Evaluation of chemical-analytical methods to trace the origin of salmon lice (Lepeophtheirus salmonis) larvae according to the wild or farmed origin of maternal host fish

Abstract Salmon lice ( Lepeophtheirus salmonis ) represent a threat to the wild Atlantic salmon ( Salmo salar ) populations, and open sea-cage salmon farming contributes to increased abundance and infestation. While models have been developed to predict salmon lice dispersal from host fish, methods able to validate the origin of salmon lice larvae —important for increased understanding of the spread, are lacking. As louse larvae on a fish, or in water samples, may originate from different maternal sources, it is essential to analyze individual larvae. This explorative study assessed several chemical-analytical methods for their potential to trace the origin of individual salmon louse copepodites, specifically to trace whether the maternal louse had been attached to a wild or farmed salmon. Lipid- and combined isotopic (δ15N and δ13C) profiles of salmon lice larvae were characteristic of the host fish’s origin, but reliable analysis of individual larvae was challenging due to the very limited sample amount. Several trace elements such as As and Rb, as well as major elements such as P, K, and Mg, were detectable in individual larvae. Elemental ratios of As/K, Rb/As, and As/P differed whether the maternal louse was attached to a wild or farmed salmon, consistent with patterns observed in adult lice and host fish. Due to their high sensitivity and the potential diet-related differences, elemental profiles represent a promising method for determining the wild or farmed origin of individual lice larvae. However, broader sampling and validation are required to verify whether these fingerprints are sufficiently characteristic and robust.