Mussel reefs, once thriving ecosystems teeming with life, have suffered global decimation due to various factors such as overharvesting, climate change, invasive species, and disease. As a result, conservation and restoration initiatives, like Revive Our Gulf in the Hauraki Gulf, have been implemented worldwide to revive these vital habitats. However, the success of these restoration efforts relies heavily on achieving and maintaining high recruitment rates. Without sufficient recruitment, degraded areas cannot recover, and transplanted mussel reefs will eventually dwindle and disappear. 

In a recent study by University of Auckland researchers a small-scale experiment was conducted at the top of the South Island, in Kenepuru Sound, an area affected by overharvesting of wild mussels, to assess the effectiveness of providing adult mussels and spat, both independently and in combination, for enhancing juvenile mussel recruitment and growth.

What is spat?

Spat is a term used to describe a young mussel during a specific stage of its life cycle. At this stage, the mussels are in a settlement phase where they are ready to attach themselves to a hard surface, such as rocks or other mussel shells. Spat have a foot-like structure called a byssus that helps them secure themselves to their chosen substrate. 

Current restoration techniques

Restoration projects aiming to revive mussel reefs often involve the direct transplantation of adult mussels into degraded areas. The success of these projects depends on establishing adequate recruitment to maintain transplanted populations and reverse declines. However, mussel recruitment is a complex process involving the settlement and relocation of spat multiple times before eventually attaching to a hard substrate or existing mussel reef. Unfortunately, restored reefs have so far exhibited minimal recruitment, highlighting potential bottlenecks in the recruitment process.

In the aquaculture industry, a different approach is taken to facilitate mussel recruitment to mussel farms, involving placing wild-caught or hatchery-reared spat onto structures in the farm. See this publication from NIWA for further information. This technique has shown promise in the aquaculture context but has not yet been thoroughly considered for restoration purposes. 

The experiment

In a field experiment conducted in Double Bay, in Kenepuru Sound in March 2022, researchers used shallow trays as experimental units and set up four different tray treatments: adult mussels only, seaweed with spat, a combination of adult mussels and seaweed with spat, and a control with neither adult mussels or seaweed. The trays were placed in the water, and after one or two months, they were retrieved to assess the recruitment of juvenile mussels.

The experiment aimed to assess the impact of providing adult mussels and spat on mussel recruitment. 

Promising results

The results were promising. The provision of spat and seaweed led to a remarkable ten-fold increase in juvenile mussel recruitment compared to the provision of adult mussels alone. Furthermore, the average size of the recruited juveniles increased in the presence of spat. Interestingly, while the provision of adult mussels did not enhance overall juvenile recruitment rates, it influenced recruitment patterns, with juveniles showing a preference for recruiting directly onto adult mussels. These findings underscore the importance of healthy seaweed populations for the functioning of mussel reefs and reveal potential recruitment bottlenecks caused by declining seaweed populations and mussel recruitment rates.

Implications for mussel reef restoration

The implications of this study for mussel reef restoration are significant. The provision of spat and seaweed could emerge as a technique to enhance mussel recruitment, complementing the current restoration approach of directly transplanting adult mussels. By increasing local recruitment rates and facilitating the recovery of degraded mussel populations, this technique holds promise for restoring and preserving these vital ecosystems for future generations. 

The findings also emphasise the need for cross-species and life cycle-informed restoration approaches and highlight the importance of healthy seaweed populations in ensuring successful mussel reef functioning.

This news article has been adapted from a publication in the Journal of Experimental Marine Biology and Ecology, Volume 566, by authors Trevyn A. Toone, Jenny R. Hillman, Emilee D. Benjamin, Sean Handley and Andrew Jeffs.