6.9 Aquatic invasive species

Freshwater ecosystems contain a high proportion of introduced fish species, but marine and estuarine systems remain relatively unaffected

There has been an increase in the occurrence of introduced species in inland waterways in NSW. Carp remains the most prominent aquatic pest species (by biomass) in many inland aquatic systems, but populations have declined significantly in some areas. Carp management in NSW focuses on programs to restore the competitive advantage of native fish, using newly developed control technologies.

Many of the introduced marine species do not appear to be having a major impact. The main species of concern is the seaweed aquarium caulerpa. Management has involved treatment of high priority areas, and extensive public awareness campaigns and community involvement in preventing the spread of the weed.

Introductions of new invasive species continue to occur and the key focus is on preventing their arrival and spread. The introduction of freshwater species through the aquarium trade requires greater management attention, as does the spread of species by coastal shipping.

NSW indicators

Introduction

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

The presence of exotic species can also have considerable economic and social impacts on activities in the aquatic environment. These may have to be stopped or modified to prevent movement of the species into new areas, where they may threaten commercial species and uses.

Current status and trends

Freshwater fish introductions

Pest fish have been introduced to NSW in several ways. These include deliberate introduction to enhance sport fishing, through contaminated batches of stocked fish, escapes from aquaculture facilities, as live bait, and the liberation of ornamental pets.

Eighteen per cent 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 at various localities between 2002 and 2005 found an overall composition of 26% introduced fish.

In the Murrumbidgee River in 2004, the proportion of introduced fish was 71% of individuals and 90% of biomass (Gilligan 2005a). In the same year in the lower Murray–Darling, the proportion of introduced fish was 14% of individuals but 56% of biomass (Gilligan 2005b). Comprehensive surveys of the Barwon–Darling, Murray and Border Rivers catchments as part of the Murray–Darling Basin's Sustainable Rivers Audit (SRA) revealed a similarly high percentage of exotic species in 2004–05. The SRA is sampling the remaining NSW inland catchments in 2005–06 and 2006–07.

Although the relative abundance of pest fish varies considerably at different locations, fish surveys regularly find that the worst-affected water bodies are regulated rivers. In the Lachlan River, for instance, nearly 40% of fish caught during the SRA pilot study in 2002 were introduced species, accounting for a third of the 18 species recorded (MDBC 2004).

Carp, tench, redfin and four species of salmonids have been introduced into NSW as fishing targets, with carp being the most disruptive. Carp compete with native fish for resources and alter stream habitat by disturbing bottom sediments, increasing turbidity, destroying aquatic plants and altering the nutrient cycle, which can promote algal blooms.

Carp are prominent in the lowland slopes and upland regions of the Murray–Darling Basin, where they are found in almost all reaches of catchments. Their numbers, however, are limited in coastal catchments (Graham et al. 2005). Although the species has been present in Australia for over 100 years, it has only become a major problem in the last 30 years, with the release of a particularly vigorous strain resulting in its rapid spread (Olsen 1998).

Carp are more abundant than native fish species in many westward-draining NSW rivers. 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). Recent surveys found carp comprised 87% of the biomass in the Murrumbidgee and 49% of the Lower Murray–Darling (Gilligan 2005a; Gilligan 2005b).

Another invasive species, tench, is now considered to be extinct in NSW, having disappeared shortly after the invasion of carp. The last individual was recorded in the lower Darling in 1988 (Gilligan 2005b).

Like carp, redfin have achieved a wide distribution. Its abundance in the Murrumbidgee River has increased significantly over the past 10 years (Gilligan 2005a) and the fish has been implicated in the decline of native fish populations through predation and the spread of disease.

Banded grunter is believed to have been introduced to the Clarence River system through contamination of a batch of stocked Australian bass. Little is known of its potential impact in NSW, but it has seriously degraded recreational fishing opportunities in several south-east Queensland impoundments. The omnivorous nature of the banded grunter assists in its ability to populate a wide range of freshwater habitats (Pusey et al. 2004).

Several other species have been introduced or translocated for commercial aquaculture purposes. In some cases these species have escaped from fish farms and formed self-sustaining feral populations. The plague minnow was introduced as a mosquito control agent but has spread extensively, causing a decline in native fish and frog populations.

Some popular aquarium species have also formed viable populations in NSW. These include koi carp, goldfish, white cloud mountain minnow, oriental weatherloach, green swordtail, plague minnow (mosquito fish) and Jack Dempsey cichlid.

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. The NSW trading ports of Sydney Harbour (Port Jackson), Botany Bay, Port Kembla and Eden contain 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 exotic species found in NSW waters, only a few are on the current schedule of key marine pests targeted in Australia. Table 6.21 shows those marine pests on the schedule which have been detected in NSW ports, together with their potential impacts. Little information has emerged since 2000 to prompt an update of this data.

Table 6.21: Introduced marine pest species detected in NSW ports

Source: Joint SCC/SC on Fisheries and Aquaculture National Taskforce 2000; Newcastle Port Corporation data September 1997; Sydney Ports Corporation data October 1998 (Port Botany) and March 2001 (Sydney Harbour); Port Kembla Port Corporation data May 2000; Department of Transport (Port of Eden) data November 1996; Ahyong 2005

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.

Of these marine pests, the seaweed aquarium caulerpa (Caulerpa taxifolia) has significant potential to adversely affect marine biodiversity. It can quickly colonise native seagrass beds that are important habitats for fish. The species also spreads easily and is tolerant of cold water. Research is underway to test whether aquarium caulerpa is capable of outcompeting native seagrasses.

