SoE 2003 > Biodiversity > 6.8 Introduced aquatic species

6.8 Introduced aquatic species

Introduced aquatic species continue to displace native species

There has been an increase in the occurrence of introduced species in inland waterways in NSW. Introductions of marine plants are also on the rise, with the seaweed Caulerpa taxifolia of considerable concern.

Species entering NSW via ballast water from international shipping are now mostly under control, but the spread of species by coastal shipping is not.

NSW Indicator

Importance of the issue

The introduction of exotic species can have substantial impacts on native species. These include degradation of habitat, predation by the introduced species and competition for food, disease, and eventual displacement of native species. Introduced plant and fish species have had widespread impacts on the biota of inland waters.

Marine introductions

Introduced species are able to invade NSW marine waters in a number of ways, including through hull fouling and boring, and ballast water. Over the past six years, a survey of the NSW trading ports of Sydney Harbour (Port Jackson), Botany Bay, Port Kembla and Eden has revealed the presence of at least 30 introduced marine taxa (AMBS 2002; Pollard & Pethebridge 2002a; Pollard & Pethebridge 2002b).

Many of the introduced species do not appear to be having an ecological or economic impact. Of the foreign species found in NSW waters, only a few are on the current schedule of key marine pests targeted in Australia. Table 6.11 shows those marine pests on the schedule which have been detected in NSW ports, together with their potential impacts.

Table 6.11: Introduced marine species detected in NSW ports

Source: Joint SCC/SC on Fisheries and Aquaculture National Taskforce 2000; Newcastle Port Corporation data, as at September 1997; Sydney Ports Corporation data, as at October 1998 (Port Botany) and March 2001 (Sydney Harbour); Port Kembla Port Corporation data, as at May 2000; Department of Transport (port of Eden) data, as at November 1996

Notes: ^(a) Cysts were identified from sediment samples but could not be germinated.
^(b) The Pacific oyster was deliberately introduced into Tasmania from Japan in the late 1940s by CSIRO for aquaculture.

The seaweed Caulerpa taxifolia is considered to have significant potential to adversely affect marine biodiversity. It can quickly colonise and smother native seagrasses that are important habitats for fish, as occurred in the Mediterranean Sea when the species was accidentally introduced in 1984. Few species appear to feed on Caulerpa and it is not readily colonised by algae or other encrusting organisms, which could smother the weed and reduce its growth. This provides Caulerpa with a competitive advantage over native seagrasses. The species also spreads easily and is tolerant to cold water.

An invasive strain of Caulerpa taxifolia was first recorded in NSW coastal waters and lakes in April 2000. It was subsequently confirmed as being present in eight estuaries: Lake Macquarie, Careel Bay in Pittwater, Port Jackson, Botany Bay, Port Hacking, Lake Conjola, Narrawallee Inlet and Burrill Lake. Infestations range from less than a hectare in Pittwater to over 150 hectares in Lake Conjola.

Freshwater fish introductions

Eighteen percent of the fish caught during the two-year NSW Rivers Survey in the mid-1990s were introduced species, although their abundance varied greatly between regions and river types (Harris & Gehrke 1997). Fish surveys by NSW Fisheries at various localities between 1999 and 2002 found an overall composition of over 25% introduced fish (see NSW Fisheries 'FishFiles' data).

Although the relative abundance of pest fish varied considerably at different localitions, these surveys found the worst affected water bodies were regulated rivers. In the Lachlan, for instance, nearly 40% of fish caught were introduced and accounted for six of the 18 species recorded.

The most disruptive alien freshwater species include carp (Cyprinus carpio) and mosquito fish (Gambusia holbrooki). Carp remains the most prominent of the alien freshwater species, particularly in the lowland streams of the Murray–Darling Basin. The species has not been found in upland sites (Harris & Gehrke 1997). Carp compete with native fish for resources and alter stream habitat by disturbing bottom sediments, increasing turbidity, destroying aquatic plants and releasing nutrients, which can promote algal blooms. Although the species has been established in Australia for over 100 years, it has only become a major problem in the last 30 years, when the release of a more vigorous strain allowed the species to spread to most major waterways (Olsen 1998). In many NSW rivers, carp is more abundant than native fish species. In a small stretch of the Bogan River, near Bourke, densities of one carp for every square metre of river have been recorded (Harris & Gehrke 1997).

The distributions of carp, mosquito fish and oriental weatherloach (Misgurnus anguillicaudatus) continue to increase, while those of goldfish (Carassius auratus), redfin (Perca fluviatilis) and recreationally important salmon species remains stable. Tench (Tinca tinca) have declined significantly.

