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Discuss about the Initial Settlement and Juvenile Recruitment.

Population estimation of Austrovenus stutchburyi

Cockels are referred to as Austrovenus stutchburyi. They occur throughout New Zealand. They live in the lower tidal mud flats which are protected by estuarine areas. The distribution basement is concentrated at soft shore below low high water which extends to the subtidal areas. The population distribution of Austrovenus stutchburyi is characterized by upper distribution limit which is bounded by submergence of 3.5 hours per tide, (Perrigault,  Buggé & Allam, 2010.). However, key factors are of concern that is the tidal distance from the open sea, substrate type, salinity, wind and tidal effects, wave actions, and predator factors. They inhabitant is characterized by a subtidal intertidal zone which has low tide makers. Predators often find it difficult to the break the shell. Often predators such as birds drop the Austrovenus stutchburyi up from sky aiming to crack the shells, but they are made of strong materials which don’t break,(Pirker, 2008).

Austrovenus stutchburyi, previously referred to as Chione stutchburyi, often burrow shallow suspension into the feeder of Veeneridae family. The presence of the Cockels is often based on the lowest high water heap to the lower shares. Evidence has shown that they can even extend up to 20m deep sea. Studies undertaken by Lohrer et al (2016) suggest that upper limit occurs within a period of 3.5 hours submergence per day. Austrovenus stutchburyi is often highly densely present in species and densities which go up to 4500m/2 which have been documented and reported in some geographical area a biomass of up to 500tonnes in Pauahanui Inlet of a total intertidal outlet. Austrovenus stutchburyi often filter up to 1.6 million cm3 of water in each occurring tidal cycle, (Powers, 2009).

Austrovenus stutchburyisexes are usually sex separated and the sex ratio is close to 1:1. Growth maturity has been observed to be functionality size rather than age factors. Sex maturity often occurs at about 18mm of the length of the shell. Cockles spawning often extend during summer persons and springtime with fertilization occurring through a platonic larval process which often lasts for close to three weeks. Larval stage depression occurs and has been reported in areas in sub rate areas which the cockles live. This could indicate a confounding factor which is affecting its growth, (Przeslawski  & Webb, 2009).

Studies show that population estimates taken in 2010 indicates that Okoromai Bay contained a larger population size of cockles. This population was about 4-5 times higher than other beaches when compared- Cockle bays and Whangateu bay. Often harvestable cockles often are rare on other beaches, (Ross, 2011).


This study sought to examine the population size of abundance and distribution of cockles Bay in the year 2018 and results compared to surveys done in 2015 and 2016. The study sought to answer the following objectives;

  1. To estimate and compare the mean density of each stratum.
  2. To estimate  and compare the global mean density with previous surveys
  3. To estimate the abundance of each stratum and total abundance.
  4. To estimate and compare the mean and mode lengths of each stratum.

Since the year 1996, sampling designs have been widely used to with a combination of other techniques in a combination of stratified random design. In the year 2009 survey, both the two sampling methods were used in combination of both to ensure efficiency.

The site review of this site was estimated using Google earth done remotely to determine the presence of any physical and environmental variables which influence spatial distribution activity. Important variables included the presence of sandbanks, streams, gross topography features, sediment size, and shell abundance. This information was taken into a priority in order to refine the strata as effectively as possible. Strata’s were further refined using channels rather than location via GPS locator.

The biomass was estimated using two methods strata random design. Stratification process involved both up the shore and alongshore variation methods. Then transects were located through random processing each of the stratum alongshore boundary. Then 0.1m2 quadrate sampled very 10 meters down in each transect. The basic unit of assessment was shellfish density per quadrate crossing the transect. In the second phase, the sampling methods were based on the precise estimates of abundance of cockles.

Weighted length frequency was utilized to assess the distribution of each species at the bay. The length measurements were weighted and weighted to account for the proportion of samples taken by stratum to this size and a total number of cockles counted divided by the measured. 

The sample extends of Okoromai Bay was split into two strata A and B having the best suitable cockles. Most substrates in one part of the stratum were covered with cockle density which was veered by the seagrass. Both states had both rocky substrates which had an equal number of cockle density which was sampled.


Average length

Average density

Per m2

Stratum A



Stratum B



Table 1Mean Density of each stratum


Population estimates




Average density per m2
















Table 2Population Estimates of Okoromai Cockles





















Table 3Weighted length frequency distribution and summary data

The increasing natural mortality of the cockerel has shown an increasing trend compared to the previous year’s surveys. There many predators which can affect these population such as birds. However despite this, still they have little impact on the abundance of cockles. cockles are further killed when they are smothered through the sediment shifts and calamities such as storms and strong tides.

Factors affecting population

The resulst obtained from this study between the two strata showed that strata A had cockles with mean length of 19.2 while strata B had an average length of 24.2. showing that strata B had longest cockles and further there was high abundance of cockles population compared to strata A. However in general there was decreased length of cockles in Okoromai bay.

These findings were similar to surveys done in Snake Bank, where the results indicated that there was an occurrence of 17-30% mortality rate, with instantaneous mortality being between a mean of 0.19-0.35 having the midpoint of M=0.28. the estimated mortality rates on cockles below the 35mm length of shell a were high.

The relative abundance found on the Austrovenus stutchburyi have shown that the mean densities could be affected by seasonability and sites. Slight significance difference s can be observed from the different sites with population’s densities which were less than 100/m2.

