How to get mussels into bed – and clean up the harbour
Posted at 9:55am Monday 11 Jan, 2021
Trying to ensure mussels survive keeps Al Alder awake at night.
Smaller mussels are more vulnerable to predators and rough seas.
Tonnes of mussels have been tossed overboard from barges into the Mahurangi Harbour in recent years. Mussels are renowned for their ability to feed on water impurities, and it's hoped that by re-establishing mussel beds, a badly polluted Hauraki Gulf can be cleaned up and marine life dramatically improved. But getting new beds to take hold presents challenges. Al Alder, a student at the Leigh Marine Laboratory, has been working on the problem …
How do you build a mussel bed efficiently? This question keeps me up at night. Here in NZ, mussel bed restoration relies on spreading large, supermarket-sized adult mussels across the seafloor, where the mussels will naturally rearrange themselves into a living mussel bed. Although adult mussels make great reefs, the process of getting them established over the seafloor needs to be made more efficient, in part by increasing the number of mussels translocated per deployment, and by decreasing the amount of time it takes to raise mussels.
Juvenile mussels take up much less space than adults and can be grown for use in restoration in about a quarter of the time it takes to raise adult mussels, which makes them an attractive alternative.
However, before we can use them to make large-scale restoration more efficient, we need to identify the factors that might limit their survival and potential methods for overcoming losses.
At the beginning of my research, I quickly realised that juvenile mussels are more vulnerable to predators and rough seas. Using the Mahurangi Harbour as an example of an area that needs restoration in the wider Hauraki Gulf, I experimented with a variety of methods to help juvenile mussels become established. I used cages to protect from predators, fences to resist the effects of rough seas, coconut matting and ropes to keep them attached to the seabed. However, despite my best efforts, I still lost heaps of mussels.
In the end, the only mussels that survived were farm-raised juveniles placed under cages and mussels sourced from a local wild population. Using cages or transplanting wild mussels for large-scale restoration isn't realistic and wouldn't make the process more efficient, but it did show me that there might be two possible ways forward: engineer a better way to protect mussels until they reach adult size or figure out what makes wild mussels resistant.
Much as certain crops are raised for specific climates (think drought-resistant grains), mussels will develop certain traits whether they're grown on the shoreline (wild) or submerged in water (farms).
Farmed mussels grow quickly – which is great for supplying market demand – however this may not translate into an ability to survive translocation to the seafloor.
Wild mussels are exposed to more disturbances during their lifetime, which generally results in much stronger shells and more attachment threads than mussels raised on farms. Over the past year, we compared the survival of mussels sourced from different populations and found distinct differences that were related to, unsurprisingly, strong shells and more attachment threads.
Mussels that sported these beneficial characteristics were able to form tightly-compacted clumps that helped groups of mussels better resist predators and rough seas. But surprisingly, not all of the mussels that demonstrated high survival were from wild populations. Two farmed populations had high survival rates, meaning mussels that express these traits can be raised using existing or modified farming techniques.
This is hugely important because it provides us with a finer focus for what to look for in a mussel when considering its use for restoration. The next step will be to find out at what size mussels begin to develop these characteristics and whether there are more farms that are already raising resistant mussels. We'd like to see whether we can use these farms as a source of mussels to continue to achieve high survival with larger deployments. If we continue to see high survival at a larger scale, that will pave the way forward to progress efficient mussel bed restoration – plus I'll finally be able to get a good night's sleep.