Cockles form a large proportion of the living organisms in the Inlet and changes in the cockle population can be a useful indicator of the biological health of the Inlet. In 1976 the former Department of Scientific and Industrial Research (DSIR) surveyed the cockles as part of the Pāuatahanui Environmental Programme. The survey found that cockles made up 80% of the biomass of the intertidal mudflat animals. In parts of the Inlet, cockle density reached 2500 per square metre. No further surveys were done until 1992.
The Inlet environment had for some years been under pressure from human activities, and the Guardians of Pāuatahanui Inlet, established the year before, was concerned that no credible measure of what was happening to the Inlet’s ecosystems existed. Monitoring the size and age structure of the intertidal cockle population was known to be such a measure. So, in 1992 the GOPI carried out the first of its regular surveys that have subsequently taken place every three years. The National Institute of Water and Atmospheric Research (NIWA) designed a method suitable for volunteer labour that would produce data comparable with the DSIR survey, and also analysed the data collected. The three-yearly surveys continue to use the same method designed by NIWA and are supported by Greater Wellington Regional Council. The data continues to be analysed by NIWA which produces a report detailing the resultant estimate of population size.
Between 1992 and 2001 the population varied between 35% and 50% of the 1976 figure. But from 2001 there was a steady rise in numbers, increasing 65% by 2013. A drop in 2016 has been attributed to the storm that took place a week before the survey of that year, but the numbers recovered drammatically by 2019. The 2022 survey results have yet to be released.
The long time-series of data now available shows that the intertidal cockle population is recovering from the severe downturn noted between 1976 and 1992.
What caused the population to fall?
We will never know exactly when or why the decline between 1976 and 1992 took place and although there has been a significant improvement since 2001 it is clear that substantial recovery towards the 1976 levels has not yet occurred. Studies of filter-feeding bivalve molluscs in many parts of the world show that a population decline can result from both natural changes to the habitat and human-induced processes. In the Inlet a reduction in the area of seagrass beds may well have been a major influence on cockle survival and abundance. Seagrass beds trap sediment particles and increase the proportion of edible particles available to the cockles. Reduction of seagrass can adversely affect the feeding efficiency of cockles. Seagrass became much less common in the Inlet during the 1980s and 1990s and the meadows also have not recovered their original extent.
There was major building development around the Inlet and in the rural catchment areas in the 1980s and 1990s, with poor control of earthworks. In heavy rain, silt from subdivisions and road works washed into the catchment streams and on into the Inlet, seriously increasing the sedimentation. Over the same period erosion in the rural areas also delivered significant quantities of sediment to the Inlet.
The combination of these two sources of deposition has caused the sediment accumulation rate to increase from 1 mm/yr in pre-human times to 3 mm/yr by 1950 and at least 4.5 mm/yr today. This increase has resulted in the Inlet’s water volume being reduced by about 4% in the last 150 years and the intertidal area increasing by about 15%, or 12 hectares. Paradoxically, as far as cockles are concerned, the increase in the lower intertidal area may provide more potentially good habitat.
Because cockles live only just below the surface of the sediment, sudden dumping of large amounts of sediment on top of them will damage their habitat and stop them reaching the surface to feed. Adult cockles can’t get back to the surface if they are suddenly buried to a depth of more than 7cm and young cockles are killed by the sudden deposition of much thinner layers of sediment. In the heavy rain events of 2004 the Pāuatahanui and Horokiri Streams deposited very large quantities of sediment in the deltas at their outlets. Up to 12 cm was added in part of the Pāuatahanui Stream delta. Data from the 2004 cockle survey show clearly that this deposition had a major depressing effect on the cockle numbers in these areas of the Inlet.
Recovery is possible. By 2007 the number of cockles in Pāuatahanui Stream delta had increased dramatically through colonisation by young cockles.
Since then there was a steady increase in population up to 2016 when, again due to a substantial storm event that cause huge erosion around the Inlet, the estimated population had declined. However recovery from this event has been rapid and in 2019 the estimated population showed a significant increase.
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 re-suspended by wave action. As a result, water clarity in the Inlet has reduced markedly in the past 30 years. The increase in the abundance of these inedible particles reduces the cockles’ ability to feed and obtain oxygen. In severe cases the particles can clog the gills so badly that the cockles die.
The increasing proportion of fine particles on the Inlet bottom also reduces the quality of the sediment as cockle habitat. Cockles require a firm substrate to support their quite substantial body weight. They simply sink and drown in fine mud and clay.
Increased organic and mineral pollution may inhibit cockle growth. Water quality in some parts of the Inlet no longer always meets public health guidelines for concentrations of bacteria derived from pollution by human sewage (via sewage pipe leakages) and farm animal excrement and the public are advised against the taking of any shellfish from these areas at these times. Whether this increase in toxic bacteria inhibits cockle growth and fecundity in the Inlet is unknown.
In recent years there has been an increase in the quantity and distribution of bright green algal mats in the intertidal area of the Inlet. These algae thrive in high nitrogen levels in the sea water. The most likely source of this nitrogen is waste from agricultural animals and fertilisers. The mats can affect cockles directly by smothering them and restricting both respiration and feeding and indirectly by stifling the growth and development of seagrass beds.
The ever expanding number of motor vehicles operating in the catchment has deposited a significant quantity of heavy metals and hydrocarbon residues in the Inlet sediments This is not yet a concern for public health but there is some evidence that these chemicals have affected the health of cockles in the Inlet on occasion by making them more prone to parasitic diseases. The ability of filter-feeding animals to accumulate metal ions and organic residues to levels where they are toxic to humans, predators and themselves is well known. It would seem only a matter of time before the Inlet cockles are affected unless the sources of the pollutants can be controlled.