For the first half of last century, 500 square kilometres of the Firth of Thames seafloor was carpeted with green—lipped mussels, but commercial dredging wiped them out by 1960. Now, a new exciting project is using mussel power in an effort to clean up and revive the Gulf. Around three and a half million live adult mussels have been deposited in a restoration area the size of eight rugby fields off eastern Waiheke Island. The drops are part of a project called Revive Our Gulf. It is the world’s first attempt to restore green-lipped mussel reefs and follows a successful trial earlier in the year.
A single mussel is able to filter up to 350 litres of seawater each day. The process of filtration means the mussel also consumes plankton which is part nitrogen- based protein, and when harvested, around six kilograms of nitrogen is removed from the ecosystem for every tonne of mussels. The commercial mussel industry removes a modest 126 tonnes of nitrogen from the firth annually, and even with increases in aquaculture, it’s unlikely to exceed 5oo tonnes. But if the re-seeded wild beds can be successfully expanded then this figure would rise substantially. It is estimated that if the old wild beds were still in place they could filter all the water in the Gulf in a single day versus two years for what remains today.
The trials at Waiheke Island show not only improved water quality but significant
increases in shelter and growing surfaces for invertebrates and plants, habitat for juvenile fish, and foraging areas for adult fish. See this poster
Monitoring of these re-seeded beds has indicated a better than 95 per cent survival rate but little recruitment. Solving this problem will determine whether the re-seeding can succeed in places with high sediment loads (most places – but the problem is much worse in the southern firth off-shore from Thames)
Excess sediment can smother mussels, interfere with the gills of small fish and clog habitats. It is estimated that 350,000 tonnes of sediment enters the Hauraki Gulf each year. The Piako carries 30,000 tonnes per year, and the Waihou 160,000 tonnes.