Introduction
The most abundant shore animal in the Pauatahanui Inlet is the Tuangi, the New Zealand Cockle ( Austrovenus stutchburyi ). Living just below the surface of the sediment, cockles are filter feeders. They draw water through their gills to get the food and oxygen they need, and so clarify the water by removing plankton and minute particles of organic debris. In 1976 it was calculated that each day a third of the volume of incoming tides passed through the gills of cockles. Cockles at that time made up 80% of the biomass of the intertidal mudflat animals. They are a major food source of fish and shore birds such as the oystercatcher, which feeds exclusively on cockles. Because they form such a large proportion of the living organisms in the Inlet, changes in the cockle population are a useful indicator of the biological health of the Inlet.
Cockle surveys
There have been surveys of cockle numbers in 1976, 1992, 1995, 1998, 2001, 2004 and 2007. Since 1992 these have been carried out for the Greater Wellington Regional Council by the Guardians of Pauatahanui Inlet, in collaboration with NIWA. These surveys record an alarming decrease in cockle numbers between 1976 and 1992. Cockle density in 1992 was only about 40% of the density in 1976.
Since 1992 cockle numbers have varied between 40-50% of the 1976 figure. Some of this fluctuation between surveys may be due to large differences in the number of juvenile cockles less than a year old. Severe fluctuation in annual juvenile recruitment is a common phenomenon in shellfish but it seldom presages a long term change in the number of adults. The juveniles in 1998 were about 7% of the total cockle population compared with only 1% in 1992, 16% in 2004 and 9% in 1976 when the adult population was much larger than it is now.
In general the cockle population appears to have stabilised at a new level that can only be presumed to indicate that the Inlet is now capable of supporting only about half of the number of cockles that existed there in 1976.
Possible causes of cockle decline
It appears that the principal ecological and human-induced processes involved are:
- Increased silt washed into the Inlet from erosion in the catchment and from subdivisions and road works by heavy rain events is the major factor in the decrease in cockle numbers. Adult cockles are unable to get back to the surface if the depth of sediment above them exceeds 7cm and young cockles are killed by the sudden deposition of thinner layers of sediment. Data from 2004 show clearly that this is a major factor in parts of the Inlet such as the deltas at the outlets of the major streams.
- Erosion of the subsoil in the catchment releases very fine clay particles into the Inlet. They do not settle out on the bottom quickly and are readily resuspended by wave action. As a result the clarity of the Inlet water has declined markedly in the past 30 years. The increase in the abundance of these inedible particles reduces the feeding and oxygen obtaining efficiency of cockle gills and in severe cases can clog them so badly that death occurs.
- Increased organic and mineral pollution may inhibit cockle growth. Water quality in some parts of the Inlet no longer meets Public Health Guidelines and the public are advised against the taking of any shellfish from these areas. Whether this is an inhibition on cockle growth and fecundity in the Inlet is unknown. Pollution by heavy metals and hydrocarbon residues from vehicles operating in the catchment is of increasing concern.
- Increase in numbers of cockles taken from the Inlet by humans. A daily limit of 150 cockles per person is permitted under current regulations with no minimum size limit and many more people use and live around the Inlet than in 1975.
- Loss or reduction of eel grass beds. This could possibly affect the quantity of small particles suspended in the water on which cockles feed.
- Poor spawning seasons producing fewer juveniles. Current data do not support this as a major factor in this Inlet.
Reports on the 1976-2007 surveys are available for study in the Porirua City Library.
Additional reports and information
- During Sea Week 2001 we invited NIWA scientist Mike Hine to give an illustrated talk "Health of cockles in the Porirua Harbour and Pauatahanui Inlet". Read the press release.
- An archaeological dig on the south side of the Inlet in 2000 yielded some significant evidence about Maori resource use and the huge size of cockles found then. Read the abstract.
- Find out about the Transmission Gully proposal and its potential effects on the Inlet.
- Read about a pollution study undertaken on cockle flesh.
For more information contact John Wells on 234 1788 or at wellsjm@xtra.co.nz.
