Studies in the Firth of Thames are revealing reduced numbers of natural beds of green-lipped mussels which is believed to be caused by overfishing and sedimentation.

NIWA scientist Dr Darren Parsons says these beds have all but gone on the Firth of Thames.

He says these wild mussel beds used to be extensive in the Firth of Thames.

In the first-of its-kind study, University of Auckland masters student Ian McLeod and supervisor Dr Richard Taylor, in conjunction with NIWA scientists, set about determining why mussel beds have not recovered despite the cessation of dredging, which removed extensive beds of mussels, nearly 50 years ago.

'Mussel populations in soft sediments are really important because they provide a habitat for other marine organisms such as fishes, and because they serve as ecosystem engineers, filtering the water.

'Overfishing and the overall decline in conditions of coastal waters through impacts such as sedimentation have definitely affected these bivalve populations globally.”

The scientists wanted to know whether adult mussels could survive on the seabed, in parts of the Firth of Thames where they had been previously abundant.

During the study, they transplanted mussels into cages on the seafloor for 500 days at three sites along the western coast of the Firth of Thames.

At each site, five cages were pinned to the seafloor.

The cages were plastic mesh, and each cage had 25 mussels, all sourced from the same mussel farm.

The experiment last almost two years and on each sampling occasion, the number of live mussels was counted and the maximum length of the tagged mussels recorded.

At the end of the experiment all the remaining mussels were measured and the average for each cage was used for the mussel growth analysis.

Results showed that 68 per cent of individual mussels survived, and on average grew 19mm in length.

Darren says the survival and growth rates didn't vary among the sites.

'These results show adult mussels can still grow on the seafloor of the Firth of Thames.

'The lack of recovery of the mussel beds is, therefore, probably attributable to processes impinging on the early life stages, such as the supply and survivorship of juvenile mussels.

He says restoration of mussel beds and the ecosystem services they provide may, therefore, be possible.”