Washing up

Could a threatened native animal help us restore our waterways? (12 minute read)

I’ve got an announcement that I’m excited to share with you about the next couple of months, but it was a bit long to combine with this article, so I’ll send it to you tomorrow, along with some bonus photographs.

We all switched off our torches and stood still in the darkest of dark nights. I was used to city darkness, where there’s always a streetlight, or a house, or a passing car. This was something different, like diving in a pool of ink. If I held my hand in front of my face, I could see nothing. We were in a deep gully, standing on a bridge, while a stream which we could hear but not see flowed beneath us. Above us, layers of canopy obliterated every trace of light from the stars.

The night walk in the forest was a highlight from the Hamilton Junior Naturalist’s camp, something I attended a couple of times as a teenager. I’d seen the forest in the day many times, but the forest at night was like nothing I’ve ever seen before. It came alive.

There were insects everywhere – wētā, moths disoriented by our torches, cockroaches. I hadn’t known that we had native forest cockroaches, but they were on every tree trunk. Cockroaches are not my favourite insect, but these were lovely, glossy in the torchlight, in shades of honey-brown. There were millipedes too, which are not insects because they have far too many legs. The ones we saw were pill millipedes, short and wide, capable of curling into a ball when disturbed.

But most remarkable were the freshwater limpets. Although not closely related to marine limpets, they have a similar habit, clinging tightly to stones under the water. They were the reason we switched off our torches as we stood on the bridge. We looked down as a few of the camp leaders waded into the water then switched off their torches as well. There was some splashing as they reached into the water to turn over a few rocks, then… magic. We saw a green glow in the water.

New Zealand’s freshwater limpets are bioluminescent, that is, they produce their own light. Producing light is an ability which is relatively common in the ocean. In fact, in deeper waters the majority of animals are bioluminescent in some way, but even at the surface it’s not rare. It’s less common on land, but some fungi do it, as do various animals, including the fireflies (despite the name, a type of beetle) and New Zealand’s glow worms (despite the name, a type of fly). But our freshwater limpets, which are found just in the North Island, are the only bioluminescent species to live in freshwater. They produce slime which glows green when startled or disturbed.

Our freshwater limpets are unique, but I think that our freshwater mussels, known as kākahi (also kāeo, ngāeo, ngūpara and torowai), are more weird.

New Zealand’s freshwater mussels have a similar shape to the marine mussels we eat, but are not closely related. They belong to a worldwide group which share an unusual life cycle. The male mussels release their sperm into the water and the females take in the sperm as they feed. It’s kind of creepy, but that’s not the really weird part. Once the female has incubated the fertilised eggs for a while, she releases them into the water. The baby mussels, which don’t yet resemble mussels, have a hook which allows them to attach to the gills or the fins of a fish. There, they grow until they have formed a juvenile mussel, which detaches from the fish and burrows into the sediment. Once mature, they can live for decades.

It gets weirder. Some species, including one New Zealand species, release their parasitic larvae attached to structures which resemble small worms, in order to attract the fish which act as hosts. New Zealand has three different kākahi species which have been identified so far. One has been found only on a single fish species, the common smelt (this species has more than a dozen Māori names, although some apply to different ages of the fish). Another has been found on a number of different fish, including introduced species, although it isn’t yet known whether it can complete its life cycle on all of these species. Science doesn’t yet know much about kākahi.

It was actually the kākahi which first prompted me to contact Deborah Hofstra, who leads the freshwater biosecurity programme at the National Institute of Water and Atmospheric Research (NIWA). As I was looking into what could be done about the state of our waterways, I came across some work which was looking at whether kākahi could be used to clean polluted water. At that stage, I barely knew anything about kākahi, but I was fascinated by the idea.

Freshwater mussels, like their marine namesakes, are filter feeders. They suck in large volumes of water and filter out particles and microbes as food. On the one hand, it means that eating shellfish such as mussels from polluted water is dangerous, because they can accumulate toxins and disease-causing microbes. On the other, it means that these filter feeders can help to clean contaminated water. It’s been done overseas. Could kākahi be part of the answer in our polluted waterways?

Alpine tarn, Kahurangi National Park. Not a habitat for kākahi, but a lovely example of a healthy waterway (image credit: Melanie Newfield)

It's an appealing idea, but there are real challenges, Deborah tells me. “All of the kākahi species in New Zealand are under threat, even the most common species that we have. Why aren’t the native kākahi doing so well? The question is, what’s happening to all of the babies? You see a lot of populations that are old or only have adult kākahi. Some of these populations have been checked, and the adults are capable of reproducing. Maybe the native fish hosts aren’t there in the right abundance at the right time? But also, if you get to the stage where you’ve got your little baby mussel dropping off the host-fish and living in the shallows, they need nice, clean sandy spaces. Degraded lakes have really thick, often flocculant [loose], sediments, and it’s not great habitat for native kākahi to be in. Then you have pest fish come along, that bioturbate, or turn over those sediments, which is likely to bury the young kākahi.

“We don’t know precisely why we only have these super old populations in some lakes, but it is linked to habitat loss in highly modified catchments. Unless we can figure out where that bottleneck is that’s preventing recruitment and correct it, those populations will just die out because there are no young. We haven’t figured out how to do it in the lab. But as far as I know, nobody is working on that because there’s no science funding at the moment.

