Mowi sites produce record harvest volume despite biology challenges

Salmon farmer Mowi Scotland’s decision to stock large post-smolts in Loch Seaforth, Harris and Lewis, has paid off with a record harvest, despite the fish being hit hard by both disease and gill health issues in the final quarter of last year.

Mowi stocked its Seaforth and Trilleachan Mor sites in the loch with post-smolts with an average weight of 427 grams in February 2025, and the last pen was harvested just 13 months later, in March this year.

The average weight for each harvested salmon came in at just under 5kg, with the total harvest reaching over 7,000 tonnes, which is a record volume for the Loch Seaforth team. The harvest was achieved despite the loss of more than a fifth of the stock, primarily due to viral disease in October and gill issues in November.

Rose to challenges

Speaking to Mowi Scotland’s newsletter, The Scoop, farm manager Stephen Maciver said: “Like any seawater cycle, we faced challenges. But with a great team consisting of 13 FTE (full-time equivalents) and brilliant resources at hand we absolutely rose to those challenges, and I am proud of everyone who played their part in achieving this record harvest.”

The Seaforth site, which was the first in Scotland to be fitted with large 160 metre-circumference pens, uses two licences, Seaforth and Noster, and has a combined maximum allowed biomass (MAB) limit of 4,330 tonnes. Trilleachan Mor has an MAB of 2,130 tonnes.

Harvest volumes on fish farms are generally higher than the biomass limit because fish are harvested throughout the farming cycle, constantly freeing up some of the allowance to be used to gow on fish to a heavier weight.

Larger post-smolts planned

The post-smolts stocked in Loch Seaforth were grown by Mowi in Loch Etive, which has lower salinity than most sea lochs and is a more difficult environment for sea lice which fare best in full seawater. Mowi says that factor, combined with synchronised fallowing of all Loch Etive sites between post-smolt cycles, mitigates lice issues for post-smolts.

A Mowi Scotland spokesperson said its strategy of deploying post-smolts is still in its infancy and it intends to stock much larger fish at the Seaforth sites in the future. Larger post-smolts are more robust and able to better deal with environmental and health challenges that may occur.