Effects of different stunning methods on blood markers and enzymatic activity of stress responses of tilapia (Oreochromis niloticus)

The objective of this study was to evaluate less stressful stunning methods for tilapia (Oreochromis niloticus). A preliminary experiment was performed to elect a chemical anaesthetic to be used as control in the following electronarcosis assays. Fish (498.8 +/- 16.7 g) were submitted to four treatments: 1 (T1) and 2 minutes (T2) of electronarcosis at 205 volts and 10.8 amperes; and immersion in benzocaine (TB) and eugenol (TE), both at 100 mg L-1. After plasma stress biomarkers evaluation, that is, ammonia, glucose and lactate, and considering use safety, eugenol was elected as control anaesthetic method. In the second experiment, fish were submitted to three electronarcosis frequencies: 400 (T400), 800 (T800) and 1200 (T1200) Hz, all of them at 203 V, similar to 3.3 A for 15sec. After electronarcosis, plasma lactate was increased by 121 and 146% in fish submitted to T400 and T800, respectively, when compared to control. Ammonia and glucose did not depict significant differences. White muscle catalase (CAT), glutathione reductase (GR) and lactate dehydrogenase (LDH) activities were also assessed. Catalase activity augmented in the measure of 25% in T400 and T800, while GR activity was increased in the measure of 41, 43 and 29% for T400, T800 and T1200, respectively. Boosts of 50 and 38% in LDH activity were observed in T400 and T800 groups. Our findings suggest that higher frequency (T1200) electronarcosis improves O. niloticus welfare during slaughter.