the seasonal flow patterns have been changed; water levels in rivers change much more quickly than under natural conditions; and flood frequencies - especially the small and medium-sized floods - have been greatly reduced with the construction of reservoirs and other storage structures. The Ovens and the Paroo Rivers are the only system without major storages and the Ovens contains significant populations of Murray and trout cod.
The reduced frequency of flooding is of particular significance, as it is the high flows that cause the migratory fish to move upstream to spawn, whilst it is the floodplains and wetlands that provide feeding, spawning and nursery areas for many species, such as Golden perch, Murray cod and silver perch. Most large native river fish spawn in spring, requiring particular water level and temperature conditions. Cold water releases from reservoirs that occur during periods of fish migration and spawning reduce spawning opportunities and survival rates. Rapid fluctuations in river levels are also deleterious for fish that spawn in shallow water, leaving fish or their eggs or fry stranded.
The many features of a river channel and its banks, such as aquatic vegetation beds and riparian plants, rocks, logs, and different types of stream beds, provide the essential habitats for fish as shelter, resting areas, spawning sites and food sources. Greater diversity of environments contributes to greater biological productivity.
However, in some areas previous river management practices have led to a reduction or destruction of habitat diversity, through the removal of snags and river-side vegetation and the realignment or reconstruction of river banks. These measures have been to the detriment of fish habitats and populations. Further land management practices have led to declines in riparian vegetation and increased sediments and nutrients entering our waterways, further impacting upon fish populations.
Some river management practices have now improved with many authorities actively restoring river banks and fish habitat. Water quality and catchment management plans are in place or are being developed by Catchment Management Authorities for much of the Basin. However, comprehensive strategies are required to improve fish habitat which address water quality, flows, vegetation, and physical habitat to enhance fish populations.
Over the period of European settlement, many factors have contributed to the decline of water quality in the Murray-Darling Basin , to the significant detriment of the fish population. The major water quality risks identified by a recent study are temperature, turbidity, dissolved oxygen and nitrogen/ammonia. Other factors listed as moderately important are salinity, pH, toxicants, phosphorus and pathogens (EarthTech 2003).
Some of the pollutants affect fish directly, others indirectly, through damage done to their habitats. Some chemicals, such as endosulfin, are directly toxic to fish and other aquatic organisms, but much less is known about the sub-lethal effects of chemicals and other pollutants.
Excessive quantities of nutrients, especially nitrogen and phosphorous, not only contribute to the growth of algae, some of which are toxic, but also result in depletion of dissolved oxygen in the water, to the detriment of fish and other aquatic organisms.
Many fish use different parts of a river system at different stages in their life cycles. For some species, such as the gudgeons and smelt, these can all be within a relatively short stretch of river, but for others, the distances involved can be considerable. For example, the golden perch spawns in flooded reaches of lowland rivers. The young use the floodplains as nurseries, and later move upstream. The golden and silver perch move over most of the Murray-Darling river system, with the Murray cod covering recorded distances of 1,000 km and the golden perch 2,300 km.
For many years, movement over such distances was only possible during periods of major floods, as numerous barriers to fish movements have been erected across virtually all of the Basin’s rivers (Figure 3).
Combined with the regulation of the rivers, these barriers have had major impacts on the life cycles of many fish species, especially the migratory ones, contributing to reductions in their numbers and distributions. For example, golden and silver perch disappeared between Yarrawonga Weir and Hume Dam following their construction. Approximately 4,000 barriers have been documented across the Basin.
Recently new fishways have opened up large river distances. Original fishways were based on designs not entirely suited to native fish and offer only limited fish passage. To address this, the MDBC has developed the ‘Sea to Hume Dam’ project as part of the Native Fish Strategy. The project aims to provide continuous passage for fish from the mouth of the Murray to the Hume Dam near Albury-Wodonga, a distance of 2,225 km. Fishways have been or will be constructed at weirs and locks 1 to 11 and 15 (Euston). New fishways were previously built at Torrumbarry (as part of the new weir) and Yarrawonga weirs. Prototype fishways have also been constructed on the barrages located between Lake Alexandrina and the Southern Ocean in South Australia.
