Why re-mussel the Gulf?

It takes a handful of kūtai (green-lipped mussels) about 30 minutes to clear the sediment from this seawater.

Imagine what the Hauraki Gulf would look like if there were hundreds of square kilometres of kūtai beds? 

Recovering mussel reef

At the turn of the 20th century the inner Hauraki Gulf was carpeted with over 600km2 of sub-tidal, soft-sediment kūtai beds[1]. That’s the equivalent of 80,000 rugby fields!

Historical mussel beds

These kūtai beds were so extensive they were considered ‘reefs’.  Billions of mussels pumping away once filtered the entire volume of water in the Firth of Thames in a single day![1]

But these brilliant bivalves do more than act as kidneys of the sea.  With all their nooks and crannies they provide a home for tiny critters, and a nursery habitat for juvenile fish.  They provide food for crustaceans and larger species like tāmure (snapper) and rays[2]. 

Fact: Mussel reefs exhibit 4x invertebrate densities, 6x productivity, 7x biomass and 10x fish abundance of bare sediment areas[3]!

They are true ‘ecosystem engineers’.

Mussel reefs stabilise the seafloor, reducing wave energy and the re-suspension of sediment. They also remove harmful forms of nitrogen[4] and lock away carbon[5].

The Hauraki Gulf, referred to by Iwi Māori as Tīkapa Moana or Te Moananui ā-Toi, was once a food basket, teeming with life.  

Tāmure (snapper), kahawai, hapuku, tarakihi, kōura (crayfish), pipi, tipa (scallop) and those kūtai. All in seemingly limitless abundance!

Fishers return triumphant 1950s

At least, we all assumed it was limitless. There is a lesson in the story of the kūtai…

Historical mussel dredge

Starting in 1910, the inner Hauraki Gulf was dredge fished for green-lipped mussels.  Fishers in the day said it ‘came up like rolls of carpet’…

Greenweight of mussels extracted

And then the kūtai ran out.  In the mid 1960’s the Hauraki Gulf green-lipped mussel fishery completely collapsed.

Barren seafloor

1,500km2 of the inner Hauraki Gulf seafloor was scarred by dredging. Over 50 years has gone by and the reefs have not recovered on their own. 

Soft-sediment mussel reefs are now considered ‘functionally extinct’.

Fact: The green-lipped mussel (Perna canaliculus) is not endangered.  You can get them at your local supermarket.  We learned how to farm them in the 1970’s. 

Adding to the pressures, sediment run-off from years of poor land management practices has turned much of the Gulf’s seafloor into a gooey mess.

Sedimentation from building works Okura

So deep in many places that you can’t touch the seafloor.

Mud up to your arm

The Hauraki Gulf is a taonga (treasure) we all love. 

Sailing on the Hauraki Gulf

But, below the waterline, there is a biodiversity crisis going on. 

Kina barrens at The Noises

Kōura (crayfish) are now functionally extinct in the Gulf. Without them, and with fewer large snapper, kina populations have exploded, turning kelp forests into barrens. Marine scientists describe an ecosystem on the verge of collapse.

What if we could re-mussel the Gulf?  Could we clear the waters and revitalise its mauri (life force)?

Kaimahi shovelling mussels

So our grandchildren and their children get to experience the kind of biodiversity and abundance our grandparents enjoyed. 

Clean water and lots of fish!

Kahawai catch

That is what the Revive Our Gulf project aims to do.

Revive Our Gulf logo

Mauri ora!

Frequently Asked Questions

  • Why haven’t the kūtai reefs recovered on their own?

    When the reefs were dredge fished it removed the kūtai along with all the epifauna (algae/seaweeds) and underlying structure. This has left nothing for kūtai larvae (spat) to settle on.  Also, without the massive reef systems, there are now much lower levels of larvae and spat in the Gulf.  Without larvae to settle and a lot fewer places to settle onto, these reef systems have lost their ability to rebuild on their own.

  • How much water do kūtai filter – and what are they eating?

    Kūtai filter feed to extract phytoplankton (microscopic plants) to help them grow and continue to build reef structures. To do this, a single adult green-lipped mussel (Perna canaliculus) needs to filter feed on around 60–150 litres of water per day. That means a small handful of mussels can easily filter at least a  bathtub of water a day! In addition to extracting what it needs to survive, the kūtai filter out other particulates in the water column, such as the kind of sediment that can leave water looking murky. These are bound-up in mucus balls and deposited onto the seafloor in the form of ‘pseudofaeces’. This kind of activity clears up coastal waters and allows for more light to reach the seafloor, which can support other important habitats like seagrass meadows! 

  • Do mussel farms have the same benefits as reefs?

    Mussel farms have some of the same ‘ecosystem service’ benefits as the reefs, but not all of them. At farms, kūtai are suspended on ropes in the water column, and will filter the water as they feed. This benefits the system by improving water clarity and can even help remove harmful forms of nitrogen. However, because the kūtai are not on the seafloor and they aren’t allowed to reach maximum sizes (up to 0.24 meters long!), you do not get the structure and habitat that develops with a natural  reef. Mussels from aquaculture are also harvested regularly, which makes it difficult for more complex fish communities to become established. Overall, this means less biodiversity and productivity benefits compared with natural systems. Unlike farmed Kūtai, those that live on the seafloor support lots of infauna (critters that live in the mud and sand) which is important kai for all kinds of fish, and can help shelter more vulnerable juvenile ones. 

Find out more Find out more about the history of how we lost the kūtai reefs, what kūtai reefs do, the scientific work underway to bring them back and how you can support our work.

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References

  1. Paul, L. J. (2012)  “A history of the Firth of Thames dredge fishery for mussels: use and abuse of a coastal resource”, New Zealand Aquatic Environment and Biodiversity Report No. 94. NIWA.
  2. McLeod, I. M. (2009) “Green-lipped mussels, Perna canaliculus, in soft-sediment systems in northeastern New Zealand”, MSc thesis, University of Auckland.
  3. Sea, M., Hillman, J. R., & Thrush, S. (2022). “Enhancing multiple scales of seafloor biodiversity with mussel restoration”, Scientific Reports 12, 5027.
  4. Hillman, J., O’Meara, T., Lohrer, A., & Thrush, S. (2021). “Influence of restored mussel reefs on denitrification in marine sediments”. Journal of Sea Research, 175(102099).
  5. Sea, M., Hillman, J. R., & Thrush, S. (2022). “The influence of mussel restoration on coastal carbon cycling”. doi:10.1111/gcb.16287

Photo credits

All photos Shaun Lee except:

Historical fishing photo: Fishermen proudly display a large catch of fish mostly snapper ship docked at a wharf in downtown Auckland. A crowd looks on. Photographer unknown. NZ Herald aboard their 1950s. d Collection. Auckland War Memorial Museum

Historical mussel dredge photo: Mussel dredge 1960’s (note the size of the mussels!). Photograph by Jack Strongman. Sir George Grey Special Collections, Auckland Libraries, AWNS-19371110-49-4

Sediment flowing out of Okura: Drone shot by Geoff Reid

Sailing photo: Project Kahurangi stock photo.

Chris and Mailee catch a kahawai: Peter Miles