Kūtai (green-lipped mussel) restoration kaupapa at Ōkahumatamoemoe (Ōkahu Bay) is part of Ngāti Whātua Ōrākei’s long-term efforts to restore the bay, with this project, launched in 2021, focusing on testing whether a raised shellhash platform could improve kūtai survival.
For those following this project, you’ll remember that it involved the creation of a shellhash platform to see whether elevating the kūtai off the muddy seafloor would improve kūtai survival, growth and whether new kūtai would settle in the bay. Three kūtai reefs were established on the muddy seafloor substrate and another three reefs were established on a shellhash platform.
Three years on, we conducted a monitoring survey at Ōkahumatamomoe using one of Revive Our Gulf’s remotely operated vehicles (ROVs). Here are the key findings on how the kūtai have fared since our 12 month update.
After three years on the seafloor, kūtai density has continued to decline with only few live kūtai remaining. There was no clear difference in kūtai survival between the mud and shell reefs. Fine sediment, which can enter the bay with runoff from the land, appears to be a major problem for the kūtai. The kūtai reefs were observed to be covered by a layer of fine sediment up to 2 cm thick (see images below). Fine sediment can smother and bury the kūtai as well as impact their ability to feed.
The transport of sediment to the Ōkahumatamomoe kūtai reefs was likely intensified by major weather events in early 2023 including cyclone Gabrielle and the Auckland Anniversary storm, as well as multiple other storm events that have impacted the Waitematā Harbour. Additionally, the kūtai may have been negatively affected by the Parnell sinkhole that caused untreated sewage to enter the Waitematā Harbour in September-October 2023.
Despite the loss of the kūtai, the kūtai shells and shellhash platform have supported the development of a diverse community of organisms living on the seafloor (known as epibiota). These epibenthic community includes sponges, starfish, sea cucumbers, ascidians, seaweeds, mobile invertebrates and fish among other organisms. Over the three years of surveys, we observed a range of different marine life! The photos here are from the three-year mark.
ROV images showing sea cucumbers and sponges on the remaining kūtai shells, with the fine sediment coating evident.
Four unwanted organisms including the Mediterranean fanworm, clubbed tunicate, wakame and the Asian paddle crab have been observed in the kūtai reefs in Ōkahumatamomoe. Although, these organisms were present in the Waitematā Harbour prior to this project and appear to remain in low numbers within the kūtai beds.
We took the opporutnity to run the ROV around the pylons in the Bay and were pleased to also see a range of marine life, including kūtai, attached to the pylons.
ROV images of pylons showing kūtai, seaweeds, and marine life on the surrounding seafloor.
This project has provided important insights into the challenges of kūtai restoration in Ōkahumatamoemoe. While the shellhash platform alone has not been enough to support long-term kūtai survival due to sedimentation pressures, it has created a foundation for diverse marine life to establish, and given us many operational learnings. These lessons are critical as we refine our approaches to restoration in the face of ongoing environmental pressures, including climate change.
Looking ahead, we will continue to explore alternative restoration methods to support kūtai recovery in sediment-impacted areas. The knowledge gained here will help shape future initiatives across the Hauraki Gulf, ensuring we adapt and innovate in our pursuit of restoring these vital ecosystems.