Stopping the invasive gold clam programme update June 2025
Update from programme lead Deborah Hofstra.
In this update we report on research activities over the last three months in the field, the lab and the development of a new resource.
Two large studies are underway that had the team in Lakes Karaapiro and Maraetai in April and again in May.
In April we took water samples from sixteen sites as a part of a multi-year monitoring project to assess the impacts of the clams on food webs within the lakes. The sampling areas included sites directly above clam beds and in open water so that we can start comparing the phytoplankton and zooplankton communities in the lakes and assess changes over time as the clam population increases.
In May the NIWA and kaitiaki field team conducted extensive sampling at Lakes Karaapiro and Maraetai, sorting and counting thousands of invasive clams and native freshwater mussels (kaakahi) from nine sites in these two lakes. The field work is an extension of the MPI funded survey of 2024, with four new sites in 2025. The study will contribute to our understanding of clam population dynamics in different habitats over the next three years. Kaakahi numbers were recorded as well, to document any changes in their abundance as clam numbers increase.
In the lab the team have been extending the research undertaken for MPI in 2024 to inform the Corbicula Check Clean Dry protocols. The recent study evaluated different products to killing young clams for the Department of Conservation. The purpose was to identify practical treatment options for decontaminating various types of field gear, with a focus on short application times that would be suitable for and reduce the time for decontamination in fieldwork scenarios.
When DOC release that report we will provide a link to it on our website.
Lab studies also include a collaboration with Dr Yu Cao (Wuhan Botanic Gardens, Chinese Academy of Sciences). Dr Cao has a research background on the structure and function of freshwater ecosystems and worked with our team to investigate native plant response to high nitrogen levels. High nitrogen levels (ammonia), anticipated with high densities of the invasive clam, can be toxic to aquatic plants. We want to understand what those nitrogen thresholds are, so that we can predict likely impacts from the clams. This work is now being repeated by Dr Cao in China, alongside a second study examining the potential impacts on native plant regeneration when the seedbank is buried by clam shells. We know already that burial depth (by lake sediments and detrital layers) can limit the emergence of native plants (charophytes) from the seedbank. So, we anticipate that there may also be a threshold density of clams, at which native plant regeneration may be impacted. We expect to report on the finding from these studies by the end of the year.
With people on the lookout for the invasive clams in their lakes and waterways we have had many enquiries about identification and in particular how clams can be distinguished from our native kākahi and sphaeriids when they are small. We have now updated the guide on our website, with this new, more detailed field guide for clam identification.
