WELFARE AND FLESH QUALITY OF SPANISH RAINBOW TROUT WHEN ELECTRICALLY STUNNED

In recent years there has been an increase in the concern over the welfare associated risks of different commercially applied fasting and stunning and killing methods for rainbow trout, especially in Europe. The best methods could also depend on the water temperature, which is normally quite high in Spain during the summer months. We carried out a short experiment using 40 commercial-sized rainbow trout that were first fasted for 5 days (114.7ºC days) and slaughtered using two different methods currently used by the industry in Spain (electrical stunning and ice chilling), to observe the effects on welfare indicators (behaviour and stress) as well as flesh pH.

To measure welfare during the fasting stage we analyzed fish movements around the feeder area using photocells. Regarding stunning, fish were either electrically stunned (removed from the home tank and placed dorso-ventrally into a V shaped stunner that provided a current at 0.4 A, 90 V, for 3 s) or ice chilled (removing the fish from the home tank and placing each one into separate 20 l buckets with an ice/water ratio of 1 at a temperature of 2ºC. The average temperature of the water in the home tank was 21ºC (performed in summer).

During fasting, trout made more movements around the feeder and changed their diurnal patterns (Fig. 1). When comparing between the fish that were electrically stunned versus those placed on ice, the slaughter weight (561 electric vs. 541 g ice), stomach content (2.0 vs. 1.1%), hepatosomatic index (1.01 vs. 1.02) and carcass yield (91.6 vs. 91.9%) were not significantly different. The hepatic glycogen (7.41 electric vs. 6.77 mg/g ice), muscle glycogen (6.33 vs. 4.93 mg/g) and thiobarbituric acid (1.65 vs. 1.47 mg malondialdehyde/g flesh) were also similar among treatments as were cortisol levels (6.21 vs. 7.07 ng/ml), glucose (115 vs. 115 mg/dl), lactate (4.68 vs. 4.31 mmol/l), triglycerides (230 vs. 220 mg/dl) and creatine phosphokinase (933 vs. 915 U/l). The only significant difference (P=0.043) was flesh pH, which was higher in the trout stunned electrically (pH = 6.49) compared to trout placed in ice (pH = 6.39).

From our data it appears as if the stunning procedure had little effect on the stress response from the fish, after approximately 115ºC days of fasting, but that electrical stunning maintains a higher pH post mortem, suggesting better flesh quality.