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CCET Cockle Bay beach shellfish monitoring April 2007 Smmary
The shellfish monitoring survey conducted on 15th April.2007 was very successful. The good low tide gave easy access to the lower shore stations where cockles are most abundant. As the weather was fine and the turn out of CCET supporters was so good again, the duplicate sampling of all eight stations that had been sampled in the previous surveys was completed fairly speedily. This allowed for sampling to be carried out for the first time along a new transect and thereby extend our knowledge about the shellfish populations over a larger area.
The report on the last survey (5th November 2006) mentioned that the Hauraki Gulf Forum ~ the advisory group that is attempting to coordinate and standardise shellfish monitoring throughout the greater Auckland region ~ was keen for the Cockle Bay survey to be extended to help meet the Forum’s “whole beach” coverage goals.
The new transect, running more or less parallel to the Boat Ramp transect, extends out from the headland at the southern end of the beach, on a line out to another headland beside Karaka Island, off shore. This will be known as the “Headland transect”. It is hoped in the future to introduce one more transect at the northern end of the beach. The whole beach will then be well covered by sampling stations.
For ease of identification at the beach, transects will continue to be known by their landmark names but for data entry on the web page maps and records, all the transects will also be known by letters of the English alphabet.
Therefore from henceforth :-
Headland transect = Transect A
Boat Ramp transect = Transect B
Toilet Block transect = Transect C
Northern transect = Transect D
Stations along each transect will continue to be know by their distance from the top of the shore.
Therefore “B 300” is the 300 metre station on the Boat Ramp transect
And “C 200” is the 200 metre station on the Toilet Block transect.
After careful ground location using satellite navigation aids and the support of DoC’s skilled map maker, Chris Wild, we now have precise site location coordinates for all of the sample stations. It is hoped that this will enable sample stations to be marked more speedily on sampling days so that the CCET group can achieve better coverage across the shore, and avoid unnecessary duplications at some stations.
As on the previous sampling day in November 2006, a contingent of keen CCET members from the North Shore boosted the numbers of samplers at Cockle Bay and their help was greatly appreciated. In the last report, the possibility was mentioned of establishing a new sampling site north of the Harbour Bridge that would be more convenient for North Shore CCET members. The site needs to have easy access, have reasonably good cockle beds that are harvested by the public and ideally some existing information about shellfish densities. One beach on the Whangaparaoa peninsula appears to meet these criteria but its selection needs to be confirmed by DoC and the Hauraki Gulf Forum. It is hoped that a decision will be made before the new year.
Patterns and Changes in the numbers and distribution of bivalve shellfish.
The object of shellfish surveys is to check if stocks of harvested shellfish are healthy or are in decline. We know that decreases in abundance can result from either over harvesting or from undesirable changes to the conditions where the shellfish live. The changes may be to the water that washes over the shellfish or to the sandy / muddy substrates that they live in.
The abundance of cockle and pipi are, of course, of most interest to those who are mainly concerned with gathering shellfish for food. In our shellfish surveys we also carefully count the numbers of nutshells and wedge shells because they are seldom or never harvested by must of the public and therefore help us identify if shellfish decline in a particular beach is due to over harvesting, environmental changes or both.
The nutshells are too small to collect and wedgeshells are generally regarded as having poor or even an unpleasant muddy flavour, so neither are gathered to be eaten. Wedgeshell generally live a little deeper in the sand than cockles but nutshells live in the to 2 to 3 cm amongst the shallowly buried cockles.
Therefore if the nutshell numbers remain constant but cockle numbers decrease then it is a sign that cockles have been over harvested. If both cockles and nutshells decline by similar amounts, it is an indication that the conditions have deteriorated. In some instances it may be possible to identify the causes of the worsening conditions and address the problems.
Patterns across the shore at Cockle Bay of the commonest bivalve shellfish
Pipi : Only ever found towards the top of the shore at the 100 and 200 metre stations and always quite scarce and small in size. Many years ago they may have been more common when the sand was cleaner, for it is known that they prefer cleaner sediments.
Cockle : The abundance and size of cockles increases with distance down the shore. At the 100 metre stations only small numbers (20 to 50 / m²) of mostly small cockles are found but at the 300 and 400 metre stations cockle numbers are much greater (250 to 450 / m²) and usually many are of good size for harvesting.
Wedgeshell : Found in similar small to moderate numbers (10 to 50 / m²) at the 100, 200 and 300 metre stations but seldom at the 400 metre stations.
Nutshells : More abundant on the Toilet Block transect (C) than the Boat Ramp (B) and Headland (A) transects. Along trancect C they are usually found in large numbers (150 to 400 / m²) at the 100, 200 and 300 metre stations but never at the 400 metre stations. On transect B they averaged only 50 to 100 / m² at the 100, 200 and 300 metre stations and less than 50 / m² at the 400 metre station.
Along the Headland transect there were 100 / m² at the 100 metre station but none lower down the shore where the sand was cleaner. There is thus a natural decline in nutshells across Cockle Bay beach from the siltier northern end, where a stream delivers sediment from the land on to the beach,
to the more open southern end where slightly stronger wave and current action may stir the bottom sediments and flush away some of the fine silt.
Changes over 18 months of sampling
The April 2007 survey is the fourth in the CCET programme at Cockle Beach. After 18 months cockle numbers across the whole shore appear to have declined by about one third although at the 400metre mark abundance is still good. This may be because this low level of the shore is only exposed on very low tides and therefore harvesting pressure is usually less because that part of the shore is less accessible to cockle gatherers.
After such a short time (18 months) it is uncertain if the declines further up the shore are real or just part of a natural fluctuation. This may take five or more years to verify. It is also necessary to take into account the proportion of juveniles. Very large numbers of juveniles (about 500 / m²) were present on one survey (October 2005) at just one station (TC 400) and this biased the data for that transect. Such a large population burst has not been seen again although small numbers of juveniles have been recorded at most stations along all of the transects.
Checking for recruitment of juveniles across the shore is an important part of the regular survey, for without good numbers of juveniles there will be no large adults to harvest in the future. It is therefore very important when samplers are sorting through their sieved samples that they do not just count and measure the large cockles but look very carefully through the shell debris to find the small and baby cockles.
On the new Headland transect (A), towards the open southern end of the beach, the numbers of cockles per square metre are low but the proportion of larger specimens is greater. This an area where local shellfish gatherers appear to concentrate their efforts so it will be interesting to follow the changes in that area through future surveys.
If good numbers of samplers turn up to CCET’s next sample day at Cockle Bay in late October it is hoped that we can again cover transect A, B, and C but also tackle transect D for the first time. This might provide some insight into the population differences along the shore from north to south and help establish some better numerical relationships between different species at different levels and positions along the beach.
JRW
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