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Lower Reservoir Restoration Project
ZEALANDIA Ecosanctuary

Lower Reservoir Restoration Project

ZEALANDIA includes some incredible waterways as well forest ecosystems. We are excited to be embarking on a long-awaited effort to begin restoring the health of Roto Kawau, our lower reservoir. Our first major step will be removing introduced perch from the reservoir and wetland areas. Through no fault of their own the perch have become over-abundant, which means the ecosystem is out of balance. Removing them will create a small, safe corner of the world for our incredible native freshwater wildlife.  

As we carry out this important restoration work, there will be the occasional day we need to close the sanctuary. We will let you know as far in advance as possible when that occurs. Further, the boat and pontoon will be unavailable, the takahē will move to the upper eastern side of the sanctuary and you will notice changes to the lake, wetlands and streams as they are slowly, but temporarily, lowered.   
 
This project is similar to the work done in 2011 when trout were removed from the upper reservoir. This work provided us with the opportunity to begin introducing threatened species such as kākahi/ freshwater mussels, to ZEALANDIA! It is worth heading up to the upper dam to see an example of a healthy freshwater environment and how beneficial this project will be once completed.  

Check out this video of Dr Danielle Shanahan explaining this project and some of the changes you can expect to see. 

 

UPDATE: Friday 19 March

We are now well underway on the journey to restore Roto Kawau. You’ll notice a few changes in the sanctuary. Over the next few days, the Te Māhanga Stream and wetland will remain rather dry. You will see standard Department of Conservation safety signs up indicating the use of rotenone, and warning people not to touch the water—these will be in place for one month. We have also put out restricted access tape in places where people could approach the water.

We want to assure you that you are safe when you visit, as the way we have used rotenone poses no risk to people or birds. Please respect the signs however, and do not touch the water.

Rotenone is the powdered root of a plant, Derris elliptica - the same plant that’s used to make Derris Dust, an organic compound that many New Zealanders use in their vegetable gardens. People and the manu/birds will not be affected by the rotenone. Their digestive systems, like ours, contain aggressive enzymes that break down rotenone’s components before it has any effect.  The lethal dose for a bird is 100 times stronger than the concentration we will be using.

We’ve been working hard over the last few months to move any native fish from this area, and during the operation, any native fish would be placed immediately in a bucket of fresh water where they would quickly revive. The water downstream will not be affected, because Roto Kawau is a humanmade reservoir and previously part of Wellington’s water supply so we have the means to simply ‘switch off’ the outflow into the Kaiwharawhara Stream. It will not be opened again until we have tested the water and it is free of rotenone.

We have been working to reduce the very high numbers of perch in the reservoir for some time. This final step of completely removing these fish is something we are aiming to do once, and do right, just like the removal of mammalian predators from the rest of the sanctuary that has led to such incredible outcomes for the sanctuary and Wellington.

 

 
FAQS 

What’s the plan?  

One major step in enhancing the health of Roto Kawau will be removing all the perch that are in it. This has been a long-planned science-based project. It will involve a four-step process:  

1.     Lowering of the reservoir level (March-May 2021)  

2.     Removal of perch from the stream and wetland (March 2021)  

3.     Removal of perch in the reservoir (May 2021)  

  

What’s the problem?  

ZEALANDIA’s lower reservoir Roto Kawau is unhealthy and out of balance. The reservoir has poor quality water and suffers an annual algal bloom. Few native species are able to hang on in there. One of the key problems is an overabundance of perch, which were introduced to the reservoir many years ago. Now, more than 20,000 are in the small reservoir. These perch alter the food chain in the reservoir by consuming very large numbers of invertebrates (or zooplankton), which normally eat the algae (in this case, a phytoplankton called cyanobacteria) keeping it in check. Perch are also predators of native fish, so to make space for our native wildlife we need to deal with this key issue.   

  

Why is it important?  

Freshwater biodiversity is in trouble across New Zealand. Water quality and habitat loss are all taking their toll on our very unique species and ecosystems. We need to take some serious steps to reverse their decline. Restoring the freshwater ecosystems at ZEALANDIA is just one of these steps.  

ZEALANDIA’s streams and reservoirs form the headwaters of the Kaiwharawhara Stream, and improvements at the source have positive effects downstream and all the way to the coast, helping restore a significant urban stream and enhancing the quality of the harbour and city beaches.  

  

What’s the goal?  

