EFFECTIVENESS OF ALARM-DISTRESS CALLS FOR FRIGHTENING HERONS FROM A FISH REARING FACILITY

We evaluated the effectiveness of using alarm-distress calls of the bl ack-crowned night heron Nycticorax nycticorax and the great blue heron Ardea herodias for frightening these piscivorous bird species from a rearing unit for rainbow trout Oncorhynchus mykiss. Heron alarm-distre ss calls reduced black-crowned night heron numbers 48% during the 11-d treatment period, compared with the pretreatment period, but did not affect the number of great blue herons. The number of black-crowned ni ght herons using the rearing unit and the proportion that remained imm ediately after the broadcast of alarm-distress calls increased from ni ghts 1 through 11 of the treatment period, which indicates that this p redator quickly habituated to the calls. We did not find any differenc es between the proportions of black-crowned night herons and great blu e herons that remained on the rearing unit immediately after playing c alls of the respective species. Alarm-distress calls were mostly ineff ective in reducing heron predation on rainbow trout fingerlings.