62 MARCH 2025 • WORLD AQUACULTURE • WWW.WAS.ORG C. elongatus may detach more frequently when mates are scarce but reduce movement once a sufficient density is reached, likely related to the increased chance of finding a mate without needing to leave the host. The influence of season on detachment rates for C. elongatus was the opposite of what was observed in salmon lice. Detachment rates significantly decreased as temperature and day length increased, with the highest detachment rates occurring during the winter. Live Free-swimming lice found in the water: The study revealed that live, free-swimming mobile and adult lice were present in the seawater surrounding the fish during delousing operations. This confirms that lice detach during the crowding process and remain viable, with the potential to infect other hosts. Interestingly the gender balance of free-swimming lice was highly male-biased with more than 90 percent of the lice of both species being males. The number of lice found in the water varied depending on environmental conditions and the intensity of crowding. In calmer conditions with lower current speeds, more lice were detected. However, stronger currents appeared to spread the lice more widely, making them harder to detect. To better understand these differences, the flux of detached lice was calculated — essentially, determining how many lice were carried away by the water currents. Notably, intense crowding had a particularly strong effect on C. elongatus detachment compared to salmon lice. In contrast, for salmon lice, it might seem logical to expect rough weather to increase detachment rates. However, the study found that detachment rates for salmon lice were quite similar in both calm and harsh conditions, challenging some common assumptions about their behavior during delousing. The Practical Impact of Understanding Lice Behavior The findings of this study offer new insights into lice detachment behaviour, with important implications for future management strategies. C. elongatus stands out for its high mobility, a characteristic well-recognized by the industry. This mobility underscores the importance of using preventive measures, such as in-feed treatments, which are favored by the industry because they can keep lice off the fish for an extended period, reducing the need for frequent interventions or to use specialized nets to catch the lice that fall off during crowding when using non-medical treatments. For salmon lice, the observed male-skewed gender balance among free-swimming lice provides another critical insight. Since it is the presence of adult female lice that typically triggers delousing treatments, the detachment of primarily male salmon lice might have a limited impact on the frequency of treatments. This suggests that the issue of salmon lice detachment during crowding might not be as significant as previously thought. Understanding the behaviors and gender dynamics of different lice species can inform more targeted interventions, minimizing both the environmental impact and operational costs associated with frequent treatments. Moving forward, continued research will help refine management practices, contributing to more sustainable and efficient aquaculture operations. Acknowledgments This article is based on research conducted in the project LiceDetached. Funded by FHF – Norwegian Seafood Research Fund. Project number 901782 Notes Gunnvør á Norði*, Birgitta Andreasen, Kirstin Eliasen and Tróndur Kragesteen are researchers at Firum, Faroe Islands. Signar P. Dam is lice management coordinator at Bakkafrost. * Corresponding author: gunnvor@firum.fo FIGURE 2. Monthly average detachment rates of mobile and adult stages of the sea lice species C. elongatus and salmon lice during sedation of fish in connection to the national sea lice monitoring program. FIGURE 3. Model results predicting the impact of day length, temperature and abundance of salmon lice and C. elongatus on the detachment rates of adult C. elongatus and the various development stages of salmon lice. The findings of this study offer new insights into lice detachment behaviour, with important implications for future management strategies. C. elongatus stands out for its high mobility, a characteristic well-recognized by the industry. Understanding the behaviors and gender dynamics of different lice species can inform more targeted interventions, minimizing both the environmental impact and operational costs associated with frequent treatments. Moving forward, continued research will help refine management practices, contributing to more sustainable and efficient aquaculture operations.
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