Beneath the surface

What's going on with Rotorua's lakes? (11 minute read)

The mirror-calm surface of Lake Rotoiti looks so perfect that it’s hard to believe this is a lake in desperate trouble. On the day I visit, a haze in the air makes the far shore look as if it has been painted in watercolour, with graceful reeds on the fringes and black swans gliding by. The voices of children, excited as they pull up a fishing net, carry to me across the water.

Beneath the surface, though, the picture of Rotoiti, and its larger neighbour Lake Rotorua, is less idyllic. The two lakes, connected to each other by the Ohau Channel, have been swamped by pollution and pests. Now, a unique biosecurity partnership between the Te Arawa Lakes Trust and the Bay of Plenty Regional Council is working to restore the two lakes, and to prevent others in the region suffering the same fate.

The children pulling a fishing net from the water in Te Wētā Bay, an inlet on the north-west of Lake Rotoiti, are part of that partnership. They’re students from a local school, and they’re learning how to be Catfish Killas. As I watch, they pull the net from the water, shake it into a blue bin – a systematic process of moving from one segment to the next – then crowd around to see what they’ve caught.

Lake Rotoiti (image credit: Melanie Newfield)

This is not the first net they’ve checked, so the children already know toi toi, a hardy native fish common in Lake Rotoiti. These make up most of the catch, and are quickly scooped up and returned to the water. William Anaru, Operations Manager for Biosecurity and Taiao Restoration for Te Arawa Lakes Trust, points out a morihana, or goldfish, too, which is also returned to the lake. Then there’s one fish left. It’s dark brown, with a wide mouth surrounded by four pairs of whiskers. This one’s a baby, at only 10 centimetres, but they can reach half a metre in length. It’s a brown bull-head catfish, first found in Rotoiti in 2016. I can hear the excitement of the children as William points it out. This is the fish that the Catfish Killas are looking for.

Catfish are native to North America and have been in New Zealand since the late 1800s, but it was the 1980s when they really began to spread. They were found in Taupō in 1985 and have spread down the Waikato River, as well as being found in Westland, Canterbury and Northland. They can survive up to 24 hours out of the water, so can be transported on boats and fishing nets. They eat aquatic insects, small fish and kōura (freshwater crayfish). They also churn up the mud on the lake bottom, in a way that no native fish does, contributing to poor water quality.

Greg Corbett, Biosecurity Manager at the Bay of Plenty Regional Council, tells me about the discovery of catfish in Lake Rotoiti. “Within a week or two of the first detection, we’d caught about 50 catfish out of Te Wētā Bay, so they were reasonably well-established. That had us scratching our heads over what to do about them.”

As the council formulated a plan, Greg recalled conversations he’d had with members of the Te Arawa Lakes Trust, the body set up to represent the interests of Te Arawa under the Te Arawa Lakes Settlement Act 2006. Although the settlement had handed title to the lake beds back to Te Arawa, they continued to see other groups, such as the Bay of Plenty Regional Council and the Department of Conservation, doing the work of caring for the lakes. Why was Te Arawa not out there? Why were they not doing the mahi?

Greg thought they had a point. And so, when the catfish were discovered in Rotoiti, he sat down with members of the Te Arawa Lakes Trust and they worked out a plan. Te Arawa would lead the community engagement work for the catfish control programme.

Te Arawa just needed the right person to run it, someone who knew about biosecurity and could share their passion for protecting the lakes. That was where William Anaru came in. He knew catfish, from fishing in Lake Taupō and the USA. And he knew biosecurity, from his work controlling pest plants and animals. With William on board, Te Arawa created the Catfish Killas programme.

I first met William via video call. He’s a busy man, fitting the conversation in between picking up his tamariki and going to a meeting, but his enthusiasm for nature tumbles out. I ask where it comes from and he tells me it was always there. “I’ve got a video of me at my first birthday, and everyone has given me all these cool-as toys and I’m playing with a bug”. But it’s more than that. His interests were nurtured by his whānau. “My koro was a big influence on me. He used to pick me before school and take me fishing on the lake and tell me all the stories of the lake, then drop me off at school. Then we’d come back to his house after school and he’d smoked all the fish that we caught.”

From his own experience, William understands the importance of building a connection with the environment in young people. Although the Catfish Killas programme involves all ages as volunteers, he’s particularly passionate about the school programme. “We’ve had 34 schools through the programme since 2018. At-risk youth too. I remember one boy, he’d never had any interest in this kind of thing, not before the Catfish Killas. Now he’s studying environmental science.”

Spurred on by William’s enthusiasm, the community around Rotoiti set their nets and helped to control the catfish. But that programme was only the start, Greg tells me. “Around this time, we were reviewing our pest management plan and looking how we improve compliance at boat ramps, ensuring that boats are clean and not transporting pests like the catfish, and also lake weed. There was this building capability in Te Arawa, and they were quick to say ‘we can do this’. So we got them involved in delivering that work.”

