Cockle survey


The 2016 triennial cockle survey was our most significant and, as it turned out, most demanding exercise of the whole year. There is always a lot of planning that goes into this task, partly because it involves such a large number of volunteers who need to know the right procedure and also require close  coordination on the day.  However, because of last month’s storm on 14 November, and the resultant conditions in the Inlet, the 27 November population count was, for the first time ever, postponed and moved to the fall-back date of 11 December. It was hoped that Inlet conditions would recover in the intervening period to allow the chance of valid data to be gained for a successful statistical analysis. If, because of the storm, the cockles suffered from the influx of sediment or high volumes of fresh water, it would be important to know this as a measure of the effect of a natural phenomenon. All we could do was hope for reasonable weather on the day.

Considering all the drama that the storm and resultant delay created, the exercise was in the end fairly successful. The main problems encountered were due to the reduced number of volunteers who turned up to help, many perhaps lost to alternative, seasonally focussed plans. Not all stations could, therefore, be measured on the Sunday and some volunteer work has been needed to fill in the gaps while the favourable tide levels lasted throughout the following week.

Now, as is normal practice, the results will be collated and passed to NIWA for full analysis before we receive the verdict, good or bad, on the current state of the cockle populations in the Inlet. However, what is apparent from the recorded numbers this time is that there has been a considerable drop in cockle population since 2013. Almost all of the 30 sampled stations are below the 2013 level and only a very few are close or equal to those recorded in that survey.

This change in recorded numbers is significant but at this stage we have no capacity to establish, for sure, any specific causes. Nevertheless we know that there have been quite radical changes to the configuration of the shores and sand banks in parts of the Inlet since 2013 which are likely to have affected the size and status of the cockle habitat. It is also possible, but yet to be established in detail, that the recent mass input of silt from the excessive November rainfall will have increased sediment deposits in the Inlet. In particular, the storm brought massive amounts of material into the harbour and the gale which occurred at the same time may well have reduced the ability of tidal flows to carry this out of the Inlet, allowing it to settle over much of the cockle habitat.

We await the final results with a degree of nervous anticipation.



Read previous NIWA reports: 1992    1995    1998    2001    2004    2007    2010

Read about cockles and cockle population trends in Cockles of the Inlet.


 The 2013 NIWA report is available as a PDF.



Listen to Alison Ballance's Our Changing World programme on the 2013 cockle survey.


The 2013 survey was held on 1 December. And what a great day was had by the 107 volunteers who turned up to dig, sieve, count and measure. We had GOPI members, Greater Wellington staff and scientists, PCC staff, Ngati Toa Runanga, Conservation Volunteers, international students from Massey University and members of the general public. The day was warm, the breeze was gentle, and the cockles came to the party too, with our initial gut feeling on the day being that the number of cockles counted was larger than in the previous survey in 2010. 

That turns out to be very true. Dr Keith Michael's NIWA report records an increase of 21% since 2010, from 277 million to 336 million, and shows that the population has increased by 87% since the 1995 survey. That's great news, but the really exciting result is the way in which the numbers appear to be growing. The shape of the curve on the graph below gives us hope to believe that the increase since 2001 is exponential and that 2016 could be a bumper year.

The long time series of data we now have (1992-2013) clearly shows that the intertidal cockle population is recovering from the severe downturn noted between 1976 and 1992. This is good news for the cockles, but it also indicates that the general ecological health of the Inlet is improving. We can take heart that efforts by local authorities, farmers and urban developers to control siltation and run-off into the Inlet are apparently having a positive effect.

The 2013 results also show that the percentage of juvenile cockles has remained relatively similar, at about 15% of the total population, since 2004. Since the population has increased some 50% in that time this suggests that both recruitment of larvae and their survival to adulthood has improved since then. Also, this time we collected a few individuals larger than 50 mm, which may indicate that the rate of survival to old age also is improving. We can only hope that in a few years we will again see large numbers of 'senior citizen' cockles in the Inlet.

Cockle numbers are increasing at most of the sites sampled. But one difficulty with the statistics is that cockles are not spread evenly over the Inlet shores - there are many patches where they are abundant and many where they are less common, even absent entirely. This 'patchiness' makes estimating the true size of the total Inlet population very difficult and results in the population size having to be expressed as a 'mean' number within a wide margin of error. In 2010 we noted that even though the trends were encouraging, the differences from survey to survey were not statistically significant. But we were confident enough to say, 'If the trend continues at the current pace, the 2013 survey may well reveal a true, scientifically acceptable increase.' With the long time series of results now available Dr Michael has been able to use more sophisticated statistical analyses to reveal that our hopes have been reflected in reality.

Our grateful thanks go to all those who came along and took part in this vital piece of work. The survey simply couldn't have taken place without you.

Our thanks also to Hu Peng from Massey University for the photos on this page. Top photo: Shyam Morar and Megan Oliver of Greater Wellington with Massey student Zhang Shanmin; photo on left Pam Watson, Dick Fernyhough (GOPI), Graham Sevicke-Jones (Greater Wellington), with Jane Kitchenman and Zhao Yan from Massey University.  

A special 'thank you' goes to members of the Kapiti Camera Club who used the cockle survey as a club activity. For a fascinating selection of the resulting photos, check Ray Lovell's photos on:

.... and other KCC members' photos on:



Last Updated: 18/02/2017 8:32pm