Key facts about freshwater restoration and eradication of trout and perch at Zealandia - Karori Sanctuary

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Background

The vision of the Karori Sanctuary Trust is to restore the sanctuary as close as possible to its pre-human state. 

The Trust’s Restoration Strategy and our Management Plan¹ identify the removal of introduced plants and animals from inside the fenced area as necessary to achieve this goal.

Since the construction of a predator proof fence in 1999, the Trust has had great success with the removal of introduced mammals and control of weeds from the terrestrial ecosystem. This has allowed the release into the sanctuary of 16 species of locally extinct birds, reptiles, frogs and invertebrates, a ground breaking achievement within 10 years of construction of the fence. Many locally extinct or rare plants have been planted to improve the natural diversity of the mainly forest ecosystem. The Trust is now ready to begin the restoration of our streams and lakes.

¹ The Karori Sanctuary Trust’s Management Plan (1997) was approved by Wellington City Council and Wellington Regional Council in January 1998. This followed extensive consultation with 95 clubs and organisations and with more than150 individuals over a period of two years.

Reasons for eradication of introduced fish

• ZEALANDIA’s lakes and streams are regionally significant because they are relatively intact freshwater ecosystems with good quality habitat; are protected within a sanctuary environment and will provide invaluable potential for restoration of freshwater flora and fauna communities that are rare or missing in the wider Wellington region.

• In order to be able to restore the freshwater ecosystem, all introduced fish (brown trout and perch) will need to be removed because they are affecting the water quality and native fish and freshwater invertebrate communities in the sanctuary.

• Brown trout were introduced into New Zealand in 1867 and are a fast growing active predator and competitor - they eat native fish and invertebrates, including the freshwater crayfish or koura, and directly compete with native fish for food.  Their adverse effects on native freshwater fish and invertebrates have been well documented in New Zealand and overseas. Trout are found in large numbers in the upper lake and tributaries and in much lower numbers downstream of the upper dam.

• New Zealand is well known for its trout fishery and trout are highly valued. However, fishing is not permitted in the sanctuary and the removal of trout will fulfil the objectives of ZEALANDIA.

• Perch were introduced into the lower lake sometime after its construction in 1878.  They are found below the upper dam in the stream, wetland and lower lake. Surveys of perch indicate exceptionally high numbers of very small sized perch. Smaller perch eat microscopic animals (zooplankton), and this is having a detrimental effect on the foodchain and water quality. If most of the zooplankton have been eaten, then the microscopic plants (phytoplankton) usually eaten by zooplankton are freed from browsing pressure and able to explode in numbers causing algal blooms. Larger perch are very active predators of fish and all life stages compete with native fish and freshwater invertebrates for food.

• Algal blooms in the lake are not new – they occurred well before the sanctuary’s existence.  The regularity and intensity is a new factor.  Blooms are visually unattractive, the smell is pungent and both the odour and contact with the bloom can be a danger to animal and human health.  It is anticipated that removal of perch will increase the density of zooplankton, and thereby lower the density of phytoplankton (and hence reduce the frequency and intensity of algae blooms).

• Native fish will be returned to the upper reservoir streams once rotenone levels have dissipated.

What will be used to eradicate the fish and why?

• Because of the size of the lakes and the length and complexity of the streams in the sanctuary, the Trust have been advised by the Department of Conservation (DOC), that the most cost effective method with the greatest chance of a successful eradication of brown trout and perch from the sanctuary will be treatment using a toxin with the active ingredient rotenone. The upper lake and tributaries will be the focus of the current trial.

• Rotenone is a naturally occurring plant extract, that is highly toxic to fish. Rotenone acts on fish by disrupting respiration at a cellular level. It has been used in fisheries management since the 1930s and as a commercial insecticide in Derris Dust for even longer. Rotenone breaks down rapidly in sunlight and air and does not accumulate in the sediments of water bodies. At 15-22°C it is undetectable in three days.

• Rotenone is approved by ERMA for use as a piscicide (fish toxin) in New Zealand. Fish are so highly sensitive to rotenone that a lower concentration of the toxicant can be used against fish than that required against insects. It has been used successfully both in New Zealand and overseas to remove fish from rivers, wetlands, ponds and lakes. The most recent use to eradicate fish in Wellington was at the Lower Whitby Lake in 2007.

