Among the criticisms of salmon farms are the facts that non-native fish can escape into the local marine environment, plus the farmed fish may become infected with parasites that spread to wild populations. A new type of salmon pen, however, is designed to address those problems.

Typically, salmon-farming pens have sides and bottoms composed simply of netting. While this does allow water from the surrounding ocean to freely circulate through, some fish inevitably get out through holes that are accidentally ripped in the net – this means that Atlantic salmon, for instance, could end up living wild in a Pacific ecosystem.

Additionally, local marine parasites such as sea lice can get in, then reproduce rapidly thanks to the population density inside the pens. This results in the whole geographical region having an increased sea louse population, with the parasites going after farmed and wild salmon alike.

That's where the Aquatraz pen may make a difference. The product of a collaboration between Norwegian companies Midt-Norsk Havbruk and Seafarming Systems, it's named in homage to San Francisco's "inescapable" Alcatraz prison.

The experimental circular pen is 18 meters deep (59 ft), has a circumference of 160 m (525 ft), and the upper eight meters (26 ft) of its wall are enclosed by a solid steel "skirt." This should reportedly keep sea lice from entering, as the organisms generally stay near the surface. The steel is also much more impact-resistant than unprotected netting, decreasing the chances of holes forming due to collisions with docks, watercraft or other floating obstacles.

In order to maintain water circulation within the pen, integrated pumps draw seawater up from below, creating a current within the enclosure. Water drawn from such depths tends to be more oxygen-rich than surface water, and of a more consistent temperature. What's more, the artificially-created current should be stronger than that of the surrounding ocean, ensuring that the fish get plenty of exercise by continually swimming against it.

The first of the Aquatraz pens was installed in Norway's Eiterfjorden fjord in October 2018. Over the next year, scientists from the Norwegian Institute for Water Research will be assessing aquatic conditions and the health of salmon caged within it. A series of other Aquatraz test pens are planned to follow, with the ultimate goal of commercialization.

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