19 August 2024
The sun is setting at Te Mata Hāpuku. The eelers of Ngāi Tahu have been hard at work digging kōawa, drains that stretch across the cobble flats between Te Roto o Wairewa and the ocean. Beside these drains sit pārua, pits dug into the earth waiting to be filled with the annual harvest of tuna, the eels that are the customary fishery for whānau members.
After sunset, the eelers settle in to wait. As the tide rises saltwater percolates through the beach cobbles reaching the kōawa. The smell of this saltwater sends a signal to the tuna in the lake. For them, it’s time to begin a remarkable journey, the tuna heke.
Te Roto o Wairewa is an ICOLL (Intermittently Closing and Opening Lake and Lagoon) on the southern side of Te Pātaka o Rākaihautū, Banks Peninsula. Each summer tuna depart from here on their heke, a migration across the ocean to the Tongan Trench to breed. But like almost all of the coastal lakes around Aotearoa New Zealand, Wairewa is in bad shape, but has been improving in recent years. It is a very shallow lake, averaging only one to three metres in depth. Forest clearance, wetland drainage, pest and weed incursion and intensification of land usage have all degraded the lake and its catchment.
When the shallow waters of Wairewa are warm, stagnant and overly rich in nutrients like phosphorus and nitrogen from fertiliser runoff or septic tank overflows, cyanobacterial blooms form. These toxic blooms make it unsafe for swimming, and some species can be lethal for local tuna populations – or for anyone who might eat them. A particularly nasty bloom in the early 2000s killed 1,000 eels on the lake.
Te Pūnaha Matatini Principal Investigator Dr Matiu Prebble (Ngāti Irakehu, Ngāi Tahu) is a tangata tiaki, one of the caretakers of the lake who issue permits locally to whānau members of Ngāi Tahu who harvest eels from January to April. “It’s difficult to make a decision on whether to go ahead with eeling when there has been a bloom,” he explains. There are three monitoring stations on the lake which the Cawthron Institute and Environment Canterbury use to sample water quality. Researchers at the Cawthron Institute analyse these samples for algal cell counts of cyanobacterial blooms, and if dangerous levels are reached, a warning is sent out through Te Whatu Ora – Health New Zealand.
“This is quite a delayed approach,” says Matiu. “We don’t get any of the data until a month later showing us what is happening in the lake. So we don’t actually know what this means for the eel fishery on the lake in real time.”
This is where the tools of complex systems can be useful. The appearance of a bloom can be thought of as a tipping point, when the complex system of the lake undergoes an abrupt transition between a clear, healthy state and cloudy, polluted state based on changes in underlying conditions such as phosphorus levels.
Matiu and fellow Te Pūnaha Matatini Principal Investigator Associate Professor Graham Donovan have seed funding from Te Pūnaha Matatini to analyse the wealth of monitoring data from the lake from a temporal and spatiotemporal tipping point perspective and develop a predictive model to inform future monitoring, predictions and potential interventions.
Graham has experience modelling bodily organs, and looking at other tipping points – such as asthma attacks in asthmatic lungs. He is interested in early warning signals that can be identified in data from asthmatic lungs that signal an impending asthma attack.
This work resonated with Matiu, as Ngāi Tahu envisage Wairewa as a bodily organ. When Ngāi Tahu ancestor Makō first laid claim to the area, he was very taken by the richness of mahinga kai or traditional foods that were available there, particularly the eels. He laid claim by saying “Taku pane ki utu, aku waewae ki tai,” or “Inland a pillow for my head and on the shores a rest for my feet.”
“Wairewa is the whakatinanatia or embodiment of Makō,” says Matiu. “For the last century, it’s been a poorly functioning organ of his body. At the moment it could be though of as a poorly functioning bladder, but what we really want the lake to be like is a highly functioning organ like a lung.”
“I saw Graham speak about his work on modelling bodily organs, and this approach really resonated with me given how we think about the lake,” says Matiu. “There’s a lot of potential in utilising his complex systems approaches to bodily organs to come up with new ideas about how we can address some of the problems in these lakes.”
Seed funding has created a unique project that could only have originated within Te Pūnaha Matatini. This funding paid for two summer interns to work on the project: Tavake Tohi (Tonga) in Auckland, and Madeleine Barber-Wilson (Ngāti Kahungunu ki te Wairoa, Ngāti Ruapani mai Waikaremoana) in Christchurch.
Tavake has a background in geographic information systems, and has been analysing satellite imagery and data from a multispectral drone to explore the spatial dimensions of blooms on the lake. He also has a personal connection to the heke of the eels, as they are also harvested in his village in Tonga at the other end of their migration.
Maddie has simultaneously been analysing years of water monitoring data to understand the lake as a temporal system. “Critical transitions between cloudy, polluted states and clear, healthy states in shallow lakes can be modelled mathematically,” says Maddie, “and our goal is to use a model to find mathematical early warning signals of changes in state for Wairewa. Restoring the health of this lake means protecting a source of mātauranga and kai for iwi and hapū of the rohe. I’m excited to be putting my maths skills to work in the real world and am hoping that the results of our project will be helpful for future kaitiaki of the lake.”
Applying modelling approaches usually used for bodily organs to a lake is pushing the boundaries of complex systems theory and its real-world application. “This project is about both extending complex systems theory, and applying this to the lake as a spatiotemporal system,” says Graham. “We have water sampling data from multiple locations at different times, and visual data from satellites and drones. Where we really want to get to is what we call spatiotemporal early warning signals, looking at how the lake changes in both time and space.”
“The lake is very long and narrow,” continues Graham. “It’s not one dimensional, but it has a very significant length from the headwaters down to the flat, and not much width. So if you can incorporate data from all these sources in a stratified structure, can you get more accurate early warning signals than you could by just looking at the temporal data alone?”
This work is of central importance to the Wairewa community, and has broad engagement from Wairewa Rūnanga, the Birdling’s Flat community, the Christchurch City Council, Environment Canterbury and the Department of Conservation. About 20 years ago, Charisma Rangipunga put forward the wero “ka haha te tuna ki te roto, ka haha te reo ki te kāinga, ka haha te tangata ki te whenua.” If the lake is breathing and full of tuna, and the houses full of language, the people will be well. But if there are no eels or language, the people will suffer.
“If we don’t have our tuna there,” concludes Matiu, “then we might as well pack up and leave, basically.”
Illustrated by Sophie Burgess.