Evaluation of carp exclusion screens at wetland inlets

Key words: pest fish control, European Carp, exclusion screen, Native Fish Strategy

Threats and Impacts: Wetlands are sites of high primary and secondary production and contain diverse flora and fauna. Indeed, many riverine species are wholly or partially dependent on wetlands for food, shelter or habitat during some part of their life cycle. Carp (Cyprinus carpio) dominate the alien fish fauna of the Murray–Darling Basin, and are believed to impact on native fish communities by increasing turbidity, disturbing and redistributing benthic seeds and invertebrates, up-rooting delicate shallow-rooted vegetation, competing with native fishes and other aquatic fauna for food and space, and indirectly promoting the development of toxic algal blooms (Fig 1).

In the Murray-Darling Basin, up to 98% of Carp are produced in wetlands connected to the main rivers. Carp Exclusion Screens (CES), which are mesh barriers that are installed at inlets to wetlands to exclude large fish from enterin (Figs 2-3), provide a management tool that has been applied to protect ecologically important areas from the impacts of Carp. However, little work has been undertaken to validate the effectiveness of CES in managing Carp, and concerns about possible detrimental effects of CES on native aquatic fauna have been raised.  

Broad aim and specific objectives: The project had three broad aims:

1. evaluate the effectiveness of CES at wetland inlets;
2. assess possible impacts of existing screen configurations on native fish communities that would normally access wetlands; and,
3. design (if possible) an optimised CES to allow small and medium sized native fish to pass in and out of wetlands whilst denying access to mature carp.

Methods: The aims of this project were achieved by undertaking six key research activities:

1. a desktop literature review and a field reconnaissance to evaluate the existing diversity in the design and management of CES within the Murray–Darling Basin;
2. analysis of available data from recent comprehensive wetlands surveys (the 2004–2007 South Australian River Murray Wetlands Baseline Surveys) to evaluate differences in the relative abundances of carp and native fishes in wetlands with and without CES;
3. identification of the species composition and sizes of fishes and other aquatic fauna that make lateral migrations through wetland inlets and which might, therefore, be affected by the use of CES;
4. modelling to establish the size range of large-bodied fish that could pass through different screen mesh dimensions;
5. calculation of ‘optimised’ mesh designs that would prevent the passage of mature, breeding-size carp (>250 mm TL) whilst allowing the passage of a majority of small and medium sized native fishes that use wetlands; and,
6. laboratory and field trials of the most common existing screen mesh designs versus the optimised designs.

Figure 1: Carp accumulating downstream of a Carp exclusion screen at Sweeneys wetland. (Photo courtesy SARDI)

Figure 2: Carp Screens installed on a channel at Riverglades SA. (Photo courtesy Leigh Thwaites)

Figure 3: Pivoting carp screen adjustment at Ramco SA. (Photo courtesy of Leigh Thwaites

Findings: The current CES designs and management regimes were noted to have been ineffective in reducing the numbers and biomass of Carp in wetlands.

A diverse and abundant native fish community (14 species) was found to utilise wetlands and wetland inlets. Some existing exclusion screen designs are detrimental to native fish (by excluding most sizes and life history stages), including species of conservation significance. Other aquatic fauna, such as turtles, are also likely to be impacted.

Two types of screens that will optimise the exclusion of large, sexually mature carp were designed:

• A square grid mesh with 44 mm gaps
• A “jail-bar” design with 31.4 mm gaps.

The jail bar design was found to collect less debris, trap more Carp and less native fish, and had little effect on flow velocity.

Lessons learned and future directions:CES may be beneficial as part of an integrated Carp management regime in some wetlands. Presently, there is no benefit in using CES in permanently inundated wetlands, unless other Carp reduction measures are also employed.

• The use of CES alone should be considered for use at seasonal/ ephemeral wetlands that dry every 1-2 years. They may also be suited to permanent shallow wetlands that remain filled for >2 years at a time, if it can be shown that all adult Carp migrate from wetlands to overwinter in deeper river water.
• The jail bar CES with 31 mm apertures between bars screen passed more native fish, including the greatest proportion of Bony Herring (Nematalosa erebi) (>90%), which are the key large-bodied native fish found to use wetlands and wetland inlets.
• All CES need to be regularly maintained to ensure that they are functioning as intended and are not altering channel hydrodynamics or impeding the passage of native fauna.

Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.

Contacts: Dr Leigh Thwaites, South Australian Research and Development Institute. Tel: + 61 8 8207 5495, Email: leigh.thwaites@sa.gov.au.

Link: Not yet published.