Double netting slashes seal-linked fish deaths in Shetland
Two-layer protection reduced mortalities by almost 80% in trial at salmon farm
Scottish Sea Farms will soon begin the second phase of a double netting trial in Shetland following a successful pilot to keep seals away from salmon stocks.
The salmon producer’s Setterness South farm has been upgraded with eight new 120-metre circumference pens fitted with new 75mm mesh outer netting and standard 18mm mesh for the inner netting, both of which offer enough space for the system to remain in place until the fish are harvested.
In the initial trial at Bellister and Swarta Skerry, a shallower inner net was installed to keep young fish away from the deeper outer net. But as the fish grew bigger, the inner net had to be removed to ensure compliance with RSPCA stocking densities.
Shetland regional production manager Richard Darbyshire told SSF’s staff newsletter, The Source, that the Setterness trial was the next stage in an evolving process of preventing predation by seals.
“As Setterness is quite a deep farm, we will now have the inner net at the usual depth and an even deeper outer net, enabling us to keep the inner net in for the full term,” he said.
Dramatic difference
During the initial trial, the number of salmon deaths caused by seals dropped from 38,060 fish to 7,826, said Ian Anderson, SSF’s infrastructure manager for Shetland.
“The difference was dramatic. However, we still had problems with seals and seabirds coming in over the top of pens, so at Setterness we will be installing high bird poles with a wall of netting, as opposed to the more traditional hamster wheel.”
HDPE (high density polyethylene) anti-predator nets from two suppliers, W&J Knox and Gael Force, will be tested on different pens, with a similar trial planned at Swarta in summer.
Meanwhile, nearby farms acquired when SSF bought Grieg Seafood Shetland in late 2021 have been upgraded and fitted with sinker tubes for the first time to better tension their nets.
“We’re trying to get all the farms as seal-proof as we can,” said Darbyshire. “Eventually, we want to roll out double netting to as many farms as possible within our Shetland estate.”
