A method of providing bubbles formed in a liquid medium on or near the surface of an animal, and exposing the bubbles to sound waves so as to induce resonance and asymmetric collapse of the bubbles. The method can be to reduce parasite infestation on an aquatic animal or a non-therapeutic method for improving the condition, appearance, meat quality or growth rate of an aquatic animal. The apparatus is also provided, as well as a composition comprising water for use in the method of treating a parasitic infestation. In another embodiment is a method for the delivery of molecules to an aquatic animal comprising stabilising the bubbles by providing a film at the gas-liquid interface or by inclusion of surfactants in the bulk of the liquid, wherein the film or liquid comprises molecules for delivery to the animal or parasite. Ultrasound exposure parameters are adapted to induce collapse of the bubbles and allow molecular delivery to the aquatic animal or to cells of the aquatic animal. Preferably the animal is a fish such as a salmon, and the parasite is non-pathogenic and comprises one or more of Lepeophtheirus salmonis, Caligus clemensi and Caligus rogercresseyi.
