Drivers of salmon louse ingestion in lumpfish (Cyclopterus lumpus): Effects of prey availability, lumpfish size, body condition and deployment strategy

Lumpfish ( Cyclopterus lumpus ) are used as a non-pharmacological method to control salmon lice ( Lepeophtheirus salmonis ) in Atlantic salmon ( Salmo salar ) aquaculture. However, their effect as cleaner fish varies substantially. This study examines how alternative prey availability, individual characteristics and deployment strategy influence salmon lice ingestion in lumpfish, aiming at improving the predictability and outcome of this treatment approach. Stomach-content data from 28,677 lumpfish sampled across multiple sites and years in the Faroe Islands (2015–2025) were analysed. The dataset included fish size and body condition, presence of alternative prey in stomachs, and cage-level salmon lice abundance on cohabiting salmon. Most lumpfish stomachs (>96%) contained no salmon lice. Due to the high proportion of zeros and large variation in salmon lice counts (0–290), a probability-based modelling approach was used to describe both the likelihood and the intensity of salmon lice ingestion. The probability of salmon lice ingestion increased with salmon louse abundance on cohabiting salmon and was strongly reduced when jellyfish were present in stomachs. Ingestion intensity showed non-linear relationships with salmon louse abundance, lumpfish size and body condition, peaking at small sizes and optimal condition. These results indicate that salmon lice ingestion is opportunistic and strongly influenced by prey availability and deployment conditions. Avoiding deployment during periods dominated by jellyfish blooms, accounting for salmon lice levels at deployment and maintaining good body condition through appropriate feeding and husbandry may improve the effectiveness and targeted use of lumpfish as a biological control agent.