Stunning techniques and climatic influence in rainbow trout: Sensibility state, welfare and recovery ability

This study evaluated the effects of climatic conditions and three stunning techniques on the stress response, sensibility state, and recovery ability of rainbow trout (Oncorhynchus mykiss). Stunning methods included ice-water immersion (ICE) and two electric shock treatments combined with ice-water immersion (E200: 200 mA for 2 s; E400: 400 mA for 0.5 s followed by 200 mA for 1.5 s). Rainbow trout were exposed to these methods in winter and summer to assess the impact of seasonality. Under winter conditions, fish in the ICE group retained sensibility, whereas electrically stunned fish lost sensibility rapidly and remained insensible for an extended period. In summer, the ICE group exhibited a gradual loss of sensibility, while fish in both the E200 and E400 groups became immediately insensible post-stunning. Blood cortisol levels were significantly higher in the ICE group, indicating a stronger stress response, whereas electrically stunned fish had lower cortisol levels, likely due to their immediate loss of sensibility. Markers of carbohydrate and lipid metabolism also reflected an intensified mobilization during summer, highlighting the influence of seasonal variation. Regarding recovery ability, winter conditions promoted higher recovery rates across all groups, with over 50 % of fish regaining sensibility. In contrast, electrically stunned fish in summer demonstrated lower recovery rates, suggesting potential irreversibility of the stunning effect, while ice-water immersion preserved recovery ability. These findings underscore the substantial impact of climatic conditions on stunning effectiveness and fish welfare in rainbow trout, emphasizing the importance of adjusting stunning techniques according to seasonal temperature changes.