These studies have illustrated that there is a sharp decline in the population of the Cockles. This unpredictability patterns have been shown to largely influence by the uncontrolled nature of the habitats. Previous studies have indicated that cockle’s population was high in years of 2010, and the population decline is being witnessed at a slower rate compared to those years, Ross, Hogg, Pilditch  & Lundquist, 2009).

Surveys in previous of 1996 and the results obtained in the year 2010 have shown that there was an estimate of around 30 million shelf fish having 36% of the population having an average length of 30mm. However, a survey done in 2012, showed that there was a decline in results obtained by about 2 million and there was also a marked decline in the size of cockles. This result depicts similar findings of this study whereby the results indicate that there is an estimate of 19 million cockles population as of 2018.

Government’s efforts to impose bans have been conducted in order to improve the cockle’s population at the bay. The Okoromai bay beach in the previous studies contained larger proportions of harvestable cockles, which is about 4 to 5 times higher than other populations. However, there is a marked decline comparing the results undertaken previously and this survey conducted, (Shears, Smith, Babcock, Duffy  & Villouta, 2008.

Increasing environmental temperatures could play a significant factor in the general cockle’s population, (Tallis, Wing & Frew, 2004). this impacts on the reproductive processes. Further the decline oxygen carrying capacity tends to affect the gaseous exchangeability and in turn, affects the general metabolism hindering the general reproductive process of the Austrovenus stutchburyi.

Efforts to improve population

Other factors which are attributed to a declining in cockles at the bay could be attributed to parasitic infection and infestation on the bivalves to the substrate levels. Parasite infection often leads to elevated uptake of contaminants. Further studies have shown that New Zealand Austrovenus stutchburyi, are prone to trematode infestation which affects its population's size, this reduces the ability to enhance burrowing thus leaving prone to predators, (Thelen  & Thiet, 2009).

Further, a more reliable approach to mitigate the declining cockles population is for the implementation of harvesting bans. In New Zealand, this can be achieved through marine protection of the sea levels through marine reservoirs approaches. Currently, New Zealand coastal line has less than 1% protection thus leaving harvesting of the cockles at the bay at risks of declining population, (Vassiliev, Fegley &  Congleton, 2010). Further recommendations  

There is a need to enhance approaches to restore the declining stock is to initiate habitat restoration activities at the bay. Further, research conducted has shown that Austrovenus stutchburyi population could be introduced into the system through transplantation, this could be a way to mitigate on the changing declining population of the cockles.


This study geared towards establishing and measuring Austrovenus stutchburyi population at Okoromai bay. The results showed declining density levels and the overall population of cockles at the bay at an alarming rate. Surveys were done in the previous years of 2015 and 2016 shows that there is an increased decline in the overall density of the Austrovenus stutchburyi at the bay. This is likely to be a potential depletion of cockle’s species at the bay. Effective measures need to be undertaken by both the authorities and environmentalists on saving the ecosystem of cockles at the bay. With these findings, there is a huge challenge ahead, with the trends observed; the population rate is declining at a high speed which calls for more ecosystem preservation for Austrovenus stutchburyi conservation.


Lohrer, A. M., Townsend, M., Hailes, S. F., Rodil, I. F., Cartner, K., Pratt, D. R., & Hewitt, J. E. (2016). Influence of New Zealand cockles (Austrovenus stutchburyi) on primary productivity in sandflat-seagrass (Zostera muelleri) ecotones. Estuarine, Coastal and Shelf Science, 181, 238-248.

Perrigault, M., Buggé, D. M., & Allam, B. (2010). Effect of environmental factors on survival and growth of quahog parasite unknown (QPX) in vitro. Journal of invertebrate pathology, 104(2), 83-89.

Pirker, J. (2008). South Island customary fisheries management and conservation. Natural history of Canterbury. Published by the University of Canterbury Press, 843-844.

Powers, S. P. (2009). Effects of Water Flow and Density on Early Survivorship and Growth of the Northern Quahog Mercenaria mercenaria L. Journal of Shellfish Research, 28(4), 777-783.

Przeslawski, R., & Webb, A. R. (2009). Natural variation in larval size and developmental rate of the northern quahog Mercenaria mercenaria and associated effects on larval and juvenile fitness. Journal of Shellfish Research, 28(3), 505-510.

Ross, P. M. (2011). The genetic structure of New Zealand's coastal benthos: using the estuarine clam Austrovenus stutchburyi, to determine rates of gene flow and population connectivity (Doctoral dissertation, University of Waikato).

Ross, P. M., Hogg, I. D., Pilditch, C. A., & Lundquist, C. J. (2009). Phylogeography of New Zealand's coastal benthos. New Zealand Journal of Marine and Freshwater Research, 43(5), 1009-1027.

Shears, N. T., Smith, F., Babcock, R. C., Duffy, C. A., & Villouta, E. (2008). Evaluation of biogeographic classification schemes for conservation planning: Application to New Zealand's coastal marine environment. Conservation Biology, 22(2), 467-481.

Tallis, H. M., Wing, S. R., & Frew, R. D. (2004). Historical evidence for habitat conversion and local population decline in a New Zealand fjord. Ecological Applications, 14(2), 546-554.

Thelen, B. A., & Thiet, R. K. (2009). Molluscan community recovery following partial tidal restoration of a New England estuary, USA. Restoration Ecology, 17(5), 695-703.

Vassiliev, T., Fegley, S. R., & Congleton Jr, W. R. (2010). Regional differences in initial settlement and juvenile recruitment of Mya arenaria L.(soft-shell clam) in Maine. Journal of Shellfish Research, 29(2), 337-346.

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