Jellyfish attacks onfarmed salmon a thing of the past

Jellyfish once posed a fatal risk to salmon farms in New Zealand, but not anymore.

An industry expert says the sheer growth of the industry in the country is the main reason attacks have stopped.

The last reported jellyfish attack on salmon in New Zealand was eight years ago, but in other parts of the world, jellyfish attacks can be devastating.

Marine farming consultant Graeme Coates, who advises salmon farmers around the world, said the changes in cage sizes was a ‘‘real gain’’ for the industry, not only for profits.

‘‘When you get some certain sea conditions, which is generally very still, little tidal current and the right temperatures for the jellyfish, they tend to explode in numbers,’’ he said.

Because of the still conditions Banjo folk artist from the UK. Highfield TerraVin Restaurant, Monday at 6pm. Bookings 5729244. Also appearing at Punga Cove on Tuesday at 7.30pm and The Captains Daughter, Havelock on Wednesday at 7.30pm. and high numbers of jellyfish, they could get caught in floating structures such as salmon cages.

‘‘They can be held close to the cages by the swimming motion of the fish, which swim around and around inside the cages, this causes a vortex,’’ Coates said.

In the 1990s, when salmon farming in New Zealand was in its infancy with smaller cages, the vortex was deadly.

‘‘There was enough vortex from the salmon to actually start the jellyfish to squeeze in on the net and the salmon get a bit spooked about it and start to swim faster.

‘‘What they tend to do then is, as they move faster the net tends to collapse and then you get the shortage of oxygen, which kills the salmon,’’ Coates said.

The worst known case of a jellyfish attack on a New Zealand salmon farm was in 1998, he said.

The attack on the Stewart Island farm killed more than 65,000 salmon.

The last reported jellyfish attack on a New Zealand salmon farm was in 2010, killing thousands of fish, again in Stewart Island.

One of the biggest international incidents of jellyfish killing salmon was in 2007, when the only salmon farm in Northern Ireland lost its entire stock of more than 100,000 fish.

‘‘At the very start we had cages that were 4 metres by 4 metres ... and now we’ve got these cages that are maybe 30m or 50m in diameter and they have massive plates holding them in place,’’ Coates said.

NZ King Salmon chief executive Grant Rosewarne said jellyfish had not posed problems for the company.

‘‘Jellyfish have not been a significant issue for our company but are something we monitor.’’

‘‘Higher flow sites outside of bays reduce the effects of any jellyfish around salmon farms,’’ he said.