The ste at Ørnfjordbotn, Senja.

SalMar culls 1.2 million fish after jellyfish invasion

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Norwegian salmon farmer SalMar had to cull 1.2 million fish after an invasion of string jellyfish (Apolemia uvaria), it has revealed. The fish had an average wight of 300 grams.

“In mid-November, our fish farmers at the location Ørnfjordbotn in Senja observed a significant and threatening influx of string jellyfish. Some dead fish were also recorded. A few days later, the extent of jellyfish increased further and the extent of damage to the fish worsened,” stated SalMar, the world’s second largest Atlantic salmon farmer.

SalMar reported that for fish welfare reasons it chose to cull all 1.2m fish on the site. Until the jellyfish invasion, the mortality rate at the location was approximately 1%.

Round-the-clock cull

“Our employees worked together with people from an external emergency boat intensively on a 24-hour basis to deal with the acute and deplorable situation that arose due to this unusual and violent invasion of string jellyfish. After 3-4 days the operation was finished,” stated SalMar.

SalMar points out that string jellyfish have also been observed in other localities in central and northern Norway, but to a limited extent. It said occurrences of so many string jellyfish are very unusual.

“Jellyfish are one of several risk factors in fish farming, but it is very rare that they cause damage to the fish as in this case. Approximately 20 years ago, a SalMar location on Frøya was exposed to a similarly powerful jellyfish attack. SalMar’s contingency plans take account of such incidents, so that these are handled in a reassuring manner and with fish welfare as an important governing principle if they occur,” the company said.

SalMar said that the incident will not affect the company financially beyond the value of the culled fish. Estimated harvest volume should also not be affected by the incident.

The 'string' with a deadly sting

The string jellyfish (Apolemia uvaria) is a highly venomous pelagic siphonophore. The species is easily recognisable by the fact that it forms long threads - hence the name. They form colonies that can vary in length from a few centimetres up to several metres.

The string jellyfish are predators with a sting which they use to paralyze and kill prey or enemies. The venom can damage the gills, skin and eyes of salmon, and has caused fish deaths.