Figure 3. Barriers to fish movement in the MDB (Source: MDBC Weir Information System, MDBC 2001)
Figure 4. Fishway design and construction at Lock 8.
The impacts of alien (introduced) species on the native fish and aquatic habitats of the Basin are still inadequately known but it is now generally accepted that these organisms are generally detrimental. Competition with and/or predation by carp, gambusia, oriental weatherloach, redfin perch and trout, amongst others, have affected native fish across much of the Murray-Darling Basin .
Reservoirs have provided ideal environments for some exotic species, such as carp, while the maintenance of constant water levels behind weirs and in wetlands has benefited alien species to the detriment of native ones.
There is growing evidence of the detrimental impact of European carp (Cyprinus carpio) on native species of fish and the overall aquatic environment (MCMC 1994; Anon. 1995; Brown 1996). Carp have spread to much of the Basin and have been estimated to account for about 70 to 90 per cent of fish numbers in the rivers of the Basin (NSW Fish Survey). Because of their feeding methods, carp are undermining the banks of rivers, killing aquatic plants, and stirring up bottom sediments, releasing nutrients that contribute to blue-green algae outbreaks, as well as killing and feeding on the organisms that graze on the blue-green algae (Gehrke & Harris 1994). The National Carp and Pest Fish Task Force is providing a focus for continuing research and community concern relating to carp.
Exotic aquatic plant species are also widespread, and new invasions are likely - these species modify fish habitat and displace native plants which provide preferred native fish habitat.
The risk of further introductions into the Basin, from aquariums and nearby waterways outside the basin, also needs consideration and contingency control plans may be needed to prevent further impacts from these species.
The overall decline in fish numbers and distribution, as well as natural fluctuations, make it difficult to determine the real effects of overfishing. It appears that this is much less of a problem than the changes to the natural habitat. However, given the limited knowledge of many species and their scattered distributions, even very limited fishing can be excessive for a ‘restricted’ or ‘endangered’ species, such as the silver perch and trout cod. Long-lived and low fecundity species are particularly vulnerable to exploitation.
It is for these reasons that the state fisheries authorities place various restrictions on what can be caught in terms of numbers and species and on the fishing equipment that can be used. At particular times and places, there are bans on fishing, such as the closed season for Murray cod and protection for trout cod in the River Murray downstream of Yarrawonga Weir. Trout cod is fully protected in Victoria and it cannot be taken. Details on these restrictions, which are frequently updated, are available from state fisheries agencies.
Pathogens and diseases arise from the introduction of exotic species and native fish from hatcheries. A number of pathogens and diseases affect native fish, for example, the epizootic haematopoietic necrosis (EHN) virus was mostly likely introduced with redfin and has been officially reported from the Australian Capital Territory , New South Wales , South Australia and Victoria . So far it has affected only redfin and rainbow trout, but it has been shown that several native fish are susceptible to the disease. There is also concern about other viruses and pathogens from aquaculture ventures, such as a virus associated with barramundi farming, to which native species have shown susceptibility and which may cause declines in native fish. Naturally occurring diseases also affect native fish and regular mass mortalities of Bony Bream occur from infections of saprolegnia.
Native fish populations can be affected by species outside their native ranges. An Australian Government policy on translocations and stockings has called upon each state to develop guidelines for managing and controlling translocations. These guidelines recommend the use of risk assessments as the best approach to reduce the risks resulting from translocations. These assessments will help to identify and manage risks associated with translocations and will consider the likelihood of escape and the consequences of escape, including establishment of feral populations, introduction of parasites and diseases, and effects on biodiversity through predation, competition and genetic shifts to wild populations.
