More marine species thanks to the muscle of the mussel

On the seabed off eastern Waiheke Island and in the Mahurangi Harbour living fish nurseries are slowly being cultivated through the establishment of green-lipped mussel, or kuku, beds. This has been achieved by Revive Our Gulf, a project run by The Mussel Reef Restoration Trust, and research funded by the University of Auckland is showing that fish and invertebrate species in these habitats are on the rise.

Dr Jenny Hillman and PhD student Mallory Sea (great name, that!) from the Institute of Marine Science at the University of Auckland, share some early findings.

In the early 1900s, extensive green-lipped mussel/kuku beds covered the seafloor of the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana. Sadly, within 60 years, dredging, overharvesting, and a combination of other human-driven factors, contributed to the collapse of the entire population. These subtidal mussel beds have never recovered.

This is unfortunate for all of us (not just those who think mussels are delicious), because the loss of these beds represents a loss of the multiple benefits we get from them. For example, mussel beds can remove excess nitrogen from coastal waters, stabilise and protect our shorelines and seabed, and clean the water as they feed, leaving us with cleaner, healthier waters.

In addition to these amazing benefits, green-lipped mussels form large, complex beds that provide food and habitat for other animals. Especially important in soft sediments, this complex bed structure provides hard surfaces and a 3-dimensional structure above the soft-sediment, increasing habitat in areas otherwise made up of sand and mud. This living habitat ultimately attracts many other forms of marine life, and we have begun to document some of these changes on our restored mussel beds.

Restoration efforts are underway to try and bring back mussels to the Hauraki Gulf, predominantly through the Revive Our Gulf project. Between 2013 – 2019, Revive Our Gulf have established green-lipped mussel/kuku beds off eastern Waiheke Island and throughout Mahurangi Harbour by depositing more than 140 tonnes of mussels.

For this research we are looking at the types of animals found on the restored beds in Mahurangi Harbour and Kawau Bay at all scales – from the small, microscopic worms living in the sediment up to the large fish and rays!

To measure how many of each animal (the biodiversity) is found on each bed, we swim along slowly and video tape measures that we lay out across the entire length of the beds. We also collect and sieve sediment samples collected along the tapes to identify the worms and other sediment dwellers that we wouldn’t otherwise be able to see. Un-baited remote underwater video cameras are then placed on the beds for a day at a time to film videos of the larger, more mobile species, such as sharks and rays. We then watch all the videos, making sure we look out for the small, cryptic invertebrates (hermit crabs, snails etc.), and identify all the animals in the sieved samples under a microscope.

We have repeated these experiments in nearby sediments with no mussel beds to see how much difference our restored mussel beds make, and where the highest numbers of species are present. While this work is still in progress, we can tell that our restored mussel beds attract many marine animals that use the beds for food and refuge. We are also comparing the beds in different places to see what kind of environment is best for the different benefits the beds provide.

We’ve documented an abundance of fish species in our underwater videos, including kingfish, parore, juvenile snapper, spotties, leatherjackets and mackerel. We’ve also seen eagle rays, squid, sting rays, and sharks on our mussel beds! While the data is still being processed, so far we have found higher numbers of both fish and invertebrate species utilising the beds.

Our research shows that mussel beds increase the biological diversity at many scales. Over the past seven years, Revive Our Gulf has returned more than 150 tonnes of mussels to the Hauraki Gulf, and they are clearly doing great things to help make local marine environments healthier. By increasing the biodiversity, these restoration projects are important both economically and ecologically and benefit all of us no matter how you enjoy our Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana.

Love it, Restore it, Protect it.