Cockles of the Inlet

Cockles of the Inlet

Just below the surface of the sediment in the Pauatahanui Inlet lives the tuangi or New Zealand cockle (Austrovenus stutchburyi). It is the most abundant shore animal in the Inlet.   The adult cockle has a rounded heart-shaped bivalved shell with strong ribs and concentric lamellae (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 can burrow 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 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 also. Middens around the Inlet show that the cockles were an important source of food for pre-European Maori. 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 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 determined 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 Maori 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 sea surface 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 widely 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 Cockles form a large proportion of the living organisms in the Inlet and changes in the cockle population are therefore 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 Pauatahanui Environmental Programme. The survey found that at that time 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. In that year, the Guardians of Pauatahanui Inlet carried out the first of the regular surveys that have taken place every three years since. The Inlet environment had for some years been under threat from human activities, and GOPI was concerned that no credible measures 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, and GOPI decided to adopt this measure, and to survey the cockles every three years. The National Institute of Water and Atmosheric 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, which continue to use the method designed by NIWA, are supported by Greater Wellington Regional Council, and the data continues to be analysed by NIWA.   The 1992 survey revealed an alarming decrease in cockle numbers. From an estimated 520 million in 1976 cockle numbers had shrunk to an estimated 220 million, only about 40% of the 1976 density. Since 1992 the population has varied between 40% and 50% of the 1976 figure. The fluctuation between 1992 and 2010 is not statistically significant and thus the population appears to have stabilized at this new level. Presumably the Inlet is now capable of supporting only about half of the number of cockles that existed there in 1976.     The proportion of juveniles (up to one year old and up to 10 mm in length) in the population has differed widely between surveys. In 1976 they formed 9% of the population, but at 12-16% the proportion in the 2004-2010 surveys was much higher. In contrast, 1998 and 2001 were poor years for the recruitment of juveniles into the population. The very low proportions in 1992 and 1995 must be disregarded as the methods of sampling juveniles were less accurate than those used in 1976 and since 1998.     We don't know why there is fluctuation but similar ambiguous patterns are a common occurrence in bivalve molluscs world wide. The most likely major factor is a cooler than normal sea temperature during the breeding season depressing the production of eggs and sperm and, possibly, survival of the planktonic larvae. In Auckland the colder sea conditions resulting from El Nino years in 1991 and 1992 have been suggested as a cause of the lack of cockle breeding success in those years.

What has caused cockle numbers to fall? We will never know exactly when or why the decline between 1976 and 1992 took place but 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 fluctuation in size of eel grass beds is the most likely natural change to have occurred over recent years. Eel grass beds trap sediment particles and increase the proportion of edible particles available to the cockles. Reduction of eel grass can adversely affect the feeding efficiency of cockles. Eel grass became much less common in the Inlet during the 1980s and 1990s but there is evidence of a resurgence over the last five to six years. We don’t know whether this fluctuation is due to natural or human-induced processes, though it is probably a mixture of both.   Sea temperature influences cockle breeding success and may have contributed to relatively poor spawning in some years, but current data do not suggest that the Inlet cockles have failed to spawn adequately.   The human-induced processes that lead to population decline relate to sedimentation rate, changes to sediment type and pollution. These factors have all been identified or suspected in relation to the Inlet.

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 earth movements. 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 to 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 Pauatahanui 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 Pauatahanui 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 Pauatahanui 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 fertilizers. The mats can affect cockles directly by smothering them and restricting both respiration and feeding and indirectly by stifling the growth and development of eel grass 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.

What about the future? While the large reduction in cockle numbers since 1976 indicates a serious decline in the health of the Inlet ecology, there is no need for pessimism in thinking of the future. The broad factors responsible for the decline have been identified and their impact can be avoided to a very large extent.    Regulations requiring the outflow of sediment from all construction sites to be minimised are now in place. Combined with continuously improving technology these regulations have the ability to bring about an almost total elimination of this source of contaminant.   Methods to control sediment and nutrient outflow from rural agricultural and forestry practices are being developed and applied throughout the catchment.   The flow of heavy metals and hydrocarbon residues into the Inlet may be the most difficult problem. These pollutants are delivered by roadside drains and stormwater systems that drain directly into the Inlet or its catchment streams. Good drainage system design and modern technology can prevent this in new developments but it is very expensive to retrofit such improvements to existing drainage systems.   Peoples’ attitude towards these issues also is changing for the better. Pressure from the general public, and especially from community groups such as the Guardians and the Pauatahanui Inlet Community Trust, has resulted in the Porirua City Council adopting A Healthy Harbour and Waterways as a strategic focus for future planning. The resulting Porirua Harbour and Catchment Management Programme now being rapidly developed will address all of the above issues.   Given this, it is reasonable to hope that the infilling of the Inlet – the inevitable fate of all such enclosed bodies of water – will at least take place more naturally than at the present human-induced exaggerated rate.

Read the full NIWA reports on all cockle surveys from 1992 to 2010.