LAKES | PONDS | RIVERS
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NZ WATERWAYS CASE STUDIES


Western Springs Lake Restoration
CASE STUDY | SEPTEMBER 1996 - JANUARY 2006

Western Springs Lake Restoration

Prior to the release of White Amur, extensive mats of floating weed were common sights on the lake and several times over the warmer months, weed harvesting boats were employed to remove the weed. The first release of White Amurinto the lake occurred in late September 1996 when 1004 White Amur were released. The Amur were able to remove the weed by August 1998 and has not been a problem since.

The macrophytes at the time of the first survey (1996) were almost entirely comprised of the oxygen weed Egeria densa smothered in an extensive growth of filamentous algae. The filamentous algae not only covered the oxygen weed, but also were floating on the surface in large mats supported by many small bubbles. These mats had covered much of the surface of the lake but at the northern end, some had been gathered and removed by the use of a boat with a boom attached comprised of prongs extending into the water. The southern end of the lake was still covered.

102 days after the first survey, the situation had dramatically changed. No filamentous algae could be seen around the lake and none was observed when samples of macrophytes along the transect lines were examined. No Egeria could be seen around the shores of the lake and very few stalks of Egeria were observed floating on the surface. The lakebed around the shore did not have any other macrophytes present except for algae growing on the surface of the rocks. No Egeria or other macrophytes were observed reaching to surface of the lake – even in the deeper parts of the lake. Normally at that time of the year, Egeria would cover a large proportion of the surface of the lake and many small white flowers could be seen.

Presently all bar an approximate 5 fish have been removed from the lake. Although the removal was not recommended by NZWR, the Council will be restructuring Parks and Stormwater so we will see what the future holds.

NZWR has also been involved in the removal of Pest Fish from the lake. As it currently stands we have removed: 1852 Bullhead Catfish (adults), 98 Goldfish and 148 Koi Carp. 

For information and a summary of reports (the first and latest report) about this project follow the link  ‘PUBLICATIONS’ to ‘WESTERN SPRINGS FIRST & LAST REPORTS’

Omapere (Northland) Restoration
CASE STUDY | 2006

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 cyclic 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.

In the 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 – White Amur (grass carp)
  • 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 White Amur. When NZWR became involved the purpose of the project was to eliminate the weed Egeria densa, by introducing White Amur into the Lake.

Commencing in 2000 and over a period of 2 years 60,000 fish were released (as the lake is over 1200 ha. and White Amur were juvenilles) 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.

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”. White Amur were the only effective tool that could eradicate the weed and stop the cycle of eutrophication occurring.

Lake Eland (Hawke’s Bay)
CASE STUDY | 1995

Lake Eland (Hawke’s Bay)

Lake Eland (Hawke’s Bay) is a 4 ha spring fed shallow dam (max depth 7 m) lake on a privately owned farm. In the 1980s when it was first reported, hydrilla covered 1 ha of the lake growing down to a 4.5 m depth. Following confirmation that hydrilla had established in Lake Eland, a trial commenced in 1988 to determine the effectiveness of White Amur to control, and potentially to eradicate, hydrilla. As it has no inlet or outlet streams and is isolated from public access, Lake Eland was ideal for the White Amur trial. The trial design included an assessment of water quality, invertebrates, vegetation, fish and birds. Water quality and invertebrates were monitored by the Hawkes Bay Catchment Board. Amongst the submerged species all but hydrilla occurred in less than 1 m of water, 5 species had less than 5% cover, the Glossostigma species and elodea had 76-95% cover and the hydrilla had 100% cover to 4 m and occurred to depths of 4.5m. Hence the native plants that were present in the lake had a limited distribution and abundance with hydrilla dominating the littoral zone of the lake bed. Triploid White Amur were stocked in December 1988. Initially 100 fish per ha at 270 mm in length were stocked in November 1988. An assessment of vegetation in April 1990 revealed a major reduction in hydrilla, 17 months after White Amur were released. At this time the native plants Glossostigma and Typha were not visibly reduced, however in April 1991 evidence of White Amur browsing on Typha was first noted, whilst the dense beds of Glossostigma and Lilaeopsis remained to a depth of ca 2 m and were abundant to 1 m. ln November 1991 extensive searches at depths of 1-1.5 m revealed occasional hydrilla plants re-growing from tubers or buried stems, predominately in areas supporting low growing turf plants and amongst fallen tree branches. An annual vegetation survey of Lake Eland has continued since then, with a single hydrilla plant last found in 2003, and more recent surveys reporting only the continued presence of the turf plant community, and young raupo. As a landlocked lake, Eland had no fish of any intrinsic value in the absence of migration pathways to the sea (eg. no self-sustaining eel population). However, trout had previously been stocked along with common bullies as a food source for the trout. Common bullies were present in low numbers at the time of the White Amur introduction, and more recently were recorded as being abundant. The number of invertebrate taxa found in the lake has remained similar. The Lake Eland White Amurtrial has demonstrated the effectiveness of White Amur at removing hydrilla, while a turf plant community is retained and so too is the habitat for a range of macroinvertebrate taxa, common bullies and waterfowl.

