Kūtai, or kuku (green lipped mussel) beds were once extensive in the harbour’s main channels. Whether due to overharvesting, deteriorating water quality or some other factor, these beds had almost disappeared by the end of last century. Concern about the future of kuku led to Professor Kura Paul-Burke to begin monitoring them in 2007. Her work was subsequently supported by the Ōhiwa Strategy partners and then by the Sustainable Seas National Science Challenge.
There were a couple of small beds still left at that point but Kura found that these were disappearing rapidly. The culprit at this stage was found to be waves of eleven armed seastars (also native) moving through the beds and devouring all the kuku as they went. Further investigation identified a further offender – the much smaller (and also native) cushion star was eating the very small juvenile kuku. A number of techniques were tried to firstly increase the kuku numbers and secondly to deal with the seastars.
Four mussel restoration stations have been successful at recruiting spat and growing mussels. The spat lines at these stations are suspended above the bed of the harbour, preventing predation by the seastars. These mussels will continue to help the regeneration of three new mussel beds on the harbour floor. In 2019 population surveys identified that there was one mussel bed remaining in the harbour with an estimated 78,000 mussels. Further population surveys in 2021 identified three new mussel beds in close proximity to two of the restoration stations with an estimated 750,000 mussels on the seafloor. Then, in July 2023, a further, huge mussel bed of 3.5-hectares with an estimated 16 million mussels was identified under these restoration stations.
The majority of the mussels are juveniles (baby and teenage sized) and will require protection from human harvesting to be able to grow to adults, reproduce and help stabilise the new bed. Ngāti Awa has issued a rāhui on the mussel beds along the Ōhope Spit. No mussels can be taken from an area beginning near the road end and extending to the east towards the harbour mouth. Signage is in place (see below).
In 2022, a feasibility study for removing eleven-armed seastars was carried out. Diver removal was effective at catching larger sized eleven-armed seastars. Trapping was more effective at catching smaller animals with 30 traps collecting 550 eleven-armed seastars and 18,900 cushion stars over an 18-day period.
We are now entering a new phase of this project. The Awhi Mai Awhi Atu project led by Kura and funded by the Sustainable Seas National Science Challenge concluded in June 2023.
We do not have the funds or resources to continue maintaining and monitoring all four restoration stations. The two most successful stations near the Ōhope boat ramp are retained while the other two on the eastern side of the harbour were recently disestablished. Regular population surveys are continuing with the help of NIWA (results of the most recent survey will be published on this website shortly) and options for removing seastars (and utilising them sustainably) are being considered. Kura, who still advises the partners on this project, says at present Ōhiwa Harbour is leading the country in mussel restoration. Watch this space for further developments!
