One of the challenges in kūtai restoration is what we refer to as the “recruitment bottleneck”. While we’ve created kūtai beds by adding adult kūtai into the environment, the natural cycle that brings in and supports new juveniles isn’t kicking in yet. Without that, the restored beds can’t sustain themselves over time.
Seaweed has been identified as being important for juvenile kūtai (you can read more about this here), and a lack of seaweed or other suitable surfaces for juvenile kūtai to settle on before moving in with the adult kūtai is thought to be one of the reasons behind the recruitment bottleneck.
We know that juvenile kūtai prefer some seaweed over others, and that they can move onto different surfaces before joining the adult kūtai bed, but we have very little information available on what other environmental conditions are important for juvenile kūtai to successfully settle and survive. For example, does wave action or the presence of predators impact the settlement process?
Kathy Burnham is completing her PhD at Waipapa Taumata Rau / the University of Auckland supervised by Dr Jenny Hillman and Professor Andrew Jeffs, investigating settlement conditions of juvenile kūtai. This research is important for learning more about the conditions that juvenile kūtai need to survive, and helping us design restoration methods and select sites for future restoration efforts.
The research
Kathy’s study is focused on a wild intertidal kūtai bed at a site in the northern Hauraki Gulf. As kūtai are found naturally at this site, this study can provide important clues about the natural conditions that are important for kūtai to survive. Her study looks at the different surfaces that juvenile kūtai settle on (e.g., seaweed, other kūtai, barnacles, rock, shell or sediment), how their settlement varies up the shoreline (as this means they would spend different time underwater, e.g., low tide zone, high tide zone, rock pool or exposed rock) and the size of the kūtai on each surface (from less than 5mm to 30mm – they are considered adults when they are bigger than 30mm). Between May and July 2023, Kathy conducted 10 surveys across the shoreline. She characterized the habitat based on the surface type and position on the shoreline, and measured kūtai in each habitat. She also recorded the number of empty paired kūtai shells (this suggests recent mortality of the kūtai) and possible predators in the area.
Results
Kathy found that there were more juvenile kūtai living on other kūtai and seaweed, or habitats where there were both kūtai and seaweed. Their preferred habitat varied up the shoreline – towards the low-tide zone juvenile kūtai were more commonly found on other kūtai, and in the high-tide zone smaller juvenile kūtai were more commonly found in seaweed. In the high-tide zone there are less predators and wave action compared to the low-tide zone, but the environment is hotter and drier as it spends less time submerged in water. Living in seaweed in the high-tide zone may offer the juvenile kūtai a refuge from these conditions. In general, smaller juvenile kūtai were often found in seaweed, and larger juvenile kūtai with other kūtai, rock and shell, which supports our understanding of juvenile kūtai moving from seaweed to other surfaces throughout their early life stages.
Conclusions
The natural conditions in the Hauraki Gulf / Tīkapa Moana / Te Moananui-ā-Toi have changed so much since there were extensive kūtai beds across the moana, so research like Kathy’s is important for understanding the conditions that are needed to support kūtai populations. Kathy’s research has revealed that both seaweed and the presence of other kūtai are important for juvenile kūtai to settle, and this information not only helps us to grow our knowledge of kūtai, but can also help us to select suitable sites for future restoration efforts.
To see the full results of this research, the journal article is:Burnham, K. A., Hillman, J. R. and Jeffs, A. G. (2025). Macroalgae and conspecifics serve as critical attachment substrates for juvenile green-lipped mussels (Perna canaliculus) during different stages of development. Journal of Experimental Marine Biology and Ecology 588: 152108.