The bottlenose dolphins Tursiops truncatus resident in the Sado Estuary, Portugal, comprise a very small, declining population. Increasing levels of dolphin-watching activity in this area may be placing this population at high risk. To investigate the response of these dolphins to boats, this study monitored the respiration and behavior of dolphins in the presence and absence of boats from August to September 2000, during the peak season for dolphin-watching. Land-based observations were made using the focal group sampling method. A high number of recreational boats were observed near the dolphins, but tourist boats spent longer periods of time around them. Boat-dolphin encounters occurred mostly during weekends.

Using respiration parameters, our results indicate that, in the presence of boats, the dolphins spent significantly less time at the surface, made fewer exhalant blows, had shorter inter-blow intervals and made longer dives. Using behavioral parameters, the increase in tailslaps, alteration of activity, changes of orientation and changes within the group were also significant in the presence of boats. Inter-blow interval, dive duration and tailslapping were all significantly correlated with the number of boats.

These results suggest that this population seems to show avoidance behavior towards boats. The creation of specific regulations for tour boat operations in the Sado Estuary is thus indispensable for the conservation of these dolphins.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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