Toxic sea slugs an ever present danger

Photo / Janna Dixon

The toxic sea slug that has kept the Hauraki Gulf off limits for a month has always been there - and may always have been deadly.

The possibility is among a range of options being considered by scientists and environmental experts who are studying the rash of dog poisonings on Auckland's east coast.

Up to 18 dogs have fallen ill with symptoms linked to a potentially deadly neurotoxin, tetrodotoxin. Four of those dogs died. Concerns grew after reports of a sick child, dead dolphins, penguins and pilchards.

It emerged the toxin was carried by a sea slug - pleurobranchaea maculata - which was mainly found on Cheltenham and Narrow Neck beaches in Devonport.

On Friday, health officials eased restrictions which had seen children and dogs warned off visiting some of the city's most beautiful beaches. Now they warn that children and pets should be supervised at all times - and not allowed to eat anything found on the beach.

Scientists at the Cawthron Institute, the independent science powerhouse in Nelson, have been studying the slugs since the problem first emerged. Paul McNabb, technical manager, said the toxin was common in many organisms. "Some organisms have big teeth, some run very fast and some are very toxic."

It was possible the slugs had always been toxic, but none had been tested for the toxin before - and "no one has eaten them".

Now that it was known toxic slugs existed, people would likely focus on them as a possible danger. "It's a little bit like getting used to managing the risk of stingrays."

McNabb said the range of explanations ran from the possibility toxic slugs had always been there but gone unnoticed, to this being the first occurrence. A middle option was most likely.

"Everyone wants to know how long it's going to last - and we just don't know."

Auckland Regional Council monitoring and research manager Grant Barnes said there was no evidence to suggest any acute changes in the Hauraki Gulf.

"We know not a heck of a lot about our environment."

Barnes said other marine deaths were part of the "natural cycle of life" and not unusual or to be concerned about. "I don't want to be glib but they are called common dolphins for a reason."

Theories about the marine deaths and dog poisonings being linked to poison drops on Hauraki Gulf islands to kill pests had been ruled out by all involved.

Department of Conservation pest control expert Keith Broome said there was a common misconception that 1080 was the poison used. It was not - instead, a poison called brodifacoum had been dropped.

The poison was similar to those sold in hardware stores for domestic use killing rats, but at a lower dose, he said. It worked by stopping blood in mammals from clotting. Blood clots keep mammals healthy by stopping bleeding inside the body caused by normal wear and tear. Without the clot, animals usually died quickly - and usually from a brain haemorrhage.

As fish weren't mammals, it was unlikely to have the same effect. A study by NIWA had seen fish fed pellets of the poison before they finally succumbed to a large dose. The poison was also not water soluble, instead depositing in mammals' fat.

And while the country was bereft of a sea slug expert, one high school teacher in the Bay of Plenty who does study the creatures was watching fascinated at the latest toxic trick performed by these "funny little things".

Paul Furneaux, a biology teacher at Otumoetai College, spent 2003 studying sea slugs through a science and technology fellowship from the Royal Society.

Furneaux said environmental conditions - even a different prevailing wind - could have a huge effect on populations of slugs by redistribution of nutrients.

On the pleurobranchaea maculata, he was inclined to believe that it had always been present - and it had always been toxic.