Welfare and quality of farmed trout fed high plant protein diets. 3 alternative indicators to evaluate stress in fish

Abstract

Previous studies reported that diets can increase tolerance to various stressful conditions in different fish species. Consequently, the present work evaluated the effects of replacing fish meal (FM group) with an 80% of plant protein source (PV 80 group) in the diet of farmed rainbow trout (Oncorhynchus mykiss) on stress response caused by two different slaughter methods (electrical stunning and asphyxia). Plasma cortisol levels are widely employed as quantitative measure of stress (Wendelaar Bonga, 1997). However, to avoid the blood sample, it would be useful to test less invasive alternative biological matrixes such as skin mucus, faeces and caudal fin to use them especially in live fish and also invasive matrixes as muscle for post-slaughter analysis. From each experimental group twelve fish were netted, six were slaughtered by electrical stunning and six by asphyxia. From each fish, plasma, skin mucus, intestinal content, caudal fin tip and muscle were rapidly collected and frozen for cortisol evaluation by RIA (radioimmunoassay). Data on plasma cortisol were submitted to analysis of variance using the GLM procedure (STATISTICA, 2006) according to a bi-factorial arrangement (2x2) with slaughter methods and diets as main variability factors. Pearson’s linear regression was used to correlate cortisol values in different matrixes. Statistical significance was taken as P<0.05. Results showed that plasma cortisol was significantly higher in groups slaughtered by asphyxia (P=0.002) and no differences were found in plasma cortisol between the different diets (P=0.34). Plasma cortisol was compared with the level in the other matrixes showing a positive correlation with skin mucus and intestinal content (r=0.57 and r=0.58, respectively). The low correlation of plasma cortisol with muscle and fin (r=0.2 and r=0.1, respectively) is in contrast with previous study on trout transport stress (unpublished data). This is probably due to the reduced vitality of fish during asphyxia or to insufficient time (20’) for the steroid to reach the highest levels that easily diffuse into muscle and fin. In summary, the electrical stunning is preferable as regard fish welfare and a diet in which fish meal is replaced by plant proteins could not influence the stress response to slaughter. Moreover less invasive matrixes such as mucus and faeces could be good indicators of the stress level in fish.