Shellfish count at Whangapoua 2011
volunteers, school children and parents get out on the estuary

by John Ogden

When the tide finally retreated on Tuesday April 19th, a hardy crew of parents and children spent the day digging up little plots of mud on a large grid of points laid out using GPS on Whangapoua estuary. In each sample they counted and measured the size of the cockles, and noted the presence of other shellfish, which they had to identify using a set of pictures. This is part of an on-going monitoring system for shellfish throughout the Hauraki Gulf. It is under the overall organisation of The Hauraki Gulf Forum (Auckland Council), but is contributed to by other organisations, especially the Department of Conservation. The data are collected by volunteers – mainly school children – but analysed independently by the Ministry of Fisheries, who report back to the Forum. The Forum produces an excellent Teachers Resource Kit, both in book form and as a DVD. The idea is that the day is fun for all, and that some learning about shellfish ecology and monitoring methods is achieved. While the number of cockles found might be influenced a bit by the persistence of the diggers, measuring shell lengths is easy and not subject to much error.  Subsequent classroom follow-ups are designed to let the children see the collated results of their individual efforts, and to impart some ideas about possible harvest­ing impacts, and ‘what eats what’ out there on the mud.  Ideas about the graphical presentation of numerical data are also introduced. The whole package is designed for children at the upper primary to intermediate level. 

Since 2007, in samples taken from the same locations (within a few metres), the number of cockles appears to have in­creased slightly (Fig 1). While this might seem good, a worrying trend is for a decrease in average cockle size (Fig 2); there appears to be a bigger proportion of smaller cockles, and a smaller proportion of big cockles now than there was in 2007.  Cockle numbers and sizes vary a lot from year to year and place to place, so these results simply mean we should be wary, and perhaps not always go for the biggest shellfish when gathering! Over the same period pipi numbers seem to have declined, and we found only very small ones this year.

Fig 1. Changes in cockle and pipi density at Whangapoua


Fig 2. Average Cockle Size