Invasive Species - Pacific Oysters

Date: 11/03/2016


by Stephen Lasham

Not so far back in Auckland’s past, the waters of the Manukau were described as having crystal clear waters with white sandy beaches; the Manukau was a food bowl and holiday playground to many.  The story today is quite different; beaches around Onehunga have seen an invasion that was first observed in the 1970’s and has continued relatively unchecked to today.  This invasion is that of the Pacific Oyster (Crassostrea gigas), a native to the Pacific coast of Asia and most likely introduced to the Manukau through ballast water and from the hulls of visiting ships.  Growing three times faster than our endemic rock oyster, they form large clumps along the intertidal shorelines and bays of the Manukau.  Farmed correctly, (although not farmed in the Manukau), the oysters represent a financial benefit, but left to their own devices they become invasive, harmful to native species, damaging to ecosystems, and hazardous to humans.  As filter feeders, they consume large amounts of food, reducing availability for other species, causing a reduction of fish in the harbour, as fish go elsewhere to eat.  The oyster reefs that form on soft mud, turn these areas into raised hard substrate, often trapping more mud, and changing the natural flows of the harbour.  When oysters die, their shells become litter, washed up on our shores, and trapped below the mud.  These shells are what pose the risk to humans; the edges are razor sharp, slicing into flesh like butter, with cuts in danger of becoming infected, requiring medical attention.  So much so is the risk that Taumanu Reserve beaches are identified for high tide swimming only, the man made beaches are sufficiently new and sandy at high tide that one is unlikely to put a foot down on broken shells.  At lower tide the swimmer is more likely over the Manukau mud, and a wrongly placed foot here could be bloody and painful.  The problem is not just for swimmers, rock hoping youth have to be careful when roaming the foreshore, as shells cling to coastal rocks; an outstretched hand to check a fall could end up lacerated by the shells.  Aotea Sea Scouts know the problem, their Ship is over the sea shore on Orpheus Drive.  Back in the 1980s the Scouts played happily in the waters off their ship; bare foot and without worries.  Today the leaders are unwilling to let Scouts go into the water due to the danger posed by the shells that now litter the beach and rocks around their building.  Thirty odd years is a very short time to have lost such a wonderful playground.  The oysters are not even a good source of food, as contaminants in the waters of the Manukau, which the oysters filter, can build up in the flesh; care should be taken to check toxin warnings before gathering oysters for eating.  The oysters are not all bad, beds well off shore provide habitat for visiting sea birds and habitat amongst the shells for small invertebrates, such as crabs, bivalves and worms, so arguably these beds remain acceptable.  This said, the birds have been coming to the Manukau long before the Pacific Oyster was introduced, and the changes to the seabed around oyster reefs can see other species depart; so are they really that beneficial in our harbour?  Eradication is likely impossible, but managing our waterfront playgrounds back to safety is.  Taumanu Reserve is a start, an example of what can be done if good people put their minds to overcoming roadblocks and making our marine environment a better place.  Our focus should be on ever improving our Manukau shorelines so that our peoples can again return to the water and enjoy it like our parents, parents did. 




MHRS has put submissions to Council to allow the removing of ‘oyster reefs’ not just removing the dead shells. 

Read the submission: MANGROVE & PACIFIC OYSTER SHELL REMOVAL - MHRS Unitary Plan Submission - May2013

Aotea Sea Scouts Submission HERE


The sharp shells of the introduced Pacific Oyster are creating dangerous conditions for people and pets on some of our inner harbour beaches. MHRS is seeking an efficient way to crush these shells and rid our beaches of the problem. We are working with AUT School of Mechanical Engineering Senior Lecturer Alan Jowitt to develop a prototype machine and hope to be trialing the prototype machine over the next few months.

Article on the use of crushed oyster shell in New Zealand  HERE


SEE ALSO - Mangrove Proliferation