Invasive Asian Carps in North America

<em>Abstract</em>.—The response of bighead carp <em>Hypophthalmichthys nobilis </em>and silver carp <em>H. molitrix </em>to conspecific skin extracts was determined during free-field observations and avoidance tests. The extracts induced freezing/motionlessness, cessation of food-searching activities, and reduced swimming activity in the fish. Silver carp tended to avoid the area where the extract was released and a strong schooling response was induced in bighead carp. In avoidance tests using countercurrent chambers, young bighead carp (5–8 cm total length) were unresponsive to the skin extract (<EM>P </EM>> 0.05). However, in both species, older juveniles (12–25 cm total length) spent significantly less time (<EM>P </EM>< 0.01) in the side of the chamber receiving skin extract solutions. The extract was effective whether freshly prepared or aged at room temperature for 24 h. Freezing did not alter the effectiveness of fresh or frozen extracts. These studies suggest that bighead and silver carp are responsive to conspecific skin extracts and that the use of extracts might be useful to repel carp from habitats of concern.