Our goal is to restore the mouri/lifeforce of Roto Kawau. We are aiming to create a corner of the world where native wildlife can thrive. We are also working to enhance the habitats within the wetland and streams to help bring back native freshwater diversity to the Kaiwharawhara catchment in the same way that we have brought back native birdlife to Wellington’s parks and gardens.   

While the work we do in the valley continues, ZEALANDIA’s second generation strategy ‘Living with Nature’ extends beyond the fence—the goal is to help create a nature-rich future for Wellington city. As part of this vision, the Sanctuary to Sea project seeks to engage the community in a whole-of-catchment waterway improvement effort, recreating an incredible reservoir habitat right in the middle of Wellington.    

This project demonstrates how many people can work together to enhance the environmental values of a waterway in an urban landscape.  

  

How will we remove the perch?  

The project involves multiple methods. We will first use netting and a fishing technique called electric fishing to get the numbers down. We are then planning on using a tried and tested substance called rotenone. A familiar product that includes rotenone is Derris Dust, an organic product that you can find in hardware shops to keep bugs away from your vegetables. This is a highly targeted substance that will be turned into a slurry and sprayed by Department of Conservation staff by hand over the Te Mahanga Stream and wetland in mid March, and then over Roto Kawau by helicopter in mid May. This will kill the fish in the treated area.  

  

Will the sanctuary / visitor centre be closed?  

Yes. The sanctuary itself will need to close for one day in mid-March (when the Visitor Centre will remain open), and one day in mid-May, when both the sanctuary and the Visitor Centre will close, for the helicopter operation.  

The exact dates haven’t been confirmed as they depend on weather conditions.  

 

What will we see?  

The project will be very visible with a number of facilities changes during the time. For example, new power cables will be installed to cross Lake Road, and part of the heritage lawn will be fenced (across the front of the boat shed) for the duration of the project to ensure we have safe working space, and that people remain safely away from the water's edge.   

Vegetation will be removed from the wetland and along the Te Mahanga Stream to ensure there are no hiding places for fish. 

The lower reservoir will be lowered by between four and eight metres, beginning mid-March; this will happen slowly, up to one metre drop per week.  The boat and pontoons will be removed. Visitors will not be able to access the pontoon area for the duration of the operation. The lowering should be complete by mid-May. We’ll be using portable pumps and discharging the water via the spillway. The reservoir’s water usually flows this way into the Kaiwharawhara Stream, and we will maintain this flow, so that there are no negative effects downstream.  

The reservoir will then become much smaller with many of the banks exposed.  

Why hasn’t something been done about it before?  

We’ve tried many approaches over the years to reduce the numbers of perch in the reservoir, for example, electric fishing. Nothing works particularly well and perch numbers bounce back very quickly, so completely removing all the perch is considered the only feasible option to make space for our native freshwater species. This has involved a massive and complex science-based planning process, in consultation with mana whenua, Department of Conservation, Greater Wellington Regional Council, Wellington Water and Wellington City Council.  

The restoration of the terrestrial ecosystems of the valley has been the priority since the establishment of the ecosanctuary in 1995. The enormous amount of work done by staff, volunteers and supporters has enhanced the valley’s habitat to the point where we can now increase our focus on the waterways. Restoration of the waterways has always been part of the vision, but has taken time to plan and coordinate. The restoration of the upper reservoir, Roto Mahanga, began in 2011 with the use of rotenone to remove perch and brown trout. This was a success and has put us in a great position to treat the lower reservoir effectively.  

  

What will happen to the native fish?  

We have begun a process of removing as many native fish as possible before we begin. Over 100 tuna/eel have been captured and released downstream, which will allow them to complete their lifecycle. As many banded kokopū as possible, and other native freshwater species such as freshwater sponges, will be moved to the upper reservoir system and returned back down later where they will be able to re-establish in the restored habitat.  

During the operation, a rescue team will follow the crew, retrieving affected native fish and where possible they will be revived in clean water. They will then be relocated to the upper streams with the others.   

How can you be sure you’ve got all the perch?  

Our goal is to do this once and well, so that we don’t need to do it again. This is why we will be managing vegetation to remove hiding places, and avoiding the time of year where there will be perch eggs. We have successfully used these methods before in the upper reservoir, and now have a much healthier ecosystem where we have reintroduced two species of kākahi, which are threatened freshwater mussels.   

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