Breadcrumbs

Freshwater bioremediation using native mussels - focussed on shallow eutrophic lakes

Joint MBIE Endeavour Smart Idea. Successful 2018.

Project Update (November 2019):

The last few months have been particularly busy in the project with a second pōwhiri at Matahuru Marae, an intense search for kāeo in Lake Ohinewai, model development and the construction of our first raft.

With some new members joining the project team there was a second pōwhiri at Matahuru Marae, Ohinewai, before we headed out to Lake Ohinewai. At the lake the aim was to get an indication of the size of the kāeo population, specifically to know if there were sufficient kāeo in the lake that could be used for placing on rafts in the future. The short answer was Yes.

The second purpose was to learn about the population structure of the kāeo in Lake Ohinewai.

Mary de Winton and Tracey Burton (NIWA divers) spent about an hour and a half snorkelling in the lake at the western end, collecting all kāeo they could find. Aareka Hopkins (AM2 and Associates), Karen Thompson and Deborah Hofstra (NIWA) tagged about half of the collected kāeo, measured, sexed, and assessed their reproductive stage, and then returned them all to the lake. The graph (below) gives an indication of their size. All were large, no young kāeo.

Work is also underway with juvenile mussels. Karen lead a team to Lake Karāpiro where brooding females were collected. The larvae (glochidia) from these mussels are cultured invitro. This methodology replaces the natural parasitic phase that freshwater mussels use whereby they attach to the gills and fins of fish to transform into juvenile mussels. This summer research will focus on different feeding rates, food and water types with the aim of improving the survival of the cultured juvenile mussels. There is currently a bottleneck where 95% of the cultured juvenile mussels “die off” at 1-2 months of age.

Image: Tracey and Aareka returning kaeo to the lake

The team will go back to the lake late November and repeat the kāeo sampling. Firstly, to see how many of the same (now tagged) individuals are re-captured, and to tag more individuals, again building a picture of the size and structure of the population.

Image: Tagged kaeo before they were returned to the lake

At the same time a small boat will be launched on the lake to put some dataloggers (sondes) in the lake. The sondes will gather data on the lake condition in terms of water quality. This will help the understanding of the conditions kāeo currently encounter in the lake and can be used for the modelling component of the project (Matt Allen, University of Waikato). Waikato Regional Council (Paula Reeves) will also be providing their monthly water quality monitoring data to inform the modelling.

The third activity for the end of November is to use sonar to map the lake basin (Dan Clements, NIWA), updating the bathymetry and developing a map of relative sediment hardness. The relative sediment hardness information will be particularly interesting to indicate other areas of likely habitat in the lake (i.e., similar sediments) where kāeo may be present.

The first prototype raft has been built, with planned deployment in the new year, so more on that in the next update.

Project Details

Partners:

  • Manaaki Whenua Landcare Research
  • University of Waikato and Waikato DNA Sequencing Facility
  • Missouri State University
  • Matahuru Marae, Ngaati Mahuta at Matahuru Marae
  • AM2 and Associates

Funding:

  • Joint MBIE Endeavour Smart Idea - Successful 2018.

Objective:

To develop native freshwater mussel (kākahi/kāeo) rafts (or suspended cages) to assist lake restoration through biofiltration. The focus of our proposal is to return degraded, turbid, shallow lakes towards their original clear, macrophyte-dominated state.

The project has four workstreams:

  1. Ecophysiology
    Lab-based work examining the mussels sublethal responses to extreme environmental conditions (e.g., low dissolved oxygen and high temperatures) that they are likely to experience on “mussel rafts” in a shallow lake. This workstream will provide design parameters for our raft construction and operation and increase the chances of the mussels thriving on the rafts.
  2. Field Trials
    Research based at Lake Ohinewai where we will trial different raft and net designs and examine their localised effect on water quality.
  3. Modelling
    Improvement of a pre-existing mathematical model of Lake Ohinewai using parameters from workstreams 1 and 2. The model will be used to explore questions such as “how many mussels per raft, and how many rafts will it take to have a significant impact on the water quality of Lake Ohinewai”? and “how can we optimize the impact of the rafts (e.g., by strategic placement in the lake”)?
  4. Kāeo Kohanga
    We are trialling improvements to our culturing system to overcome the bottleneck where 95% of our cultured juvenile mussels “die off” at 1-2 months of age. We are also examining genetic parameters around “eco-sourcing” mussels from local populations in situations where donor populations are needed to restock lakes where mussels have completely died out.

Status: Year 2 of 3-year project

Project Background

Freshwater mussels (known as kāeo or kākahi) throughout much of Aotearoa are considered a “Cultural Keystone Species”. They were an important mahinga/hauaanga kai resource – and are still collected today. They feed by filtering large quantities of water at an average rate of 1 litre per mussel per hour. There are indications that populations are declining in many streams and lakes (including Lake Ohinewai) – they are however, thriving in other locations (e.g., Lakes Karāpiro and Taupō).

Meanwhile many shallow lakes, especially in the Waikato are severely degraded as a result of factors such as increasing nutrient inputs. The lakes have “flipped” into a state where rooted aquatic plants can no longer survive. The lake sediments are no longer held in place by the aquatic plants and so the lake waters remain permanently muddy (or turbid) – and this is made worse by the presence of pest fish species like koi carp and catfish.

Our team is inspired by the idea that freshwater mussels could be part of the restoration process in shallow lakes. At present, it is thought that mussels are slowly dying out in shallow lakes when oxygen levels decrease at the lake bed. We would like to test the idea of moving mussels from the lake bed, up onto rafts or into baskets, to enable them to survive and continue filtering the water. If the mussels can survive, we would then like to understand what would happen if we increased the numbers on rafts up to thousands. We believe we can learn more by running small scale trials in a shallow lake and in the laboratory.

Contact: Deborah Hofstra