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 and with short siphons, 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.
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 major
food sources of fish and shore birds, especially the oystercatcher, which
feeds exclusively on cockles. Because cockles are such a large part of
the living organisms in the Inlet, changes in the 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 and 2004. Since 1992, the surveys have been carried out for the Greater Wellington Regional Council by
the community group Guardians of Pauatahanui Inlet, in collaboration
with NIWA. These surveys record an alarming decrease in cockle numbers
between 1975 and 1992. Cockle density in 1992 was only about 40% of the density
in 1976 (see graph below).
Since 1992 cockle numbers have varied between 40% and 50% of the 1976 figure, though the numbers are not statistically significantly different over this period. Much of this fluctuation is probably due to large differences between surveys in the number of juvenile cockles less than a year old. Severe fluctuations in annual juvenile recruitment are a common phenomenon in shellfish, but they seldom presage 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 and 16% in 2004.
In general the cockle population appears to have stabilised at a new level. This level can only be presumed to indicate that the Inlet is now capable of supporting only about half the number of cockles that existed there in 1976.
Fig.1.Trend in cockle population within Pauatahanui Inlet, 1975-2004
Possible causes of cockle decline
Research indicates that, world wide, there are five principal ecological and human-induced processes affecting cockle abundance and health. Not all of these seem to apply in the Pauatahanui Inlet.
