Effects of dietary L-tryptophan enrichment on stress response, growth and survival in lumpfish (Cyclopterus lumpus) following sea transfer

L-tryptophan (TRP), the precursor to serotonin (5-HT), modulates stress responsiveness and social behaviour in vertebrates. In cleaner fish such as lumpfish ( Cyclopterus lumpus ), these traits are integral to sea lice control in Atlantic salmon ( Salmo salar ) farming. We tested whether a TRP-enriched diet attenuates the stress response during commercial sea transfer and affects early cohabitation performance. Lumpfish were fed a control (5.2 g kg −1 ) or TRP-enriched diet (24 g kg −1 , 4.6× control TRP) for 7 d, fasted for 2 d before transport, and then fed for 14 d after deployment in commercial net pens. Post-transfer stress was assessed using plasma cortisol concentrations. Telencephalic 5-HT neurochemistry was quantified as [5-HT], its metabolite 5-hydroxyindoleacetic acid [5-HIAA], and 5-HIAA/5-HT. Performance was evaluated based on growth, stomach content, and survival. TRP feeding increased 5-HIAA/5-HT after the pre-transfer phase, indicating increased serotonergic turnover, and reduced post-transport plasma cortisol, consistent with reduced HPI-axis reactivity. During 16 d of salmon cohabitation, TRP-fed fish showed a higher proportion of empty stomachs and slightly reduced growth, while cumulative survival did not differ between treatments. By the end of cohabitation, telencephalic 5-HIAA had converged back to similar means, while 5-HT remained significantly elevated in the TRP group. Overall, short-term dietary TRP enrichment reduced acute handling and transport stress in lumpfish without compromising survival, but prolonged exposure was associated with reduced feeding and growth. Dietary TRP enrichment thus appears promising as a pre-conditioning strategy to enhance lumpfish robustness during critical husbandry stages, although potential trade-offs with feeding motivation, growth, and cleaning behaviour require further study. • Seven days of TRP feeding increased telencephalon serotonergic activity. • TRP-fed lumpfish showed a clear reduction in transport-induced cortisol. • Extended TRP exposure during cohabitation lowered growth despite elevated 5-HT. • TRP-treated fish displayed signs of altered feeding motivation at sea. • Short-term TRP supplementation appears promising to build stress resilience