Without the filtering power of mussel reefs the waters of the Hauraki Gulf have become murkier and more vulnerable to the effects of sedimentation.
Less than 1% of what was thought to be up to 500+ sq.km of the original mussel / kūtai reefs of the Hauraki Gulf Marine Park remain. The complete collapse of what marine scientists believe to be a key habitat for the Hauraki Gulf was triggered by dredge fishing in the first half of the 20th century. Although the dredging stopped over 50 years ago, the damage was done and permanent. There’s been no evidence of the sub-tidal reefs naturally regenerating.
Mussels are filter feeders pumping water, phytoplankton (microscopic plants) and other particles through their gills. A single mussel can filter up to 350 litres of seawater daily. This means that at their prime seabed mussels could have filtered the water of the Firth of Thames every day! Now it would take two years for the few remaining reefs to do this.
The knock-on effects of losing the reefs are profound. Mussels not only filter the water, they provide a sheltered habitat for juvenile fish, small invertebrates and other species; and food for octopus, starfish, eagle rays, and tāmure / snapper. Fishers know that snapper love mussels, which is why mussel farms have become such popular fishing spots! In fact, mussel reefs are one of the most productive and biodiverse marine habitats we have in Aotearoa / NZ, with ten times more fish and six times the productivity of bare sediment.
The rapid expansion of Auckland and Waikato has heaped more pressure onto the Gulf, and there’s more need than ever to bring the mussels back, with sediment and nitrates from land development and farming flowing into waterways and out to the Gulf.