Lake Omapere (Northland) Restoration

Lake Omapere was once a source of water for the Kaikohe region in the early 1970’s and 80’s. In December 1985 residents complained of stomach upsets after drinking water originating from the lake. Since then the lake suffered cyclinc eutrophication producing lengthy periods of poor water quality, offensive odour, death of native fish and fresh water mussels which are the lakes natural filters. The cyclic eutrophication produced from prolific weed growth triggered a toxic algal bloom which poisoned food and water sources.

Lake Omapere is a large shallow lake almost circular in shape occupying an area of c.1200ha. Recreational activities on the lake had virtually ceased and the Northland Area Health Board prohibited the taking of water and advised against bodily contact. The Lakes Trustees needed to act.

Options to get rid of Algae

Algal problem
Following the algal bloom of 1985, a task force consisting of staff from Northland Catchment Commission, Ministry of Works and Development, Ministry of Agriculture and Fisheries, Wildlife Services, Bay of Islands Acclimatisation Society and Northland Area Health Board was set up to formulate possible solutions to the water quality problems. The main solutions considered included:

• Creation of buffer zones around the lake margin which would trap agricultural
  run off and reduce nutrient inputs to the lake.
• Removal of nutrients from the lake using the planktivorous fish,
  silver carp (silver amur) to feed on the phytoplankton.
• Chemicals controlling algae using alum or cooper salts.
• Constructing artificial wetlands to strip nutrients and algae from water.

Silver Carp success on Algae
The Lakes Trustees showed a visionary approach. Silver carp were further evaluated and following an Environmental Impact Assessment, 100,000 silver carp were liberated into Lake Omapere between 1988 and 1989. NIWA evaluated the lake water quality in February 1992 and clarity had improved markedly. Data for chlorophylla also reflected this change. The silver carp were effective in filter feeding on the cyanobacteria algae and reducing its presence in Lake Omapere.

Eutrophication Cycle
Following its 1985 algal collapse Lake Omapere remained in a de-vegetated state for c.8 years when Egeria Densa weed was noted in increasingly dense clumps. By 1996 Egeria densa has spread and formed large patches covering 50% of the lake floor.

Paul Champion NIWA explained "the weed beds re-established after 1985 but collapsed again over the past two summers. This event also killed off many freshwater mussels that play a key role in filtering the lake clean"

If weed growth was allowed to continue the Lake Trustees feared a repeat of the conditions that led to the 1985 collapse and the decision was made to try and break the cycle of eutrophication.

Options to Break Cycle of Weed Growth

In their 1996 report, Paul Champion and David Rowe of NIWA assisted the Lakes Trustees in assessing the options:

• Herbicide – (an annual programme of spraying was already in place).
• Mechanical Harvesting – (expensive and anticipated as an ongoing need).
• Habitat manipulation, suction dredging – (not acceptable environmentally).
• Biological control using herbaceous fish - Grass Carp (White Amur).
• Do nothing (Lake too important to community).

The various options were assessed and with the requirement to eradicate weed, the recommendation from NIWA was to use grass carp.

"Previous experience indicates that the use of grass carp is the most cost effective and environmentally acceptable option for Egeria control in the case of Lake Omapere".

Aims / Objectives

The purpose of the project was to eliminate Egeria densa weed from Lake Omapere and break the eutrophication cycle.

How, who, when?

NZ Waterways Restoration (NZWR) is a privately held company that works in conjunction with Councils, DOC, Ministry of Fisheries, NIWA, Fish and Game Council and local Iwi to restore fresh waterbodies using biological control methods. NZWR represents and provides grass and silver carp on behalf of licence holders. The steps NZWR took in eradicating weed from Lake Omapere.

• NIWA site visit and identification of systemic problem. The total vegetated ha was calculated at 50% = c.600ha.
• Environmental Impact Assessment (EIA) prepared – by NIWA)
• Approval of EIA by DOC
• [Instalment of containment screens was not required due to rock barrier already installed for Silver carp]
• Fish delivery – implementation - NZWR
• Ongoing management, monitoring, review – NIWA, NRC

Commencing in 2000 and over a period of 2 years 60,000 fish were released by NZWR. In 2002, the herbicide spray of weed was called off as there was less than expected weed growth. Weed appeared to have been eradicated from most of the lake, with 80% of the lakebed having less than 5% weed cover. The biomass of the weed in the lake was estimated to have reduced significantly. NZWR continued for a further 2 years after the weed was eradicated.

Results / evaluation

In April 2004 NIWA surveyed the lake and the subsequent report concluded "No plants of Egeria densa were found within Lake Omapere". The report also noted "that the lake has probably been effectively de-vegetated for at least a year"

In the future and with sound management, this lake will produce potable water much needed in the Northland region. Grass carp were the only effective tool that could eradicate the weed and stop the cycle of eutrophication occurring.

Testimonials

In their role as Kaitiakitanga the Lake Omapere Trustees are working closely with local bodies and government agencies in delivering a lake management strategy. Not only is this holistic approach good for the environment but it can also deliver direct financial benefits to iwi. "The use of white amur (grass carp) has been such a success, attention now can be given to the managing and control of the blue-green algae problem" - Mr Kelleher, Chair Lake Omapere Trust.

Contact NZWR

Today, NZWR specialises in biological restoration of lakes. This includes

• Pest weed eradication
• Toxic blue/green algal management
• Pest fish eradication

NZWR has the complete solution in biological control tools to stop the cyclic eutrophication of lakes and help promote the regrowth of native flora and fauna. For an assessment of fresh water management needs.

NZWR
Tel: 021 908 676
Email : info@nzwaterways.co.nz
www.nzwaterways.co.nz