A number of species have also been introduced to be farmed, as well as those species brought to NSW for sport, stock enhancement and pest and weed management. In some cases these agricultural species have escaped from fish farms and formed self-sustaining feral populations.

Response to the issue

Once introduced aquatic species are established, it is very difficult to eradicate them. As a result, the focus of responses has been to manage existing species and prevent the entry of new pests. The key management tools being used are:

• listing of species, either as noxious or target pests
• developing strategies for specific existing pests, which may include techniques for control and/or eradication
• systems to prevent new entrants.

In NSW, undesirable species are able to be declared noxious under the Fisheries Management Act 1994 (FM Act). Once declared, these species can be seized and destroyed. Many aquatic weeds, including alligator weed, cabomba, salvinia and water hyacinth, have also been declared priority or noxious weeds under terrestrial weed control mechanisms (see Biodiversity 6.4).

Caulerpa taxifolia has been declared Noxious Marine Vegetation under the FM Act. The NSW Government has committed $1 million for aquatic pest and weed management, with Caulerpa as the primary focus.

A variety of control techniques, such as hand-picking, smothering, dredging and treatment with swimming pool salt, have all proven successful in removing Caulerpa from small areas. However, the trials have shown that a large-scale eradication program would be a long and expensive process and unlikely to be completely effective. Complementing these trials, NSW Fisheries has marked known Caulerpa beds, restricted commercial fishing in those areas and mounted an extensive public awareness campaign. Local communities also play a role in controlling Caulerpa by reporting sightings and checking anchors and propellers for the weed.

A taskforce on the prevention and management of incursions by marine pests was established in 1999 and is working towards a nationally consistent approach to dealing with the issue.

The National Management Strategy for Carp Control 2000–05 provides a broad policy framework for future carp management actions in Australia (CCCG 2000). The NSW Fisheries Carp Assessment and Reduction Program (CARP) is aligned with the framework and focuses on:

• the transfer of information and expert advice through local and regional workshops to community groups, industry, State agencies and staff
• documenting the distribution and abundance of carp and identifying environmentally valuable areas and those at high risk or most likely to respond to investment in carp control
• developing alternative fishing methods to improve the efficiency of commercial catches of carp while minimising the catch of non-target species.

In conjunction with the CSIRO, NSW Fisheries has also been helping to develop 'daughterless carp' technology as a potential long-term means of carp control. This technology involves genetic manipulation to breed the male fish out of the species.

One of the key threatening processes listed in the Threatened Species Conservation Act 1995 is the feeding on frog eggs and tadpoles by mosquito fish. A draft threat abatement plan for mosquito fish has been submitted for approval after its public exhibition.

Certain introduced terrestrial species have significant impacts on aquatic ecosystems, notably wetland areas, and also require specific control efforts. For example, feral pigs are efficient predators on some waterbirds, such as ibis and Australian pelicans. Control programs in the Macquarie Marshes and Nocoleche Nature Reserve are aiming to keep their populations low to minimise their impact on waterbirds and other plants and animals.

In July 2001, the Australian Quarantine and Inspection Service implemented a computer-based decision support system that categorises vessels according to the risk they pose in bringing in unwanted aquatic species.

Effectiveness of responses

Introduction of marine species from overseas via ballast water is now under some management, but other sources such as hull fouling are less controlled. Populations of marine pests already established in Australia continue to expand their ranges, as do many inland aquatic pests.

Activities to control introduced aquatic pests have had mixed success. The application of salt to Caulerpa taxifolia beds has successfully reduced the seaweed's biomass. In contrast, trials using specially formulated rotenone pellets to control carp have failed, and attempts to control speckled mosquito fish (Phalloceros caudimaculatus) from a location in North Sydney appear also to have been unsuccessful so far.

Future directions

Land managers and industries, such as aquaculture, can use preventative measures to restrict the spread of introduced aquatic species into native ecosystems.

Commercial and private users of large boats can improve their management of ballast water and hull condition to prevent the spread of species along the coast. Smaller boat users can improve management of ballast and bilge water, and check equipment such as anchors and chains to prevent the spread of pest species between estuaries.

The management of commercial and domestic aquariums can improve, especially of wastewater and the disposal of stock to prevent the introduction and spread of aquatic plant weeds and exotic fish.

Linked issues

2.1 Population and settlement patterns

4.1 Land-use changes

5.1 Freshwater riverine ecosystem health

6.6 Aquatic ecosystems

6.7 Aquatic species diversity

6.9 Aquatic harvesting