The Cooks Beach Lakes were created as part of a subdivision development begun in the mid to late 90s. Construction work was carried out by Hoppers. Prior to the development the area was swamp land and part of the Harsant farm.
The lakes A and B were constructed as a storm water detention system with lake B being the forebay for the system, essentially a catchment lake.
However, the developers marketed these sections as “lake front” and were priced accordingly. All residents of the area therefore consider this system to be “lakes” which are classified for “passive recreation.”
Resource consent for the lakes was issued to Waikato Regional Council (WRC) who in turn vested ownership to TCDC, who has management and control of ongoing maintenance.
The main method of weed control for more than 20 years has been the use of an aquatic spray, Diquat with a gel added. This spray has been in widespread use in NZ waterways and rivers for 60 years and is approved by NIWA (now Ocean Earth Sciences). The name sounds ugly but this is a desiccant type spray, which essentially breaks down the leaf structure and the plant drops to the bottom. It does NOT kill the root structure and creates a composting effect for the plant to regrow in spring and summer. The decaying vegetation will result in a decrease in oxygen levels on the lake’s bottom resulting in changes to the benthic fauna.
The main weed type in the lake is Potamogeton ochreatus commonly known as blunt pondweed. Considered indigenous in NZ it is also widespread in Australia and SE Asia therefore is not native. During the warmer months an algal bloom forms and this can cause botulism in the local wildlife if consumed. Over the years residents have either had to remove dead ducks or call council to do this.
Up to now most methods of weed control have been attempted including mechanical harvesting, which is similar to mowing lawns and therefore needs to be done on a regular basis. This is a very expensive approach to maintenance and only has a short window of effectiveness before the weed comes away again. Mechanical harvesting results in small eels, bullies and other native fish being trapped in the harvested weed and dying.
There have been suggestions of riparian planting and the installation of aerators however neither of these ideas are practical as a 2 metre wide strip of planting will be ineffective and the use of expensive, noisy aerators would do absolutely nothing to alleviate the high level of nutrients feeding the weed that flows into the system from the farm land up the back.
In July 2022 a submission was made to the then Mercury Bay Community Board to consider the use of grass or silver carp as a weed control measure. The idea was adopted with the request that TCDC investigate this as a weed control measure as distinct from the annual diquat aquatic spray programme.
Differentiating the types of Carp
At this point it is important to distinguish between grass or silver carp and koi carp. Koi carp are omnivorous and have a reputation for destroying riverbanks and waterway edges.
And consuming anything they come across. Conversely grass or silver carp are herbivores being totally weed eaters. The usual reaction from people when talking about carp is one of total ignorance by not knowing the difference between the two species and consequently this brings a negative, unfounded reaction to any suggestion of grass carp being used for weed control.
Grass carp are known as the “cows of the ponds” as they tend to graze and move around a waterway rather than eat it out and are stocked according to the visible weed-over rate.
As the weed cover is reduced and with careful management, the fish stock numbers can be reduced, still controlling the weed but resulting in further potential cost savings to Council. You could say a one-off cost as the fish can live for upwards of 10 years or longer.
The use of grass carp is widespread in NZ and one of the most successful biodiverse methods of weed control used today. Grass carp are widely used in neighbouring councils, Tauranga, Western Bays, Auckland and throughout the South Island with great success.
Dr Robert (Bob) McDowall, a leading expert on NZ freshwater fisheries, stated that grass carp were one of the most carefully introduced species into NZ.
Another expert in the field said, “In my 45 years involvement with grass carp, I have not seen any adverse environmental effects with the use of grass carp for weed control. If there were any, the carp would have been mostly removed.”
The process for grass carp applications are subject to the Conservation Act and handled by DoC. Starting with a public notification process in major newspapers, in the TCDC case there were NO objections but a lot of support for the initiative.
The DoC process is quite thorough and there is a Government legislated requirement to consult with local Iwi.
This does not give Iwi the right of veto but is a consultation process only.
Any release of grass carp is strictly controlled by DoC with security screens being required to contain the fish. These fish will not survive in salt water with the only access to the sea being the stream behind the Cooks Beach hall. Salty or briny water comes up well behind the Cooks Beach camp ground so containment of the fish is critical for their survival.
In over 40 years of releasing these fish they have never bred in the wild as local conditions do not meet their needs. Testament to this is the fact that there is only one company approved by the Government to breed these fish, were they to breed freely then that company would have gone out of business long ago!
The grass carp were featured on a Country Calendar programme a few years ago and it is a very worthwhile watch. See the link: Country Calendar Episode 26 ‘The Soil Beneath’ – NZ Waterways Feature
The lake is home to natives such as eels, the common bully, inunga and grey mullet, but also, sadly, has an unwanted population of red eared slider turtles. These get dumped when no longer required as pets. They are omnivorous eating small fish and carry salmonella. They are considered in the world’s most 100 invasive species. The local resident’s group has managed to catch about 80 in recent years and hand over to WRC for disposal. If it wasn’t for the local resident’s group catching these pests then they would be out of control.
Over the last few years particularly there has been a lot of money spent on the lakes’ maintenance (as a requirement under the Resource consent) with nothing really to show for it.
The only method of weed control NOT tried by Council is the use of grass carp, but is by far, result-wise undoubtedly the most successful, eco friendly, method available today.
Therefore it is difficult to grasp why a tried and proven, cost effective method of weed control wasn’t instigated years ago by TCDC.
