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Cockle mystery investigated

Posted at 8:00am Wednesday 17 Feb, 2021

Rebecca Gladstone-Gallagher will check on the cockles over three years.
Rebecca Gladstone-Gallagher will check on the cockles over three years.

A three-year study into why the cockles in Whangateau Harbour aren't growing to adulthood starts this month.

The cockles were decimated in 2009 when extreme tides exposed the seabed to the midday sun. More than half of cockles at monitoring sites perished, including 80 per cent of large (over 30mm) cockles.

The population has failed to recover despite a Ministry of Primary Industries ban on taking cockles, which has been in place for 11 years.

Data collected by marine biologist Dr Karen Tricklebank, and her team of student citizen scientists, has consistently shown that, while numbers of juvenile cockles have returned, they are not maturing.

Researcher Rebecca Gladstone-Gallagher, from the Auckland University Institute of Marine Science, says the population ought to have recovered within five or six years.

She is testing two hypotheses in an effort figure out why it has not.

One of her theories is that the sudden loss of large adult cockles has affected the environment in which juvenile cockles grow.

She says adult cockles release nutrients which stimulate microbial growth in the seabed, which the juveniles feed on.

Adult cockles also stabilise the seabed, stopping it from eroding.

Therefore, without a significant adult population already existing, it is possible that juveniles are unable to mature.

Ms Gladstone-Gallagher plans to test her theory by collecting adult cockles and placing them in a single marked patch, mimicking a high-density population.

She will mark juvenile cockles using fluorescein, a dye compound which can only be seen using a UV light. When she checks on them after 12 months, it will allow her to see how much the juvenile cockles have grown.

Ms Gladstone-Gallagher's other hypothesis is that the environment has degraded due to silt, clay and nutrients.

She says it is possible that adult cockles were ‘hanging on' despite poor environmental conditions, but after the die-off event, the population was unable to recover.

For the study, she will test this by monitoring multiple sites in the area to see if different environmental factors have an impact on growth.

However, Ms Gladstone-Gallagher says she does not suspect this will be the case as the harbour is known to efficiently flush out sediment.

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