One of the challenges that Revive Our Gulf currently has in its restoration efforts is how to effectively stimulate ‘recruitment’ at our restoration sites in the Hauraki Gulf/ Tīkapa Moana / Te Moananui ā Toi. Recruitment is the term we use to describe successful reproduction when we see baby green-lipped kūtai (mussels) settling into the restored kūtai beds.
Kūtai are broadcast spawners; the eggs and sperm of adult kūtai are released into the water column and combine to form larvae that float in the ocean for weeks until ready to settle.
These late-stage kūtai larvae typically settle on structures like seaweeds, and then make further migrations as they grow. In a healthy system, the juveniles find their way to an adult kūtai bed or other suitable soft-sediment or rocky reef habitat.
We know that kūtai larvae exist in the water column of the Gulf, but they aren’t settling in the adult beds. This means there is a ‘bottleneck’ in the lifecycle, and it’s possible that a lack of settlement structure (like seaweeds) could be a limiting factor.
Revive Our Gulf is dedicated to understanding and addressing these challenges and, alongside the Kelly Tarlton’s Wildlife Marine Trust and the Institute of Marine Science at the University of Auckland, has been undertaking a study into what local seaweed species could act as “settlement cues” for baby green-lipped kūtai.
Called Project Spat, the research is focused on identifying what seaweeds are most favoured by baby green-lipped kūtai (spat) and whether there is the potential for transplanting those seaweed species with kūtai spat attached into the moana.
The experimental design was developed by Alanta Loucks, Dr Mallory Sea, Dr Jen Hillman and Professor Andrew Jeffs to test the settlement of kūtai spat on six species of seaweed:
- Xiphophora chondrophylla
- Jania sp.
- Pterocladia sp.
- Cystophora torulosa
- Carpophyllum maschalocarpum
- Carpophyllum flexuosum
The trials used seaweed samples collected from the inner Hauraki Gulf, and kūtai spat from SpatNZ in Nelson. The experiments were conducted in tanks with temperature control at Kelly Tarlton’s Sea Life Aquarium, with design input from aquarium expert, Craig Thorburn.
The experiments started in March 2023 and continued until August 2023, undertaken by Craig and Mallory for the first two months and then handed over to Honours student Alanta Loucks.
Promising findings
Project Spat found that Xiphophora chondrophylla, Jania species and Pterocladia species had the highest settlement rates of kūtai spat among the tested seaweeds. The study also found that X. chondrophylla and Pterocladia species had significantly higher settlement rates than the artificial mussel spat rope controls, while Jania species had slightly lower rates.
The next phase of the research will delve into how long spat remain attached on preferred seaweeds after this initial settlement, and whether we can observe how they move from seaweed structures to other reef structures. We’ll be investigating factors like growth rates and survivability to ultimately determine if spat-encrusted seaweeds can be effectively reared and used in our restoration methods.
The Kelly Tarlton’s Marine Wildlife Trust will continue to lead this research, working closely with the Institute of Marine Science at the University of Auckland.
Photos supplied by Kelly Tarlton’s Marine Wildlife Trust.