Acidic oceans threaten shellfish industry

New Zealand's multimillion-dollar seafood export industry is under threat from increasingly acidic oceans, a Niwa study has found.

Ocean acidification, the seaborne sibling of climate change, is caused by the ocean absorbing carbon dioxide (CO 2) from the atmosphere, reducing pH levels - and affecting the way shellfish such as mussels and paua grow.

Paua is a multimillion-dollar export industry for New Zealand and greenshell mussels are New Zealand's biggest aquaculture business and the largest single species seafood export, accounting for more than $200 million a year.

Based on a model for the Wellington coastal region that predicts increased CO 2 in the water over the next 40 to 90 years, the study saw a decline in paua fertilisation and larval development.

A four-month experiment found the shell and flesh growth of juvenile paua was also affected by changes to temperature and CO 2 levels. More dissolved CO 2 in the water results in the presence of less carbonate required to construct the paua shells. More energy is therefore expended building shells than growing flesh resulting in a smaller animal.

The study found 14-month-old paua lost flesh weight as pH and temperature scenarios increased.

Ocean acidification is expected to reduce ocean pH levels by 0.2 to 0.5 units, from present levels of 8.0, by 2100.

During the experiment, juvenile growth fell off at pH 7.85 and temperatures above 17°C.

Initial increases in temperature had short-term positive effects for growth, similar to vegetables in a greenhouse, but these were only temporary.

Niwa ecologist Vonda Cummings said: "By 2100 the negative effects on the different stages of the population outweigh the benefits and we are expecting declines in population," Cummings said.

Preliminary, evolving research on greenshell mussels by University of Otago, also showed adverse effects on reproduction and growth from ocean acidification.

Mussel development from egg through to shell growth was distorted and slowed under pH scenarios predicted for 100 years' and 300 years' time.

"You don't want an organism having to spend more energy dealing with a changing environment, when that energy could be used to grow meat which is what they are trying to sell," said Dr Miles Lamare, of the University of Otago's Marine Science lab.

Ocean acidification is a gradual process and the animals used in research have not been given an opportunity to adapt.

Next week an ocean acidification workshop hosted by the University of Otago will bring together a network of researchers to plan for the effects on New Zealand's marine environment.

Sunday Star Times