Gear, I., & Hofstra, D. (2011). The Efficacy of Caged Grass Carp. Wellington: NIWA.

Lake Opouahi (Hawkes Bay)
CASE STUDY | 2008

Lake Opouahi (Hawkes Bay)

Lake Opouahi is the smallest (6ha) and highest of the currently hydrilla affected lakes and is located in the hills north and inland of Lake Tutira. It is situated in the Department of Conservation administered Opouahi Scenic Reserve, which has, under a joint venture arrangement with ECOED (Environment, Conservation and Outdoor Education Trust) also become home to the Opouahi Pan Pac Kiwi Creche following the construction of predator proof fencing by ECOED. Lake Opouahi has high scenic values and is surrounded by native bush and swampland with 20 to 30% of the lake’s catchment (44 ha) in farm land. There are several small inlet streams that pass through the northem wetland and the main inflow to the lake from the swamp is the Waipapa Stream. The lake outflow on its southern side is the Awatamatea Stream which eventually joins the Waikoau River. Hydrilla was first noted in Lake Opouahi in 1984 , and is likely to have established at some time between 1970 and its first record, because it was not reported in a lake vegetation survey in 1970. At the outset of the MAF hydrilla eradication response hydrilla formed discrete clumps of vegetation within the lake littoral zone, as well as a dense weed bed near the jetty. The lake and its surrounds are visited for hiking, bird watching and picnicking, and the lake itself provides water for adjoining properties. Water takes from the lake were problematic for the proposed use of herbicide to reduce hydrilla near the jetty. As a result, an alternative solution was devised. A containment fence was constructed around the hydrilla at the jetty and a number of the White Amur scheduled for release into the wider lake for hydrilla control, were released inside the fence. The intention was to retain the fish within the fence for four months, by which time it was anticipated that the hydrilla would be reduced to an acceptable level. However, the White Amur acted more quickly than expected and were released into the rest of the lake in two months, as all vegetation within the contained area had been consumed. This result highlighted the potential for White Amur to be used for aquatic weed control and/or eradication through a contained or ‘mob-stocking technique’.

Gear, I., & Hofstra, D. (2011). The Efficacy of Caged Grass Carp. Wellington: NIWA.

Lake Kereta
CASE STUDY | 2012

Lake Kereta

White Amur (grass carp) were stocked into Lake Kereta, (Auckland Region) to improve amenity values of the lake for boating and water skiing. This lake had been invaded by the submerged weed hornwort prior to 1999 and had dominated the open lake area. In a privately funded lake restoration programme, four grass carp stocking events were made between March 2008 and April 2009. Recent observations by local land owners suggested grass carp grazing effects were evident and NIWA were asked to assess the current status and success of weed management by grass carp.

Lake Kereta was visited by NIWA on 27 February 2012. A combination of surface observations, snorkel diving, sonar, bottom rake and grapnel drags were used to detect hornwort and describe vegetation characteristics. As well as spot checks for hornwort at c. ten sites in open water, the entre margins of Lake Kereta were inspected for likely refuges for hornwort from grass carp grazing, such as backwaters, swampy margins and dense marginal vegetation.

From the results it was apparent that most of the weed biomass (>99.9% of previous levels) had been removed from the lake, and the aim of an improvement to amenity values of the lake had been achieved. Observations made by NIWA in 2008 (de Winton et al. 2009) showed hornwort extended at >75% cover over most of the open water areas of the lake, with heights of between 0.5 and 2 m.  Given the area of the lake at 23.58 ha given by Freshwater Ecosystems of New Zealand, NIWA estimated approximately 20 vegetated hectares of hornwort had been removed, or a biomass probably exceeding 35 tonne and less than 298 tonne dry weight of weed.

NIWA have recommended that another assessment of hornwort status once the marginal wild rice beds have been removed from the lakeside, and to achieve eradication, the fish should be left in the lake for at least two years following the last detected weed fragments.

For more information and the full NIWA report about this project follow the link  ‘PUBLICATIONS’ to ‘HORNWORT REMOVAL IN LAKE KERETA’