Lake weed problems in Lakes Rotorua and Rotoiti first began in the 1930s. Because the lake weeds grow fully submerged, they weren’t immediately obvious. But they were quietly smothering the growth of native plants and making the habitat less suitable for species such as kōura. They first came to public attention in the late 1950s, when stinking masses of weed began washing up on Rotorua’s shoreline. There was a public outcry, followed by calls for the government to act and arguments over who was responsible. A partial solution was found in the form of mechanical harvesters and the chemical diquat, which was used to spray the extensive weed beds. But the weeds kept coming back.

The problem with lake weed is aggravated by another problem facing the lakes. For decades, nutrients, especially nitrogen and phosphorus, have been pouring into Lake Rotorua, from sewage, from fertiliser, from livestock manure and from industry. From Lake Rotorua, they have flowed down the Ohau Channel and into Lake Rotoiti. We tend to think of nutrients as things which are good for us, but the same doesn’t apply to lakes. A healthy lake is low in nutrients. Add too many, and the environment gets out of balance.

Excessive growth of lake weed is one sign of that imbalance. Another symptom is the blooms of algae which plague the lakes. The algae occur there naturally, but high nutrient levels and high temperatures encourage proliferation, turning the water a lurid shade of green. They aren’t just unsightly, they can also be toxic to humans, livestock and dogs. At times, they can deplete the oxygen in the water and kill fish.

Lakes are more sensitive to excess nutrients than rivers and streams. In a river, the sediment which carries a lot of the nutrients is washed out to sea and eventually sinks to the depths of the ocean. In a lake, the sediment tends to stay around. Anything which churns up the bottom of the lake, like a catfish, releases a flood of nutrients back into the water, encouraging even more weed growth.

When catfish were detected in 2016, Te Arawa, the Bay of Plenty Regional Council and the Rotorua Lakes District Council had already been working together for some years to manage the nutrients in the lake. In 2008, they had taken the drastic step of diverting the flow of the Ohau Channel away from Lake Rotoiti and directly down the Kaituna River. This stopped water from the more polluted Lake Rotorua affecting Lake Rotoiti. They were applying alum to some streams flowing into the lakes to remove phosphorus. They were connecting properties near the lakes to the municipal sewerage system rather than relying on septic tanks. They were protecting and restoring wetlands on the margins. But until William and the Catfish Killas, Te Arawa had limited practical involvement.

William’s skills offered increasing opportunities for Te Arawa to actually do the work. Wetland restoration requires weed control and William had experience with agricultural spraying. So his team took on more staff to do that work. Then there were the lake surveys. Rotorua and Rotoiti have four different species of exotic lake weed, but other lakes in the region are free from some of the most troublesome species. It’s no simple matter to detect one species of lake weed among others when they’re under the water, but some of William’s team were divers. They did the training and qualified to become occupational divers, and now do most of the lake weed surveys.

New ideas began to surface. One method for controlling lake weeds is to smother them with hessian mats. It’s a method which can’t cover as large an area as spraying, but it’s ideal for the most sensitive environments and can allow native water plants to re-establish. Instead of hessian, Te Arawa wove uwhi mats from harakeke and began trialling their use. William tells me, “They work really, really well but we just need to go bigger.”

But one major biosecurity programme was not managed by the Bay of Plenty Regional Council. The lake weed spraying was the responsibility of Land Information New Zealand, the agency which holds title to most of the lake and river beds in New Zealand. They used a single contractor to manage any lake weed spraying around New Zealand. William thought they could do that too, and opened negotiations with Land Information New Zealand. In 2022, the Te Arawa Lakes Trust took over that work and are now able to make decisions on when and where the spraying is done.

Te Arawa are not unique in having a treaty settlement which gives them a greater say in the management of natural resources in their rohe. What is impressive, though, is the degree to which they have become practically involved in managing pests and weeds. They are now doing almost all the biosecurity work on the lakes that was once done by the council, and more. Greg Corbett is full of praise for their work and the partnership. “It has brought new and fresh thinking. It’s a very effective and fluid partnership. We have a good ability to respond fast and very good open lines of communication.”

William, too, is passionate about what has been achieved. “The best thing over the last few years is being able to give more of our whānau work. For generations, there’s always been that noise that our people should be doing more work on the lakes. Now I have 40 people in the team I manage and all but three of them are Te Arawa. And seeing the change in a lot of the kids too when we work with them, that has been really, really cool.”

There are still some major challenges. Recently, areas sprayed for lake weed have suffered algal blooms. Greg tells me that scientists have struggled to show that there’s a link between the spraying and the algae, but they can’t rule it out. The catfish, too, are an ongoing problem. Surveys have shown that it is in Lake Rotorua as well as Rotoiti, and the numbers are astounding. In 2019, nearly 62,000 were caught. The hard work of the Catfish Killas is making a difference – only around 11,000 were caught last year – but William isn’t likely to be out of a job soon, even if he would like to be one day.

“My kids and their kids might have to do this job for a little while longer but I think as technology advances, somewhere in the world they’re going to develop some real awesome solutions for a lot of these problems. So I’m hopeful that the day will come when no one will have to do this job.” In the meantime, though, he’s achieved something special.

Lake Rotorua (image credit: Melanie Newfield)