WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2025 61 problem that accounts for all stages of lice behavior to ensure that treatment strategies remain both effective and economically viable. Understanding Lice Detachment: Insights from Recent Research A recent study conducted at aquaculture farms in the Faroe Islands sheds light on the critical issue of lice detachment during non-medicinal delousing treatments. The research focused on how different stages of salmon lice, as well as the species C. elongatus, behave when farmed salmon are crowded before being pumped onto delousing vessels. Study Overview Multiple approaches were employed in this study. The research involved counting lice on salmon before, during, and after delousing treatments using the thermal delousing method Optilicer. To complement these observations, a comprehensive dataset from the national sea lice monitoring program, which included data on lice that detached during the sedation of fish was analyzed. In addition, water column sampling was performed to detect and quantify live, free-swimming mobile and adult lice. This multi-faceted approach provided a more detailed understanding of lice behavior during crowding and delousing. Key Findings Lice Detachment During Crowding: During the crowding process, large mobile salmon lice steadily detached from the fish, as about 21.6 percent of the lice detached each hour. This was a significant finding, showing that a large portion of lice are lost before the fish even reach the delousing vessel. C. elongatus proved to be even more prone to detachment as 77 percent had detached toward the end of the crowding process, making this species particularly difficult to manage. The study also revealed that the number of attached lice varied greatly over time depending on the stage of development. For (CONTINUED ON PAGE 62) instance, while large mobile lice showed a clear and consistent drop in numbers, adult females and small mobile salmon lice didn’t follow the same predictable pattern. In some cases, their numbers even increased. Meanwhile, C. elongatus showed sharp changes, peaking around an hour into the crowding process before suddenly dropping off. In Figure 1, we present observations from delousing of nine cages at various fish farms. The group large mobile salmon lice includes pre-adult II of both genders and adult males, while small mobile salmon lice are the pre-adult I stage. Seasonal and lice density effects on detachment rates: The large dataset, comprising over 45,000 individual cage observations from the national sea lice monitoring program, revealed how season and lice density influence detachment behavior during increased fish densities. Notably, the detachment patterns observed during sedation of the fish closely mirrored those seen during crowding. C. elongatus had the highest detachment rates, while, among salmon lice, the large mobile stages were the most prone to detach from the fish (Figure 2). For large mobile salmon lice, warmer seawater temperatures led to a clear increase in detachment rates (Figure 3). They were also more likely to detach when there were fewer lice on the fish. As lice density increased, they tended to stay attached, likely because their chances of encountering a mate on the same fish were higher. For C. elongatus, detachment rates also depended on the abundance of their own species on the fish. Detachment rates increased significantly as the number of C. elongatus rose from none to two per fish, but then slightly declined as more lice were present. This pattern suggests that, similar to salmon lice, FIGURE 1. Lice attached on fish during crowding, relative to the abundance at the onset of the crowding process. PHOTO 3. Pumping and sieving sea water samples from the crowded fish.
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