High sea temperatures up the agony of sea lice on wild salmon populations

Sea lice on a sea trout displaying lesions around the anal fin area.

August’s hot weather and warmer seas were a welcome boost for wild swimmers, but not for wild salmonids. Such conditions significantly increase the speed that parasitic sea lice reproduce on salmon farms, and data gathered by farmers and published by SEPA shows that during the first three weeks of August more farms had already gone over the Fish Health Inspectorate’s 2.0 lice per fish limit, than in all of July, say WildFish, who analyse SEPA’s data.

WildFish warns that in warmer water, sea lice complete their life-cycle faster, one study showing that an increase in water temperature from 9°C to 11°C doubled the infection pressure on Atlantic salmon farms from sea lice.

It quotes another study, which found lice were most successful at infecting salmon between 12°C and 15°C, assisted by the increased vulnerability of the Atlantic salmon, which are not adapted for higher temperatures.
The average water temperature in Scotland in July ranges from 12°C to 16°C, but August has the warmest sea temperatures of the whole year. Even today, on September 30, sea temperatures off the Isle of Islay on Scotland’s west coast were 14.7°C.

WildFish warns that as winters get warmer, there is no reprieve for salmon from these parasites and they can continue to proliferate throughout the winter, which means that, due to climate change, wild salmonids will be at a greater risk to dangerous infestation pressure from farms for even longer periods of the year in future.

Figures for the first three weeks of August showed 20.8% of fish farms were in breach of the industry’s Code of Good Practice, and 16 farms were recording above the two lice per fish limit, and that the company with the highest average lice number across all farms was MOWI Scotland, recording 0.6 lice per fish.