Ultraviolet-C light suppresses reproduction of sea lice but has adverse effects on host salmon

Sea-cage salmon farming creates ideal conditions for ectoparasites such as the salmon louse Lepeophtheirus salmonis, with high lice densities leading to welfare challenges for stock and increasing lice burdens on wild salmonids. New treatments with low environmental impacts and minimal handling are needed to complement existing strategies. Irradiation by ultraviolet-C light (254 nm, UVC) at specific doses renders fertilised lice eggs inviable. We tested if this treatment can be applied directly to female lice with eggstrings while attached to salmon. We treated fish with attached adult lice with UVC light while they swam freely in tanks, to achieve a cumulative dose of similar to 0.1 J cm(-2) on each side of the fish within a 6-day period. To compare to fish in tanks with no UVC (control), we collected and incubated eggstrings to measure survival of resulting larvae at the infectious copepodid stage. The UVC treatment resulted in up to a 99% reduction in copepodid production relative to the control. However, UVC negatively impacted fish welfare, and was associated with early-stage cataract-like pathologies, poorer skin condition and behaviours indicative of discomfort. While UVC is highly effective at suppressing lice reproduction, the exposure regime tested here led to unacceptable animal welfare outcomes. More conservative exposure regimes may be acceptable with careful testing and calibration, but applications that do not expose host fish are preferable.