60 MARCH 2025 • WORLD AQUACULTURE • WWW.WAS.ORG confined space before treatment, which can induce stress and inadvertently cause lice to detach from their hosts even before the fish are pumped onto the delousing vessel. This premature detachment means that a significant number of lice may enter the surrounding water, posing a risk of reinfection for both the treated fish and neighboring farms. Studies have shown that detachment rates can vary significantly, with some reports indicating that up to 50 percent of lice detach under certain conditions. Environmental factors, such as water temperature, current speed, and fish density, all play a role in how lice behave during these treatments. For example, observations from farms in the Faroe Islands reveal that C. elongatus is particularly prone to detachment, which presents unique challenges in regions where this species is prevalent, however, few studies have thoroughly quantified lice detachment during crowding, leaving a knowledge gap. The cost of delousing operations is already high, including both direct expenses and indirect impacts, such as reduced fish welfare and appetite, and increased mortality. If detached lice reattach to the fish or spread to nearby farms, the effectiveness of these expensive treatments could be compromised, potentially requiring more frequent and costly interventions. This highlights a need for a better understanding of this potential In salmon aquaculture, the fight against sea lice is an ongoing challenge. In the North Atlantic, the main species causing problems are Lepeophtheirus salmonis, better known as salmon lice, which specifically target salmonids, and Caligus elongatus, a more adaptable species that infests over 80 types of fish. Managing salmon lice is particularly crucial because they must be kept at low levels to protect wild salmonid populations, which are already struggling because of human activities, such as river development and pollution. As global demand for salmon continues to grow, with millions of tons produced each year, tackling the sea lice problem has become a top priority for aquaculture operations worldwide. A Global Shift in Treatment Strategies Over the past decades, salmon farming operations have moved from chemical-based delousing to non-medicinal methods, driven by concerns over chemical resistance and environmental impact. Techniques like thermal, mechanical, and freshwater treatments are now widely used, but these non-chemical approaches have introduced new challenges, especially regarding the crowding process required before delousing. The Complexities of Crowding Crowding involves gathering the fish into a The Hidden Challenge: Understanding Lice Detachment in Salmon Farms Gunnvør á Norði, Birgitta Andreasen, Kirstin Eliasen, Tróndur Kragesteen and Signar P. Dam PHOTO 1. Onboard the delousing vessel. Photograph by Sandra Ljósá Østerø. PHOTO 2. Crowding fish for lice treatment. Over the past decades, salmon farming operations have moved from chemical-based delousing to non-medicinal methods, driven by concerns over chemical resistance and environmental impact. Techniques like thermal, mechanical, and freshwater treatments are now widely used, but these non-chemical approaches have introduced new challenges, especially regarding the crowding process required before delousing.
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