Cockle Surveys
2025 Cockle Survey
It is good news for the Inlet and the population of cockles. A summary of the findings follows.
Current Status - The 2025 Survey
Our most recent survey of the cockle population was our twelfth in the long-running citizen science series and conducted on 2 November 2025. A staggering 18,676 individual cockles were examined and measured in all but one of the 31 transects around the shores of the Inlet. This equated to 51.9 shellfish per quadrat, each quadrat measuring .1 sqm. The full set of measurements was compiled by our team and then the raw data sent to NIWA. After several months of analysis, we now have the results, reported in the full publication from ESNZ which can be read here.
The key results are that the total number of cockles in the intertidal zone which we sampled have increased by just over 26% (to around 519 million) since the last survey in 2022. This continues the generally steady increase in numbers since 1992. The other general increase seen was the proportion of juveniles (less than 10mm) in the samples – steadily increasing since 1992 to 21.4 % in 2025.
To summarise, the 2025 report shows that:
- The estimated mean population of cockles in the Inlet as of 2025 is 519 million. This figure is the highest recorded estimate since 1992 representing an increase of 102% over that first survey conducted by GOPI. This estimate also is a figure representing 84% of that obtained from the first and highest population survey undertaken by the New Zealand Oceanographic Institute in 1974.
- Most transects in 2025 had higher total counts of cockles than in previous surveys.
- The highest number of cockles recorded per 0.1 sqm quadrat in 2025 was 229, which was fewer than in 2019 and 2022 but more than in 2016.
- Mean cockle density over the intertidal survey area in 2025 was 51.9 per 0.1 sqm with a 99% Confidence Interval (CI) and this was higher than all previous GOPI surveys.
- The percentage of juvenile cockles sampled in the population has increased markedly from 1.0% of cockles sampled in 1992 to 21.4% in 2025. From 2004, the juvenile cockle population has increased between 16.0% and 21.6% of the cockles sampled. The high percentage of juveniles, and the increase in the population over this time, suggests regular recruitment and good survival of newly settled spat.
There is no information on the likely levels of the movement of juvenile cockles between intertidal and subtidal areas. The increase in terrestrial sediments in 2016, considered deleterious to cockles, do not appear to have had a long-term effect on the intertidal population. This trend is not consistent with the expectation of relatively high mortality of small cockles due to high levels of mud and suspended sediments caused by the floods in 2016.
All the GOPI surveys have used the same design and methods, allowing site, transect, and tide-level comparisons between surveys. In all the analyses carried out by ESNZ (formerly NIWA) since 1992, two methods have been used to estimate cockle population size and both show upward trends and significant differences from 2007 to 2013, a decrease between 2013 and 2016, and a marked increase between 2016 and 2025. The low Coefficients of Variation (CV) of 3-6% suggest statistically significant increases. The 2025 mean population size using Method 1 (519 million cockles, 99% CI 477 – 562 million) is 99.2% of the first and highest population estimate from the 1976 survey, and over twice that in 1992 (222 million cockles).
Below, under History of the Cockle Surveys, you can see the latest, updated, graph of the population estimates which clearly show the significant increases that have occurred since 1992.
A video, created on the day of the 2025 survey, has been provided by Rachel Knight, Predator Free Porirua.
Click on the link to watch.
The Cockle
The cockle, a filter-feeding bivalve mollusc, is a robust keystone species of sheltered coastal habitats and an indicator species of ecosystem health. A significant and long-term decrease in the abundance and size distribution of cockles is likely to represent changes to environmental factors that affect the ecology of the habitat with probable loss of ecosystem function.
In Pāuatahanui Inlet, cockles are present in huge numbers and it is estimated that together they filter at least a quarter of the volume of seawater with each incoming tide. Understanding the trend in population is therefore a very helpful indicator of the state of the marine environment in the Inlet.
A Cockle Survey or Cockle Count is a scientifically-based method of establishing an estimate for the number of cockles to be found. It involves teams of people sampling the intertidal regions of the shoreline to pick out live cockles from a large number of randomly selected small square patches of sediment and measuring the size distribution, i.e. how many of each size are found in each sample. The results are then analysed and extrapolated to produce an estimate of the total number of cockles present in the whole Inlet.
History of the Surveys
The Guardians of Pāuatahanui Inlet have now carried out 12 (twelve) triennial surveys of the intertidal cockles (Austrovenus stutchburyi) in Pāuatahanui Inlet since 1992. These surveys now provide an important time-series data-set to monitor trends in cockle population densities, and their sizes, over space and time, in the intertidal zone.
The first systematic sampling of the cockles in the Inlet was undertaken in 1976 by the then New Zealand Oceanographic Institute of the DSIR (Department of Science and Industrial Research). It was conducted as part of the wider Pāuatahanui Inlet environmental study, coordinated by W.B. Healy and published as a book in 1980.
Since 1992 GOPI, with the assistance of other community volunteers, have undertaken regular triennial surveys of the cockle population at a representative selection of sites around the shores of the Inlet with overall oversight by NIWA (National Institute of Water and Atmospheric Research). NIWA then analyses the raw data and reports on the findings. These surveys were specifically designed to be conducted by members of the general public, with supervision by members of GOPI and other experienced team leaders.
The first survey in 1992 found there had been a significant decrease in the population since 1976. The second, in 1995, showed that the decline had continued, albeit at a slower rate. Potential causes of such decline include increased sedimentation, poor or insufficient nutrients, and increased harvesting pressure. Since then, however, the population has increased survey-on-survey, with just one exception in 2016.
The relatively stable population size of cockles in the period between 1992 and 2010, and the relatively high proportion of juveniles in following surveys, suggested that the intertidal habitats of Pāuatahanui Inlet had not changed greatly over this period. Since then, the population has been increasing significantly.
By 2004 these regular surveys had reached a significant milestone when they were then thought to be the longest running and most comprehensive community science project in New Zealand. By now, in 2026, this has become an accepted accolade.
Estimates of total cockle population size and 99% confidence intervals for Pāuatahanui Inlet, 1976–2025.
The initial survey in 1976 (Richardson et al. 1979) used a different survey design from that of GOPI (Method 1). Surveys since 1992, carried out by the Guardians of Pāuatahanui Inlet, have all used an alternative survey design and method (Method 2).
Estimates using the Method 1 are shown in pink and estimates using weighting factors for transect length, Method 2, are shown in blue.
Trends in population size are shown as dashed lines.
The survey in 2019 showed a marked jump in population size, recovering from the decline in 2016, indicating the population of cockles in the intertidal zone of Pāuatahanui Inlet is in an improving state. The consistently high percentages of juvenile cockles since 2004 (12.4–17.4% of the total populations) suggests successful settlement of larvae and good survival of spat, or the immigration of juvenile cockles from subtidal areas. High percentages of cockles (more than 50%) are above spawning size (larger than 18 mm in length) and that should maintain larval production in the Pāuatahanui Inlet for the foreseeable future.
On 1 July 2025, NIWA merged with GNS Science to become Earth Sciences New Zealand but has continued to do the data analysis and publish the triennial report.
The latest survey, in 2025, has shown that the population of cockles is still increasing despite concerns over the ongoing deterioration of the Inlet health.