Ethological observations of aggressive social interactions reveal multiple targets for non-invasive monitoring of stress and welfare in farmed Atlantic Salmon (Salmo salar).

• Main Author: Green, Evelina Andrea Løsnesløkken
• Other Authors: Øyvind Øverli, Mikkel Gunnes, Inger Lise Andersen
• Format: Master Thesis
• Language: English
• Published: Norwegian University of Life Sciences 2024
• Subjects: Atlantic salmon; Salmo salar
• Online Access: https://hdl.handle.net/11250/3148384

New observations regarding cognitive abilities and emotional processes in fishes challenge traditional perceptions and emphasize the need for ethically sound practices in aquaculture. Similar welfare legislations applies to fish as to mammals, but there is lack of consensus on how to define and assess fish welfare. Furthermore, current health and welfare monitoring of fish is mostly based on indicators that yield a signal only after a problem has developed (e.g. physical condition, disease, parasites, and mortality). Non-invasive operational indicators that reveal acute threats are therefore needed for proactive continuous assessment of fish welfare. Here we utilize close observation of social interactions in juvenile Atlantic salmon to reveal hitherto unreported behavioral indicators of stress. In particular, we noted that release of gas bubbles from the swim bladder through the mouth increased distinctly and predictably in stressful situations. Firstly, a transient increase in such “bubbling” behaviour was observed after transfer from communal rearing to a novel environment (social isolation). This phenomenon was virtually absent after two weeks of habituation when isolated individuals had adapted and started to feed in new territories. During subsequent social interactions, stressed subordinate individuals again showed this behaviour, to a far higher degree than dominant fish. Most notably, stress induced swim bladder venting was individually variable, increasing linearly in response to the amount of aggressive attacks received from dominant fish. Hence, this indicator appears to accurately reflect the severity and temporal dynamics of on-going stressors. Another phenotypic trait in salmonids that is amenable for non-invasive visual screening is melanin-based skin pigmentation patterns. In this particular experimental environment, test fish quickly exhibited changes in the number of black eumelanin spots and skin coloration. Preliminary data suggest that this process was also affected by stress coping ...