Introduction
In intensive production of Atlantic salmon, the fish are subjected to various handling procedures such as transport, vaccination, and mechanical treatments for sea lice. T hese events induce acute stress in the fish . Add itional stressors include sudden changes in water quality or environmental changes during the transition from freshwater to seawater. St ressed fish exhibit reduced disease resistance and diminished appetite [1, 2], leadin g to decreased growth and economic losses for fish farmers. A recent study estimated that a single mechanical treatment against sea lice could result in a 200-gram growth los s per fish [2] . Feed that enhances appetite following a stressful treatment can improve growth and robustness, ultimately reduc ing the time required to reach the desired market weight. To accurately a ccess feed efficacy, it is essential to move be yond controlled lab-scale trials and consider real -world challenging environments.
Materials and methods
A challenge model was developed to emulate the handling of Atlantic salmon prior to mechanical delousing in well boats . The handling procedure consisted of mild crowding for 1.5 hours, followed by netting, sedation, individual weighing, and manual delousing. This model was validated through a nine-month feeding trial conducted at LetSea (Dønna, Norway) , which compared stressed and non-stressed treatments among the salmon. Throughout the trial, the handling procedure was performed four times. This challenge model has been used in assessing functional feeds at LetSea . The test feed in this trial contains additional magnesium and specific plant raw materials, which are hypothesized to enhance appetite. The primary objective was to mitigate stress-related growth loss in Atlantic salmon post-handling. The experimental feed was administered two weeks preceding, and during the week of handling.
Results
The challenge model was verified by a significant stress response shown as increased levels of cortisol in fish scales, serum biomarkers such as the secondary stress response parameters lactate, glucose, magnesium, calcium and potassium , and altered expression of stress-related genes in the liver of stressed fish compared to the non-stressed control treatment. In addition, the cardio somatic index (CSI) and heart length and roundness were significantly affected by the stress treatment, with unstressed fish having lower CSI and wider and rounder hearts , and stressed fish had increased skin hemorrhages and snout wounds.
During the feeding trial the fish grew from approximately 1.5 kg to 4.3 kg. Comparing stressed control treatments with stressed functional feed treatments, we have seen no significant differences in cortisol levels in scales or stress response serum markers. This indicates that the fish fed the functional feed had a normal adaptive stress response. Additionally, we have seen an increase in appetite (increased feed intake) and growth after the handling events for fish fed the functional feed, as evidenced by higher TGC (thermal growth coefficient). Despite the stress induced by the challenge model , the fillet color (pigmentation) was improved reflecting the increased feed intake.
Conclusion
The findings of the current study underline the importance of addressing stress and growth in fish through scientific interventions. The challenge model tested effectively simul ates the handling of Atlantic salmon prior to mechanical delousing operations and has been verified through extensive feeding trials. The new functional feed significantly improves appetite and growth in salon following stress-induced handling procedures.
References