“But at Lake Tutira [in the Hawkes Bay], as part of the hydrilla response (by MPI), there has been a regular monitoring programme. Over the time that the hydrilla eradication response has been going, we’ve seen juvenile kākahi in our monitoring samples. By juvenile kākahi I mean individuals as small as about five millimetres in size, so very small. If you weren’t sitting there sieving through sediment samples to pick out the bugs (macroinvertebrates), you wouldn’t have known that they were there. In the two years before the hydrilla was removed, we did surveys and we didn’t find baby kākahi in the samples but we did see them after it. We can’t prove cause and effect because we don’t have that kind of data (successful breeding is described as sporadic in the science literature) but this is one of the interesting and positive things that we have seen over the years of monitoring in Lake Tutira.”

The idea of using kākahi to clean our waters is appealing, but we need more of them. So Deborah and her colleagues looked at what they could do to help. “The question was whether you could take those kākahi that are living in a challenging environment, put them on rafts, give them a nice environment to live in up in the oxygenated water. Would they survive? Would they thrive? And, could you find enough mussels to move when there is an at-risk and declining population?”

So far, the results are promising, but there’s still work to do, Deborah says. “Yes, they survived and yes they thrived. So the concept works. Part of that project was then looking at whether or not they could be bred in the lab, since they need a native fish host. The team in the lab could rear babies without native fish. But only up to the age of about three months old, and then after that it seems as though there’s something in nature that they were missing in the lab, because they did not live beyond that time. That was part of the project that didn’t really get to where we wanted it to before the funding ended.

“But it certainly shows the potential of what can be done and I’m sure somebody else at some stage will crack that science, the bit that we were missing that will mean it is possible.”

But cleaning the water with kākahi is only one of the projects that NIWA is doing on restoring our waterways. Deborah tells me about the work they are doing to re-establish native plants in lakes where they have been lost. This turns out to be far more complicated than digging a hole and planting a seedling. It’s much closer to the challenge of replanting seaweed, perhaps even trickier than that.

“The big lakes where there is a lot of wave action, you can have a whole lot of turbidity [cloudiness] that makes them really difficult and challenging for re-establishing planting. There is some science from Te Waihora, Lake Ellesmere, in the South Island where there were barriers put in place to mitigate the wave action to see if they could then create a safe space to restore some native plants. The wave barrier that was put in place in that lake was considered to be over engineered when it was installed, but it failed. The forces in the lake were significant, so no wonder it’s difficult for the poor plants to re-establish.

“Even if you were to actively replant, what is your sweet-spot for replanting? If the system is turbid, you need your plants to be tall enough, or to plant them shallow enough, so they can still get light. But a lot of the lakes that are highly degraded and have a long history of algal blooms, also have a highly flocculant sediment (it is sloppy and loose), so it’s also difficult for roots to secure plants in place. It’d be like trying to plant your pot plant into soup, there’s nothing solid in there. How do you get more consolidated sediments so that you could plant into them? Some of the things that we’ve looked at in the past but haven’t tested yet is using products like hessian on the sediment combined with “plant bombs”.

“Hessian is biodegradable, and it’s used to smother some of the invasive waterweeds. We know when that’s used, it breaks down over time and that the native plants come back through it from their seed banks. What we’ve started testing is could we use something like hessian on top of soft sediments to help consolidate them. Then, if you’ve got a lake that’s shallow or gently shelving you could just wade and re-establish plants. But if you have a lake which drops off with steeper slopes, you can’t necessarily just wade out there and plant. But could you sit on a boat or a kayak and drop plants in there? How would you make them sink? What if you made little bombs of clay and gravel with your plant shoots in them? The bomb concept actually works in experiments. The bombs can stay intact, the plants get held there and they grow through the small ball of clay and gravel.

“We’ve actually got another project going on at the moment called RotoTurf which is also about growing plants in mats. What matting products can you use? Could you use those to roll out in a lake, like Ready Lawn? Could you just roll out your native water plants? We’re doing that work at the moment.”

There are lots of ideas that sound hopeful, but there are still some big challenges. “If you gave me a lake and you told me you’ve got whatever budget you want to fix it, you still have to start on the land. I don’t think there’s any point in taking action in the water until you’ve taken action on the land. What’s happening in the catchment is so important. Until the sediments and nutrients are managed, to reduce the external loads that go into our waterways, then the time isn’t right to start in the water. That’s because even when external loads are reduced there will still be legacy nutrient loads in-lake to address as part of the restoration process.

“So, the catchment needs to be dealt with in terms of what’s happening on land and what’s going into the water. Your biosecurity pathways need to be addressed too. If you’ve got a lake that doesn’t have invasive species in there, what is the probability that they’re going to get there, and how can the lake be protected and kept pest free? What is the probability of invasive species being introduced? How connected is that lake to other systems? Can barriers be put in place that will allow native fish migration and not invasive fish to move into the systems? Some of that science has been addressed, people know what to do..”

It’s also a matter of time, Deborah tells me. “When you look at a lot of the European scenarios, even when they’ve cleaned up their catchment inputs, they’re still waiting for decades for aquatic plants to come back. Can we accelerate that process to restore the health of our aquatic ecosystems?”

Listening to what Deborah has to say, I can see some exciting possibilities. But I can also see where the real challenges lie. As long as we continue to put things into our waterways which don’t belong there – excessive nutrients, excessive sediment, invasive species – then we are going to struggle to fix our water.