Treatment Methods

The first step in developing an effective eradication or control strategy is to check literature and database sources to accumulate as much information as possible about management options for this species. Managing an invasive species is not the management goal, but a tool in the process to maintain or achieve resource management goals such as habitat restoration, a sustainable fishery or ecotourism sector, the preservation of an undisturbed ecosystem, etc. In most cases, best practice management of an invasive species involves an integrated management system tailored to the species and location. Thus, it is important to accumulate the available information, assess all potential methods, and use the best method or combination of methods to achieve the target level of control.

Four approaches have been applied for both the eradication and control of pest species:

• mechanical/physical (e.g. removals by hand, divers or mechanical harvesting);

• chemical (e.g. chemical dosing, toxic baits, application of an inorganic or organic herbicide, larvicide or other pesticide);

• biological – such as use of a target-specific pathogen, parasite, predator, biopesticide (e.g. Bacillus thuringiensis [Bt]), genetic manipulation, reproduction manipulation or habitat modification (e.g. salinity change by salt dosing or freshwater inundation); and

• Integrated Pest Management (IPM).

5.2.8.1 Mechanical and Chemical Treatments

Various mechanical and chemical treatments have been used by natural resource management and conservation agencies in various countries in attempts to eradicate or control invasive species, and there are differing rules on the use of some of these treatments according to local legislation. The CSIRO Rapid Response Toolbox, the GISP database and other websites and

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published documents provide information sources on this topic (Appendices A and C). It is also worth noting that national and/or regional legislative instruments pertaining to the permissibility and specific method/s of a treatment can be present in occupational health and safety and public health regulations as well as those pertaining to environment protection (Appendix B).

Treatments used for the eradication or control of freshwater species in lakes and waterways have some application for marine situations such as coastal lagoons, barred estuaries and enclosed breakwater harbours. These include poisons such as rotenone used for fish, specific larvicide (TFM) for lampreys, and aquatic weed mechanical harvesting or herbicide spraying.

While herbicides typically used for freshwater plants (e.g. glyphosphate, 2,4-D) do not kill algae species, there are several algicide alternatives. Second generation electrical barrier systems hold less promise for containing the spread of introduced migratory species in marine or brackish-water areas owing to the problem of salinity conductance which greatly reduces their effectiveness and requires dangerous, costly power levels to overcome. However they may be useful for controlling spawning migrations of unwanted anadromous and diadromous non-native fishes in particular watersheds (see Box 6).

Box 6: Developments in Electric Barrier Methods

Since the first attempts in the 1950s-1960s (Section 3.1), there have been several improvements to the efficiency of electrical barrier designs for preventing non-native migratory fishes from entering rivers and watersheds for spawning and spread (e.g. sea lampreys, salmonids, Asian carp etc). Systems trialled in various North American watersheds during the 1990s typically use a Graduated Field Fish Barrier (GFFB; as developed and promoted by Smith-Root, Inc). The GFFB provides a substantial improvement over the earlier alternating current and single-field direct current barriers. By using an electrical array mounted in an insulated concrete pad to produce the graduated field, they can also be installed without posing a major navigation obstacle and are less prone to debris damage. As a fish swims into the electrical field, the increasing voltage inhibits the swimming ability of the fish and the fish are quickly swept clear of the field by water flow. The most efficient electric field pattern for blocking or guiding fish has electric field lines running parallel to the water flow. The advantage of this parallel field orientation is that fish which turn crosswise to the electric field receive almost no electric sensation or muscle paralysis. Fish learn very quickly that by turning side ways to the flow they minimize the effect of the electric field. In this orientation, upstream migrating fish are swept clear of the field by the water flow. The figures below show reactions of migrating fish when they try to migrate across a GFFB. In slow or static water a high percentage of fish learn to turn and swim away from the electric field. However water velocities >0.6 metres/second are required to avoid fish kills by helping to quickly sweep the fish downstream and out of the field. Details on designs and locations of of electrical barriers trials in the US are available as downloadable files from www.smith-root.com.

