Shellfish under threat from over-collecting

DECLINE: Stephanie Hampson has researched the health of shellfish in the estuary and it's not looking good. Picture Martin Hunter

Cockle numbers in the Avon-Heathcote Estuary are in serious decline.

A six-month study shows the population has fallen dramatically since a 2012 survey, from around 400 per sq m to between 50 and 80 per sq m in some areas.

Biological sciences student Stephanie Hampson has been researching the extent and impact of harvesting on the health and sustainability of estuary shellfish beds – an important feeding grounds for birds.

The results suggest the current limit of 150 cockles per person per day is too high and needs to be reduced or even a ban put in place, Ms Hampson said.

Auckland’s limit is 50 per person per day, which she believes would be more appropriate if harvesting is to continue.

Ms Hampson measured the number and size of cockles in areas near the Mt Pleasant causeway, the Beachville Rd jetty and the Mt Pleasant Yacht Club.

“I was really surprised,” she said, “I wasn’t expecting it to decline that much, I thought we might find a little bit less but not like this . . . a lot of them are really small because there’s not enough good nutrients. We had cockles eight or nine years old that were only 20-30mm which is smaller than they should be for their age.”

Cockles can live for 20 years but eight to nine years is typical as there is high juvenile mortality rate.

Beds alongside the causeway and yacht club were heavily depleted.

“The areas near the causeway are hard-hit by collecting so there’s not enough time for them to establish and the area closest to the Mt Pleasant Yacht Club was also low which might be due to disturbance and unhealthiness from cars driving near to the beach. We struggled to find more than 40 across that entire site – about 10m right beside the water line.”

A survey of 100 estuary users found some were collecting in groups as large as nine; collectively they could take more than 1000 cockles in one day and still be within the limit.

Most people didn’t know what the limit was.

“Some thought it was similar to Auckland, some thought it was a bucketful, some thought it was between 150 and 400.”

Signs put up with catch limits “kept disappearing.”

Ms Hampson attributes the decline to a combination of poor nutrients and pressure from collecting.

“With the earthquakes, the silt was blocking the cockles’ gills and I think that affected their health; if they’re not as healthy they won’t be producing as many baby cockles. And then because people are taking so many, there’s not enough replacement in the system; they don’t have the chance to recover and grow.”

“I think for people to still be able to collect there needs to be a lower limit, especially in the hardest hit areas. At this rate, in the channel they’re going to die. Eventually there will be so few the only ones that are big enough to reproduce will be taken.”

The research was funded by Canterbury University and the Avon-Heathcote Estuary Ihutai Trust.

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