Abstract: A deterrent system is provided for repelling selected species of aquatic animals from an underwater deterrence zone, for example, from the opening of a municipal or industrial water intake. A conduit for fluid, generally water, has an outlet at the deterrence zone. Intermittent flow means cause one-way fluid flow through the conduit, visiting at the outlet as pulses of fluid alternating with periods of insubstantial fluid flow. The pulses have a volume and flow rate sufficiently in excess of the insubstantial fluid flow to repel the selected aquatic animals from the deterrence zone. The conduit outlet does not serve also as a substantial fluid inlet to the conduit and, rather, the system provides substantial net positive outflow from the conduit outlet over one or more full cycles of the aforesaid pulsed fluid flows alternating with periods of insubstantial fluid flow.