“Houston, we have a problem.” A case of cockle déjà vu.

It’s happening again! Our wonderful cockles in the Whangateau Harbour are dying in what appears to be an event similar to that in 2009 when 80% of our cockles died.

A dead and gaping cockle lies moribund and stinking on the sand surface over the cockle beds off Lew’s Bay in the Whangateau Harbour.

A dead and gaping cockle lies moribund and stinking on the sand surface over the cockle beds off Lew’s Bay in the Whangateau Harbour.

In the summer of 2009 Whangateau Harbour residents noticed an awful smell coming off the harbour flats. It soon became obvious that it was caused by many thousands (in fact millions) of dead cockles rotting in the sun.

The cause was eventually attributed to two pathogens – a coccidian parasite and a mycobacterium – which were able to attack the cockles because they became heat-stressed on a series of summer days with low spring tides near the middle of the day, no wind, no clouds, and hot searing sun.

That unprecedented event led to closure of the beds for three years to allow the cockles to recover. At the end of that period, although there was noticeable recovery of the smaller cockle sizes through growth of recently-settled young, there was still no increase in the larger sizes which people like to harvest.

The larger sizes also happen to be the best breeding stock. There was a risk that if the beds were opened again to harvesting, the few remaining large cockles would be stripped quickly and we may end up with no breeding stock to replenish the beds. So the ban was rolled over for a second three-year term. We are currently one year into that second term, and had anticipated that by the end of this term there would be plenty of cockles in the over 30mm size range to allow harvesting to resume.

But now we have a new problem. While Dr Karen Tricklebank and I were helping students from Mahurangi College monitor shellfish east of Horseshoe Island on Friday 28th March, Karen noticed that on the outer shell banks there were numbers of fresh “cluckers” or open cockle shells lying around, in larger numbers than usual. She also found a few large cockles lying on the surface, as well as a few which were gaping open and in a moribund state.

Numerous “cluckers”, or freshly open cockle shells, were present on some of the outer banks, reminiscent of the dead shells which were abundant after the serious cockle die-off of 2009.

Numerous “cluckers”, or freshly open cockle shells, were present on some of the outer banks, reminiscent of the dead shells which were abundant after the serious cockle die-off of 2009.

Karen holds two large but very sick cockles found lying on the sand surface.  She also found a few gaping open in a moribund state and starting to stink.

Karen holds two large but very sick cockles found lying on the sand surface. She also found a few gaping open in a moribund state and starting to stink.

Remembering the serious cockle die-off back in the summer of 2009, and fearing the worst, Karen contacted MPI (Ministry of Primary Industries) in Wellington to warn them of the event. She also asked for a special permit so she could collect samples to send to Wellington for analysis to check if it is a return of the same problem which caused the previous die-off and closure of the beds. Karen collected the relevant samples and they are now being tested in Wellington by fisheries scientists.

Over the last few days of hot sunny calm weather and low tides during the heat of the day, the situation appears to have got worse. Increasing numbers of dying cockles are present on the sand flats out from Whangateau Domain and Lew’s Bay.

Today I visited the cockle beds with Unitec student Dyllan Cochrane, who is completing a B.Sc. degree and needs a research project. Dyllan was brought up in the Whangateau and has fond childhood memories of collecting shellfish and fishing in the harbour. He is considering studying the present cockle die-off and comparing it to the 2009 event for his student research.

A recently dead and slightly gaping large cockle lies on the sand off Lew’s Bay.

A recently dead and slightly gaping large cockle lies on the sand off Lew’s Bay.

Unitec student Dyllan Cochrane examines a gaping dead cockle. Dyllan is likely to study this cockle die-off event as part of his undergraduate research.

Unitec student Dyllan Cochrane examines a gaping dead cockle.
Dyllan is likely to study this cockle die-off event as part of his undergraduate research.

A close view of the dead and stinking cockle Dyllan is holding.

A close view of the dead and stinking cockle Dyllan is holding.

Dyllan prizes open a dying cockle with his fingers and thumbs.  There is no way he could  open a healthy cockle like this.  The shell would be held firmly shut by the adductor muscles.

Dyllan prizes open a dying cockle with his fingers and thumbs. There is no way he could
open a healthy cockle like this. The shell would be held firmly shut by the adductor muscles.

The low tides are now very late in the day and cloudy or rainy weather is forecast, so it is hoped the event may be coming to an end. There may, however, still be a lot more mortally damaged cockles below the surface which could show as surface gapers over the next week or so.

So far there has been only one report from the public of bad smells coming from the harbour. If many more cockles die, however, they may overwhelm the scavengers like whelks, crabs and small fish which normally gobble up dead shellfish and keep the harbour clean. That is what happened in 2009, leading to abundant rotting shellfish and a horrible stink!

Whelks home in on a dying cockle and finish it off, like good scavengers keeping the harbour clear of dead fish and shellfish.  But if too many cockles die, the natural scavengers are overwhelmed by the abundance of food and the cockles simply rot and create a horrible smell.

Whelks home in on a dying cockle and finish it off, like good scavengers keeping the harbour clear of dead fish and shellfish. But if too many cockles die, the natural scavengers are overwhelmed by the abundance of food and the cockles simply rot and create a horrible smell.

We have good data on cockle abundance and sizes from the ongoing Community Shellfish Monitoring Programme supported by Auckland Council and the Hauraki Gulf Forum, at Whangateau carried out by the Whangateau HarbourCare Group in conjunction with Mahurangi College. So there is a good basis for comparison with recent data if the monitoring is carried out again soon after this event is over. We should be able to determine just how many cockles have died and in what size ranges.

With samples collected early during this event for pathogen analysis, there is every chance that MPI scientists can conclusively figure out what is causing this very unfortunate loss of large numbers of the best cockles in the Auckland Region.

Comments

  1. Darryl Torckler says:

    Bugger, has the low tide being in the middle of the day with really hot sun and no wind?

    • Roger Grace says:

      That seems to be the trigger for an event like this. The cockles get heat-stressed then pathogens can attack them when the cockles are in a weakened state with little resistance.

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