Microscopic jellyfish challenging, but not curtailing, Scottish salmon production

Salmon swimming inside an aquaculture net pen.

Despite government reports confirming high farmed salmon harvests of over 169,000 metric tons (MT) last year and a projected total for 2023 just shy of 188,000 MT, Scottish salmon production still has its challenges, and the industry is continuing to battle obstacles like small harvesting size, sea lice, and microscopic jellyfish blooms that have posed significant problems for farmers.

Sea lice is a well-known parasitic scourge of the industry, costing producers in Scotland around GBP 40 million (USD 48.8 million, EUR 46.3 million) annually.

More recently, though, microscopic jellyfish have become a competitively serious issue, and this year, higher-than-average seawater temperatures have exacerbated the threat of such blooms.

Fish breathe in the jellyfish, which sting their gills and reduce their capacity to breathe properly. These blooms were a major contributing factor for 2.8 million salmon deaths that occurred in September 2022 – the worst month for fish mortalities since farmers began publishing mortality statistics in January 2018. Those mortalities amounted to 4.6 percent of salmon farmed in Scotland that month.

Historically, September survival rates typically drop to their lowest annual level because this is when seawater temperatures peak, and this year has seen record temperatures, partly due to the El Niño weather-warming phenomenon. These high temperatures contribute to an increase in naturally occurring organisms, which can reduce oxygen levels and also compromise the health of the salmon in areas such as the west coast of Scotland.

Luckily, the latest figures on Scottish salmon survival rates show

Photo courtesy of Salmon Scotland

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