Successful Shellfish Survey Day at Whangateau Harbour

 

Sieving the sand out of the samples to enable the shellfish to be counted and measured.  Eagleray feeding holes filled with water were handy on the upper shore.  Without them samples would have to be carried several hundred metres to water for sieving

Sieving the sand out of the samples to enable the shellfish to be counted and measured. Eagleray feeding holes filled with water were handy on the upper shore. Without them samples would have to be carried several hundred metres to water for sieving

The annual survey of shellfish, particularly cockles, on two transects east of Horseshoe Island in the Whangateau Harbour, was successfully completed on Friday 28th March by ten students and several adult helpers. Mahurangi College science teacher Wendy Dunn organized the students into pairs for the task. As the shellfish beds in the Whangateau Harbour are currently closed, a special permit is required to carry out the surveys.

The shellfish survey is part of an Auckland-wide programme supported by Auckland Council and the Hauraki Gulf Forum. Technical assistance and practical help on the day was provided by Sophie Barclay and Shelley Hackett from Auckland Council, who also collate the data which is then sent to Ministry of Primary Industries fisheries scientists who use the data in their annual assessment of the state of shellfish resources throughout the Auckland Region.

Sites were spread 100 metres apart on two parallel transects running east of Horseshoe Island right down to the low tide channel. The transect lines crossed wide expanses of cockle beds.

At each site two replicate samples of 0.1 sq.m. in area were marked on the sand, and then the sand was carefully dug from the marked site and placed in a sieve. The nearest water was found in which to sieve the sample to remove most of the sand, leaving dead shells and live shellfish to process further. On parts of the shore eagleray feeding holes containing water provided very handy spots to sieve the samples. Each cockle was then measured and counted, and other animals like crabs, wedge shells and whelks counted. All the data were carefully recorded on pre-prepared data sheets.

Markers for each site were set out earlier in the day by Whangateau HarbourCare members and local marine biologists Dr Karen Tricklebank and Dr Roger Grace. This was the fourth year this particular area has been surveyed, though other sites off Lew’s Bay have been surveyed for ten years.

Students digging sand and shellfish from the marked quadrat site and placing it in the sieve

Students digging sand and shellfish from the marked quadrat site and placing it in the sieve

Walking out over the sand flats in the morning, we came across some very clearly defined eagleray feeding holes, where the rays had been feeding on shellfish on the previous high tide in the middle of the night. The outline of the ray was still very obvious on the sand, including the hole beneath the head where they ray had dug deep to get at whatever it was feeding on.

I have watched eaglerays feeding underwater at Tawharanui marine reserve, and their feeding technique is fascinating. The ray pumps its head up and down and blows water out of its mouth in a vertical jet, liquefying the sand under its head then wafting the sand backwards using its large pectoral fins or “wings”.

A very clear hole left by an eagleray which would have searched for food at this site just hours before on the previous high tide in the middle of the night

A very clear hole left by an eagleray which would have searched for food at this site just hours before on the previous high tide in the middle of the night

A very different feeding mark was also seen on the sand. Broad trails about 15cm across and up to three or four metres long occurred in a few areas of damp sand, the trails lying parallel to each other. These trails are left by red billed gulls which create them by “puddling” their feet and shuffling slowly backwards. The gulls carefully watch for small sand hoppers disturbed by their puddling, and quickly snap them up when they become exposed. All the trails were parallel because while doing this the gulls face into the wind and shuffle backwards. This is what seagulls are supposed to do, rather than beg for chips or a sandwich in the local park!

These parallel trails in the damp sand are left by red billed gulls “puddling” their feet to disturb and expose sand hoppers which they then snap up for food.  The trails are parallel because the gulls face into the wind as they shuffle backwards

These parallel trails in the damp sand are left by red billed gulls “puddling” their feet to disturb and expose sand hoppers which they then snap up for food. The trails are parallel because the gulls face into the wind as they shuffle backwards

Another very common feeding mark on the sand is that left by wedge shells below the surface. The marks look very much like bird footprints and are often mistaken for them. A wedge shell lies on its side about 50mm below the sand surface and extends a feeding siphon up to the sand surface where it “vacuums” the top of the sand. It collects a mix of sand and micro-organisms, sorts them out in its gills, expels the sand and eats the micro-algae as food. Just as the tide is receding and the sand surface is still wet, the feeding action of the siphon leaves the funny marks that we see on the sand surface.

What look like bird footprints in this photo are really marks left by the feeding siphons of wedge shells lying on their side beneath the sand surface.  While the sand is still wet as the tide recedes, the siphon sucks the sand surface for micro-algae, and leaves these distinctive marks as the sand dries out

What look like bird footprints in this photo are really marks left by the feeding siphons of wedge shells lying on their side beneath the sand surface. While the sand is still wet as the tide recedes, the siphon sucks the sand surface for micro-algae, and leaves these distinctive marks as the sand dries out

After all the shellfish sites were completed, Dr Grace explained to the students how Horseshoe Island itself is slowly migrating westward under influence of wave action and storms from the east. In the past 50 years the sand bank comprising the island has moved about 60 metres, stranding now large mangrove trees east of the sand bank where they are not “comfortable” in the higher wave action and their long radiating roots are now sitting well above the sand surface. Meanwhile the sand bank itself is partly smothering other mangroves which would have started growing in the shelter to the west of the bank.

Further along the sandy island to the northwest a small pohutukawa tree has died in the last few years because its roots have been eroded out as the island migrates westward. The next pohutukawa is about to suffer the same fate as its roots are exposed on the beach and sand is clearly moving past the tree toward the interior of the island.

This small pohutukawa tree died a few years ago as its roots became exposed by the retreating sand as the sand bank comprising Horseshoe Island slowly migrates westward under influence of waves and storms from the east.  Sand-binding vegetation on the right is clearly eroding, but slowing down the rate of natural migration of the island

This small pohutukawa tree died a few years ago as its roots became exposed by the retreating sand as the sand bank comprising Horseshoe Island slowly migrates westward under influence of waves and storms from the east. Sand-binding vegetation on the right is clearly eroding, but slowing down the rate of natural migration of the island

The next pohutukawa to the west is suffering the same fate as its roots are being left stranded by the moving sand

The next pohutukawa to the west is suffering the same fate as its roots are being left stranded by the moving sand

Horseshoe Island is a very dynamic environment, and while it is eroding on the east side, it is extending westward on the other end where a small sandspit is actively accreting. Truly a fascinating place. It is even more interesting at high tide and a wonderful place to go snorkeling when the conditions are right – usually a high spring tide with little or no rain or wind for several days. Then the water is magically clear for snorkeling amongst the mangroves.

Horseshoe Island is a wonderful place to go snorkeling amongst the mangroves.  On a spring high tide after a week with little rain or wind, the water is remarkably clear, partly because huge numbers of cockles filter the water, leaving it clearer on leaving the harbour than when it came in

Horseshoe Island is a wonderful place to go snorkeling amongst the mangroves. On a spring high tide after a week with little rain or wind, the water is remarkably clear, partly because huge numbers of cockles filter the water, leaving it clearer on leaving the harbour than when it came in

Back at the carpark, Shelley, Karen, Sophie and Wendy checked all the field sheets to see they were all in order and the data were clear, before taking the students back to school in time to catch their bus home.

Shelley, Karen, Sophie and Wendy check the data sheets after the shellfish survey to make sure all the records are clear

Shelley, Karen, Sophie and Wendy check the data sheets after the shellfish survey to make sure all the records are clear

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