An apparatus (10) for repelling sharks or seals, has a pulse generating circuit (11) arranged to selectively connect to an output switching circuit (23) to control it to switch a power supply (51) having a direct current voltage of from (80) volts and including capacitance with stored charge capacity of at least 8,000 μF, to deliver pulses to at least two electrodes (19, 20). The pulses may be unipolar or bipolar, each unipolar pulse having a pulse width of from 50 μS to 1000 μS, at said direct current voltage less any voltage drop inherent in said output switching circuit. The apparatus (10) can be connected to arrays of electrodes and associated sensors to protect fish pens from predation and swimming beaches from sharks.
