Living Under an Aquatic Freeway: Effects of Boats on Irrawaddy Dolphins (<I>Orcaella brevirostris</I>) in a Coastal and Riverine Environment in Indonesia

Abstract

Interactions between boats, and coastal and freshwater Irrawaddy dolphins (Orcaella brevirostris), were studied in East Kalimantan, Indonesia, during 2001. The goal was to determine the conditions under which dolphins reacted to boats and to recommend conservation actions. Both coastal and freshwater Irrawaddy dolphins surfaced less in the presence of boats, but the avoidance reaction lasted longer for the river dolphins. River dolphins surfaced significantly less often in the presence of motorized canoes ( 1,000 hp). Coastal dolphins only reacted to speedboats, and only when they approached at a 50-m distance. River dolphins reacted within a maximum distance of 250 m before and 300 m after a speedboat passed. Besides surfacing changes, river dolphins actively avoided container tugboats. The strength of reactions did not depend on the dolphins’ behavior, group size, or age. Hypersensitivity by river dolphins to intensive boat traffic could explain the different responses between coastal and river dolphins. To prevent dol phin displacement from their core areas, an action plan currently is being developed by a nongovernmental organization in cooperation with Indonesian governmental institutions and residents. Speedboat owners will be urged to reduce boat speed in areas indicated on sign boards.