• While rotenone has been used successfully in lakes, wetlands and ponds, this will be the first use of rotenone in flowing water (streams) in New Zealand. DOC is interested in trialling this technique at the sanctuary because:

  • it can be trialled in the upper tributaries and lake which can be effectively closed off to eliminate downstream discharge of rotenone;

  • it supports the restoration of a regionally significant freshwater ecosystem with significant potential for native freshwater fish and invertebrates.

  • if successful, rotenone could be a useful tool for protecting native fish in situations where other methods would not be effective.  This will not lead to widespread use of rotenone.

• Rotenone has low toxicity to birds and mammals. When birds and mammals drink water and eat dead fish that contain rotenone they are not affected because they have natural enzymes in the digestive tract that neutralise the toxicant. In Central and South America the indigenous people use the plant roots that contain rotenone to kill fish for food, collecting the dead fish from the water to be eaten.

• When used according to instructions, rotenone poses negligible hazard to public health. The United States Environmental Protection Agency has approved the use of rotenone in waters used for irrigation, livestock and swimming. The greatest risk to human health is to those handling the toxicant so full protective clothing is worn to reduce exposure to it.

• Alternative techniques such as netting, electric fishing and low pH have been investigated to see if these would be feasible. Using low pH was unsuccessful and other techniques, while useful for control, are not effective eradication techniques. They are labour intensive because of the widespread distribution of trout up most of the small tributary streams and there would be a high risk of failure because only one trout pair needs to survive to re-colonise the lakes and streams.

When will the eradication trial happen?

• The trial has received Resource Consent from Greater Wellington Regional Council to allow the treatment of the upper lake and its tributaries. The trial will take place in late summer 2011 when low stream flows and warm temperatures will help in reducing the amount of rotenone we use and how quickly it breaks down.

• The waterways below the upper dam will not be treated in this operation.

•  If the trial is successful then planning will begin for the removal of brown trout and perch from the lower lake and streams below the upper dam.

• The Trust has no plan to eradicate introduced fish species outside the sanctuary.

How will the eradication trial happen?

• There are several steps to the operation in the upper lake and its tributaries.

  • To protect the native fish and koura already present, as many as possible will be caught and transferred to live cages in untreated streams so that they can be released back into the upper lake and streams after the operation.

  • The upper lake will be lowered and its outlet sealed to reduce water volume and prevent any discharge of toxin downstream.

  • The rotenone will be added to the streams as a slurry and pumped via hoses onto the lake to kill the trout.

  • When the water is no longer toxic, the outlet will be reopened and water once again allowed to flow downstream into the lower lake.

• There is no chance of any rotenone leaving the sanctuary during the trial.

• Detailed monitoring will be undertaken to determine the effects of the toxin on the freshwater communities, the effectiveness of the operation and the effects of trout removal on native freshwater fish and invertebrates. These studies are being undertaken by researchers from DOC, University of Otago and Massey University.

What happens to native species present?

• Only one species of native fish (banded kokopu) is known to live in the upper lake and tributaries. Some banded kokopu live in streams where trout have been unable to penetrate and these will not be exposed to the rotenone. However, in the area to be treated with rotenone, as many kokopu as possible will be caught and transferred to live holding cages in untreated streams before the operation. During the operation any native fish affected will be rescued and revived if possible and placed into the live cages. Once the waters are deemed safe after the operation, these fish will be released back into the lake and streams.

• Aquatic invertebrates are less sensitive than fish to rotenone, but will also be impacted when exposed to rotenone. However they are also found in nearby streams or upstream of the operational area that will not be exposed to rotenone.

• In addition, in the area to be treated with rotenone, as many koura as possible will be caught and transferred to live holding cages in untreated streams before the operation. Re-colonisation is expected to be rapid by invertebrates, either being washed downstream, or by many species having a mobile adult life stage. The effects of the operation will be closely monitored.

• Birds are much less sensitive to rotenone than fish and the likelihood of birds receiving a lethal dose is extremely small because the lethal dose for a bird is 100 times higher than the discharge concentration proposed. Scavenging birds may be affected indirectly by bacterial infections caused by eating rotting fish, but dead fish will be collected for off-site disposal to minimise risks.