Snorkel technology to reduce sea lice infestations: efficacy depends on salinity at the farm site, but snorkels have minimal effects on salmon production and welfare

Abstract

Sea lice are a critical health issue in most salmonid farming regions. New cage-based technologies can prevent infestations from occurring, such as the 'snorkel', which introduces an impermeable barrier that separates salmon lice Lepeophtheirus salmonis from Atlantic salmon Salmo salar in the surface waters where lice are most abundant. While snorkels provide protection from lice, their lice-reducing effect can vary under different environmental conditions. We conducted production-scale sea-cage experiments at 2 sites with contrasting salinity environments in Norway. At the coastal site, with a weak and unsystematic halocline, snorkels reduced lice infestations by 76%. However, at the fjord site, with brackish surface waters and a strong halocline, snorkels did not reduce lice relative to control cages, likely because both lice and salmon remained deeper in the water column below the brackish layer, and infection rate was similar. At the fjord site, as lice numbers between snorkel and control cages were similar, we tested for differences in the absence of the potentially confounding effect of different lice levels. Snorkel cages at the fjord site modified swimming speeds (1.14 times faster), surface breaching behaviours (2.8 times less), and total echo-sounder signal strength of fish (an index of swim bladder fullness; 30−40% less) relative to control cages. Production parameters remained similar, but snout condition was poorer in snorkel cages, suggesting more frequent contact with cage netting. Our results suggest that salinity is a significant environmental factor that alters the lice-reducing efficacy of depth-based technologies such as snorkels. Further, snorkels affect salmon behaviour, which must be considered in welfare assessments of their use.

Prevention not cure: a review of methods to avoid sea lice infestations in salmon aquaculture

“…Brackish surface water can also reduce the efficacy of skirts and snorkels by causing both lice and salmon to reside below the level of the barrier (Oppedal et al . 2019), while there is evidence that barrier technology may reduce the performance of cleaner fish when used in combination (Gentry et al . 2020).…”

Parasite management in aquaculture exerts selection on salmon louse behaviour

“…This was most pronounced in spring–summer at inner‐fjord sites, where a relatively thick, stable brackish layer accumulated (Figure 1 ). Experimental trials have shown that seasonal and spatial variation in salinity has a strong effect on the efficacy of depth‐based preventions (Geitung et al, 2019 ; Oppedal et al, 2019 ).…”

Biofouling on Salmon Pen Nets and Cleaner Fish Shelters Does Not Harbor Planktonic Stages of Sea Lice

“…A possible retaining effect by lice skirts or biofouling might be linked to the same factors that that make a lice skirt effective in shielding the salmon from copepodites. The effectiveness of the lice skirts can be reduced by environmental conditions like the presence of a cline deeper in the water than the lower edge of the lice skirt, which have been shown for the presence of a halocline (Samsing et al, 2016;Oppedal et al, 2019). We investigated the water profile for haloclines, thermoclines, and pycnoclines (only the latter is shown in this paper, because the density of seawater is a function of salinity and temperature, Helber et al, 2012); however, we found no clines for the Liceskirt7 site.…”