Sea transfer and net pen cleaning induce changes in stress-related gene expression in commercial Atlantic salmon (Salmo salar) gill tissue

Abstract

Stress is a major concern in aquaculture production and more knowledge is needed on physiological responses towards different operational events. Few studies have been performed on fish reared in an actual commercial setting. Transferring salmon from hatchery to sea involves handling, crowding, pumping, transport as well as adjusting to a new environment. This case study investigates the relative expression of selected stress related-genes in farmed Atlantic salmon (Salmo salar) post-smolts that were relocated from land-based breeding tanks with few environmental stressors to sea phase with numerous environmental stressors and major management operations e.g. net pen cleaning. Gill tissue for analysis (n = 60) was harvested at four distinct time points: before, during, and 3 and 6 weeks after sea transfer. RT-qPCR was performed on a panel of 12 genes involved in different cellular pathways (alox5, cyp1α, hif1α, il4/13a, muc2, muc5, muc18, nrf2, pcna, phb, p38 and tnfα). While the transport process itself did not appear to induce notable stress levels, metabolic gene markers showed significant changes in expression after transfer to sea, implying cellular adaptations to sea phase. The occurrence of net pen cleaning induced a strong upregulation of pro-inflammatory markers (alox5, tnfα) and mucins (muc2, muc5 and muc18), suggesting their gene products to be relevant during this operational event. As p38 expression was significantly elevated during transport and after cage cleaning, we cautiously propose p38 as an interesting stress marker for future exploration. The study provides insight into the lives of farmed Atlantic salmon and demonstrates that timing of major operations is crucial to avoid accumulation of stress.