COCKLES OF THE INLET

What is a cockle?

Just below the surface of the sediment in the Pāuatahanui Inlet lives the New Zealand cockle. It is the most abundant shore animal in the Inlet.

Known to Māori as tuangi and to science as Austrovenus stutchburyi, the NZ cockle was scientifically described by the English surgeon and naturalist William Wood in 1828. He named it after the geologist Samuel Stutchbury, who collected cockles and other sea shells while he was the naturalist to the Pacific Islands from 1825 to 1827. The cockle is a dominant member of the fauna of estuaries and harbours right around New Zealand. Our last triennial survey, in 2016, estimated the Pāuatahanui Inlet population to be between 259 and 316 million.

The adult cockle has a rounded heart-shaped bivalved shell with strong ribs and concentric ridges. The cockle’s shape and external structure anchor the animal in the sediment so it can’t be scoured out by waves and currents. The cockle’s large mobile foot allows it to burrow under the surface, although its rounded shape and short siphons mean it burrows only deep enough to cover the shell.

Cockles live throughout the subtidal and intertidal areas of the Inlet but are most abundant between the shallow subtidal (permanently covered by about 1 metre of water) and the middle of the intertidal areas. They are filter feeders, drawing water through their gills to get the food and oxygen they need. Those in the intertidal area must be submerged, and therefore able to feed, for at least three hours per day (1½ hours per tidal cycle).

The filter-feeding habit of cockles makes them essential to the water quality of the Inlet since they remove plankton and minute particles of organic debris and help to keep the water clear. It has been calculated that up to one-third of the volume of incoming tides passes through their gills.

How do cockles feed?

When the cockle is submerged, it draws water into its outer body cavity through its ‘inhalant’ siphon, straining the water through its gills to remove potential food particles. It then pushes the water out through its ‘exhalant’ siphon. The captured particles are moved down to the base of the gills and on towards the mouth by cilia (fine hairs). Large particles are removed by the palps and redirected to the exhalant water stream. Small particles are directed into the mouth. When cockles are feeding, their siphons can be seen above the surface of the sediment.

Cockles as a food source

Cockles' shallow living arrangements make them vulnerable to predators. They are a major food source of fish and shore birds such as the oystercatcher, which feeds primarily on cockles and other shellfish. Small carnivorous snails are important predators of young juvenile cockles.

Cockles have long been a food source for humans as well. Middens around the Inlet show that the cockles were an important source of food for pre-European Māori. An archaeological dig on the south side of the Inlet in 2000 yielded some significant evidence about Māori resource use and the huge size of cockles in the past.

The life cycle of the cockle

A few cockles may live for 20 years but the average age at death is 5-8 years for those that survive the high juvenile mortality rate. Age is estimated by counting the annual growth rings in the shell. Age is closely correlated with size only for the first 2-3 years of growth, by which time the cockles have reached a length of 18-20 mm. The largest cockles now seen in the Inlet are 45-50 mm but records from Māori middens indicate that cockles grew much larger in the past.

Cockles spawn from October to December. They release eggs and sperm into the water on an outgoing tide. The resulting larvae spend 3-6 weeks in the plankton before settling on the sediment. Since many of the newly spawned larvae are carried out to sea by the ebbing tide it is not known how many of those that settle in the Inlet are derived from its resident population and how many come from neighbouring populations in Cook Strait. Thus the number of juvenile cockles in the Inlet is not necessarily an indication of the reproductive output of the resident population.

After settling, juvenile (up to 10 mm in length and one year old) and pre-adult (10-20 mm) cockles move around the Inlet, either passively through tidal and wave action or actively by dragging themselves over the sediment surface. It is common for settlement to be much higher in some parts of the Inlet than others (the actual areas of high density vary from year to year) but redistribution of the young as they grow results in a more even, (although still patchy) distribution of young adults (18-25 mm) throughout the Inlet.

Juvenile mortality is high. Predation by carnivorous snails, birds and fish may take 70-90% of juveniles in the first three years post-settlement. Once this phase is over the mortality rate from natural causes other than senescence is very low.

Sexual maturity is determined by size (18-20 mm) rather than age.

Monitoring the cockles

What has caused cockle numbers 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 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 but there is evidence of a resurgence over the past few years.

Sedimentation rate

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.

Sediment type

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.

Pollution

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 heavy metals 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.