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

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.