Physiological and behavioral responses to an electrical stimulus in Mozambique tilapia (Oreochromis mossambicus)

Abstract

Consumer awareness of the need to improve fish welfare is increasing. Electrostunning is a clean and potentially efficient procedure more and more used to provoke loss of consciousness prior to killing or slaughtering (reviewed by Van de Vis et al. in Aquac Res 34:211-220, 2003). Little is known how (powerful) electrical stimuli, which do not stun immediately, are perceived by fish. We investigated responses of hand-held Mozambique tilapia (Oreochromis mossambicus) to a standardized electric shock applied to the tailfin. The handling with the resulting unavoidable acute stress response was carefully controlled for. Fish responses were analyzed up to 24 h following the shock. Electric shock resulted in slightly higher levels in plasma cortisol, lactate, ionic levels, and osmolality, than handling alone. Plasma glucose had significantly increased 6 h after shock compared to handling, indicative of enhanced adrenergic activity. Mucus release from the gills, branchial Na⁺/K⁺ ATPase activity, and chloride cell migration and proliferation, parameters that will change with strong adrenergic activation, were not affected. Decreased swimming activity and delay in resumption of chafing behavior indicated a stronger and differential response toward the electric shock. Responses to handling lasted shorter compared to those to an electric shock. The differential and stronger responses to the electric shock suggest that fish perceived the shock potentially as painful.

Fig. 1

Scheme of the electrical system used to provide the standardized electroshock. 1, V-shaped box covered with a wet towel to avoid desiccation. Fish were gently and manually restrained; 2, anode; 3, cathode; 4, pliers; 5, spring, adjusted to ensure standard stimulus; 6, stimulator with digital indication of the current delivered; 7, electronic integrator with fixed value of voltage delivered (15 V per pulse)

Fig. 2

Quantification of the general swimming (crossing) activity in Mozambique tilapia following several treatments; a electric shock versus its control; b handling stress versus its control situation; analysis for a period of 24 h. Data are presented in number of crossing events per fish and per minute per periods of 1 h (10 fish per tank). Controls were analyzed similarly over a period of 5 days prior to the stimulus application. Results are presented as averages of 4 periods of 15 min per hour, and SEM. Tanks are divided into two distinct zones (covered vs. uncovered). Fish were considered to cross when their entire head was in the other compartment