Smash and grab predators threaten Norwegian farms

Large bluefin tuna breaking into salmon cages have been causing serious disruption at a number of Norwegian fish farms over the past few weeks.

While the problem is not on the same scale or of the same severity as the jellyfish attacks, the tuna – also known in Norway as “mackerel sturgeon” – have nevertheless created quite a headache and a lot of extra work for the affected farms.

Four incidents were recorded during October and at least three more in the preceding few months.

Mowi was one company affected while the farming company Sulefisk spent at least four days trying to free a 348 kilo (770lbs) tuna. They finally succeeded thanks to help from divers and underwater vehicles.

Some of these fish are particularly large and powerful, often the size of a small shark. They can cause a lot of damaged as they try to break into the cages.

The attacks appear to come mainly from the species known as eastern mackerel sturgeon which spawn in the Mediterranean.

The veterinary authorities say there has been an increase in the number of tuna moving into Norwegian waters during the late summer and early autumn months, where they graze intensively in the coastal waters in which the farms are located.

The authorities say that overall, more and more tuna are being observed over large areas from the North Sea right up to the southern tip of Svalbard.

The Norwegian Directorate of Fisheries has issued a guidance list to salmon companies on how to handle the tuna if they do get into a cage, including the advice that entry holes must be repaired as soon as possible once the tuna is out.