Rainbow trout welfare: comparing stunning methods in winter and summer

Abstract

This study investigates the effects of three stunning methods: thermal shock (TS), electric shock at 200 mA 2 s (ES2), and electric shock at 400 mA 0.5 s followed by 200 mA 1.5 s (ES4) - both electric shock treatments followed by thermal shock - on the stress response and energy metabolism of rainbow trout (Oncorhynchus mykiss) during winter and summer. A significant interaction between stunning method and season was observed for blood cortisol levels. In summer, cortisol concentrations were elevated regardless of the stunning method, whereas in winter, the ES4 method resulted in the lowest cortisol levels. Seasonal variation significantly influenced stress response in trout, with summer conditions leading to higher cortisol levels, darker skin pigmentation, and reduced liver glycogen and lipid reserves. Increased metabolic activity during summer was associated with enhanced reactive oxygen species (ROS) production, which triggered the upregulation of key antioxidant enzymes (sod, gpx, cat, gst). The results suggest that the choice of stunning method is critical in mitigating stress, with electrical stunning combined with thermal shock being more effective than thermal shock alone, particularly under elevated temperatures. These findings underscore the importance of optimizing stunning practices to improve fish welfare, especially in the context of climate change. Understanding the interplay between seasonal and procedural stressors provides valuable insights for improving aquaculture management and sustainability.

Keywords: Electric shock; Rainbow trout; Seasonality; Stress response; Thermal shock.

Fig. 1

Temperature profile during the acclimation period (14 days) and the trials (2 days) in both seasons studied

Fig. 2

Blood cortisol levels (µg/dl) of rainbow trout stunned by three different methods in summer and winter. St: Stunning; Se: Season; TS: Thermal shock by ice water immersion; ES2: Electric shock at 200 mA for 2 s; ES4: Electric shock at 400 mA for 0.5 s and 200 mA for 1.5 s. Data are presented as mean ± SEM. Different lowercase letters indicate significant differences between groups (p < 0.05)

Fig. 3

Liver gr1 (glucocorticoid receptor 1) and mr (mineralocorticoid receptor) gene expression of rainbow trout stunned by three different methods in summer and winter. St: Stunning; Se: Season; TS: Thermal shock by ice water immersion; ES2: Electric shock at 200 mA for 2 s; ES4: Electric shock at 400 mA for 0.5 s and 200 mA for 1.5 s. Data are presented as mean ± SEM