Characteristic current flow through a stocked conical sea-cage with permeable lice shielding skirt

The characteristic current flow field around a 55 m deep full-scale stocked conical Atlantic salmon sea-cage equipped with a 10 m permeable skirt was studied experimentally using acoustic Doppler velocimeters and profilers. The weakest current speed was inside the cage at 6 m depth and the highest reduction downstream was recorded behind the shielded volume. Downstream of the cage the reduction in speed became little to non-existing at 22 m depth, probably due to the decreasing diameter of the cage with depth. To reduction in current speed through the cage was compared with estimated reduction from theoretical expressions. The results compared reasonably well downstream of the shielded cage, while the reduction inside the cage was higher than the estimates. The difference in current flow field behind a conical cage compared with a cylindrical cage may have implications for the dispersal of waste, feed pellets and microorganisms from the cage influencing the benthic impact of the farm

Lice shielding skirts through the decade: Efficiency, environmental interactions, and rearing challenges

Lice shielding skirts are a preventative measure against salmon lice in Atlantic salmon farming. The skirt is wrapped around the top meters of the net cage to divert the current flow around the cage, and thereby keep the salmon lice out. Despite these skirts being used actively in Norwegian aquaculture for the past decade, there is no standardised way of using them, and therefore type, depth and operating procedures vary between sites. The academic literature on the lice shielding efficiency of these skirts is not extensive and reported efficiency varies across studies and sites with some reporting favourable results, while others find none. Some also report of welfare related issues, with dissolved oxygen levels being the most prevalent, but this too varies across sites and through the production cycle. The aim of this paper is to provide a comprehensive overview and summary of relevant academic and grey literature from the last decade to identify knowledge gaps that must be filled to achieve optimal use of lice shielding skirts. This paper focuses on three main topics: lice shielding efficiency, interaction with the current flow and rearing challenges. The positive results from some sites indicate that skirts have potential as a tool against salmon lice, however, to create a best practice recommendation for skirt use, more knowledge is necessary on the interaction between skirt and the environment, and sufficient monitoring procedures and decision-making tools must be established.publishedVersio