An invasive strain of caulerpa was first recorded in NSW coastal waters and lakes in April 2000. Its presence in 10 estuaries has subsequently been confirmed: Lake Macquarie, Pittwater, Port Jackson, Botany Bay, Port Hacking, Lake Conjola, Narrawallee Inlet, St Georges Basin, Burrill Lake (Creese et al. 2004) and most recently Brisbane Water in April 2006. Infestations have ranged from a few hectares in Narrawallee Inlet to over 150 hectares in Lake Conjola. In May 2006, no caulerpa was found at any of the previously affected sites in Lake Macquarie.

The remaining organisms in Table 6.21 have limited distribution and are not considered to pose a serious threat to NSW biodiversity. Some species of toxic dinoflagellates, such as Alexandrium minutum and Gymnodinium catenatum, have been introduced, while the origin of others is unclear. Under certain conditions these organisms may form algal blooms known as 'red tides'. The toxins they produce may bioaccumulate in marine organisms, such as planktivorous fish and shellfish, and consumption of affected shellfish can cause poisoning in animals and humans. Routine monitoring since 2003 of phytoplankton introduced into NSW shows that algal blooms occur periodically several times per year.

The European shore crab is a potential aquaculture pest that has been reported in small densities south of Sydney. The Mediterranean fanworm, currently restricted to Eden, has the potential to smother native species and alter habitat. The Pacific oyster is a threat to the oyster farming industry in NSW. It outcompetes and kills native oysters and other marine life by dominating intertidal habitats.

Response to the issue

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

In NSW, undesirable species can be declared noxious under the Fisheries Management Act 1994 (FM Act). Once declared, these species are able to be seized and destroyed. A list of fish species prohibited from importation into NSW under the FM Act was gazetted in 2004.

Two key threatening processes listed under the FM Act (see Biodiversity 6.8) are 'the introduction of fish to freshwater rivers outside their natural range' and 'the introduction of non-indigenous species to coastal waters', but threat abatement plans have not yet been prepared for these. 'Predation by the plague minnow' (mosquito fish) is listed as a key threatening process under the Threatened Species Conservation Act 1995 and a threat abatement plan has been finalised.

Aquarium caulerpa has been declared 'noxious marine vegetation' under the FM Act and $1 million annually has been committed for aquatic pest and weed management, with caulerpa as the primary focus. The NSW Caulerpa Control Plan sets out a strategic framework for future management of the weed.

A variety of control techniques has been trialled, including hand-picking, smothering and dredging. Intensive treatment with swimming pool salt has also been attempted in several estuaries (Glasby et al. 2005). Complementing these trials and to prevent the weed's further spread, known caulerpa beds have been marked with 'no-anchoring' buoys, and commercial and recreational netting restricted in affected areas. An extensive public awareness campaign has involved local communities reporting caulerpa sightings and checking anchors and fishing equipment for the weed.

Many freshwater 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).

The National Management Strategy for Carp Control 2000–05 provides a broad policy framework for carp management in Australia (CCCG 2000). The NSW Carp Assessment and Reduction Program is aligned with this framework. Several promising technologies are emerging to control carp. 'Daughterless carp' technology involves genetic manipulation to bias the sex ratio and eliminate female carp from the population. 'Hot spots' for carp reproduction are being identified for use as control areas. Other strategies currently under investigation include the use of carp-specific toxins and pheromone attractants, and the introduction of a carp-specific disease.

Projects to restore degraded habitats and provide native fish with a competitive advantage include the removal of obstructions to fish passage, installation of carp-selective traps on fishways, restoration of riparian vegetation, re-snagging of rivers, and the exclusion of terrestrial animals from stream banks.

A 2005 management strategy for freshwater fish stocking incorporates measures such as 'no-go' areas for salmonid (trout) stocking, including permanent and temporary closures and buffer zones around areas known to contain threatened species; a new accreditation scheme for hatcheries supplying fish for stocking; and an improved framework for assessing stocking applications and managing the activity into the future.

In 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.

The statewide natural resource management (NRM) target relevant to this theme is 'By 2015 there is a reduction in the impact of invasive species'. The available data may be used as a baseline measure of this target in some parts of NSW, but there is a need for further data on a statewide basis to provide a full picture of progress toward the target. NSW is also engaged in national processes to address marine incursions and control ornamental fish.

Future directions

With the growth in global trade, it is inevitable that new and potentially invasive species will continue to arrive in NSW. Prevention and eradication of new arrivals is critical in order to avoid further additions to the current array of widespread invasive species.

The preparation and implementation of threat abatement plans provides a model for better strategic management of invasive species. Incorporating monitoring programs into the plans will provide feedback on the effectiveness of on-ground programs.

The preparation of AusBIOSEC, the Australian Biosecurity System for Primary Production and the Environment, will provide an opportunity to integrate all relevant plans and actions addressing the environmental impacts of invasive species across all levels of government.

Measures are now in place that afford some control over the introduction of marine species from overseas via ballast water, but some refinement may be needed in the future. Hull fouling is less well controlled.

Populations of marine pests that are already established can be spread along the coast by coastal shipping and recreational boating. Management by operators of ballast and bilge waters, and hull and anchor condition may be required.

Measures should be developed to prevent the inadvertent introduction of exotic fish and aquatic weeds into native ecosystems from the aquaculture and aquarium industries. In particular, care is needed in the disposal of wastewater and surplus stock.

Further research is needed into methods for identifying new arrivals and better techniques for suppressing or controlling the species that are already here.