Figures from Smith-Root website

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5.2.8.2 Biological Treatments and Integrated Pest Management

These are organisms or agents which are typically sourced from the natural range of the harmful species and have a very high level of target specificity. By exerting negligible side effects on native species, ecosystems and human health, these biological controls can be used in all sensitive areas to provide an efficient and self-sustaining long term response. The main disadvantages of biological controls are:

• the time and expense required to identify, screen and test candidate control agents;

• the time required for the released agent to multiply and cause the required effect throughout the targeted population;

• uncertainty about the level of control the agent will ultimately bring to bear on the targeted population;

• the potential for the agent to exert an unexpected effect on native species or communities; and

• the population regulation mechanism underlying the principle of biological control, which does not anticipate eradication but reduces the invading population density and fitness, with the prey/host or predator/parasite relationship achieving a dynamic balance.

Depending on the situation, eradication of pathogens and parasites may focus on isolation, treatment or culling of infected hosts (if the latter comprise cultured populations contained in ponds or open water pens), and/or the culling of local wild populations. Confirmatory post-eradication monitoring should be implemented as a routine, precautionary measure.

Habitat modification includes altering the characteristics of the water column as well as modifying the benthos (e.g. increasing the salinity to increase the osmotic stress and reduce the growth rate of the aquarium strain of Caulerpa taxifolia, as has been used for coastal lagoons in South Australia (Box 7).

Integrated Pest Management (IPM) frequently forms the basis of control programme approaches. IPM arose from terrestrial programmes where environment and human health issues arising from the long term use of broad spectrum pesticides for controlling agricultural pests led to the adoption of mixed treatment strategies. Thus IPM typically employs one of several pre-agreed mechanical, chemical and biological methods depending on the locations and circumstances. Optimum control strategies are likely to be location specific, and need to be trialled and fine-tuned for different areas. If the introduction is an edible shellfish or finfish, one control method in the toolbox is targeted recreational and commercial fishing.

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BOX 7: TREATING CAULERPA TAXIFOLIA IN SOUTH AUSTRALIA

An invasive population of the seaweed Caulerpa taxifolia was discovered near Adelaide in South Australia in March 2002. Surveys established the incursions were limited to three localities: an urban wetland (West Lakes), part of the upper Port River, and an isolated small patch further downstream in Eastern Passage (Barker Inlet).

West Lakes is an artificial wetland located in an urbanised coastal area near Adelaide and was the site of the largest incursion. West Lakes has a surface area of approximately 1.2 km² and holds approximately 3 gigalitres at usual fill levels. Most of West Lakes is relatively shallow but in one area extends to approximately 6 m deep. The system drains into the infected area of Port River, and water exit/entry can be controlled for managing lake levels. In the upper Port River (which is a tidal estuary system), the weed was present along an approximately 2 km long section, with most in the southernmost 800 m stretch. The Eastern Passage outbreak comprised a small patch located approximately 7 km further downstream from the nearest edge of the upper Port River incursion area.

After information exchanges and consultations with managers and researchers engaged in Caulerpa incursions in coastal estuarine areas of New South Wales, the South Australian government agency responsible for responding to aquatic invasive species decided to eradicate the outbreak in West Lakes by reducing its salinity. This was achieved by diverting very low salinity urban storm water, sourced from the Torrens River catchment. A successful eradication was achieved during 2003 by reducing bottom water salinity to values to ~10 PSU and maintaining these for several weeks (longer in some areas), To date this treatment appears to have been successful.

The small patch located in Eastern Passage was treated by smothering with heavy duty PVC sheet which was pinned down before injecting with chlorine. Large scale removal of the weed’s biomass in the upper Port River was achieved using a large, diver-operated airlift connected to barge-mounted settlement and filtration systems. Significant portions of the Port River incursion were also treated using smaller portable suction devices and by smothering with black PVC Further large scale treatment strategies for this area are currently being developed and assessed.

Regular extensive surveys throughout the area have located no other incursions to date.

(from communications and information kindly supplied by John Gilliland, Primary Industries and Resources South Australia (PIRSA), Adelaide, Australia [Gilliland.John@saugov.sa.gov.au])