The Effect of Dolphin Watching Boat Noise Levels on the Whistle Acoustic Structure of Dolphins in Bocas Del Toro, Panama

The resident bottlenose dolphin community at Dolphin Bay in Bocas del Toro sustains the largest dolphin watching (DW) industry in Panama. Our previous work shows that dolphins significantly respond to DW boat presence by modifying their whistle frequency and duration particularly when engaged in foraging activities. The usual assumption is that the noise associated with the DW boats is responsible for the change in whistle parameters. In this study we evaluate the effect of noise levels on whistles acoustic structure by analyzing recordings obtained in 2007, 2008 and 2012 under various boat interactions. We measured ambient noise levels (RMS values, 100Hz to 48kHz) for each of these recordings and for each whistle within those recordings we measured a number of standard frequency variables and duration. Noise levels increased with boat presence; however, there was also significant variation among years. After adjusting the level of significance for multiple comparisons to α= 0.006, our results indicate that 8% of the variation in whistles minimum frequency is explained by the interaction between noise levels and year. In contrast, 16%, 22%, and 11% of the variation in whistle ending frequency, peak frequency, and duration was explained by year, respectively. In agreement with recent studies these results indicate that while annual variations in noise levels can significantly affect dolphin communication, dolphins show great plasticity in coping with these changes. Furthermore, it is important to highlight that changes in noise levels only explained a small percent of the variation observed in dolphin whistle structure suggesting that other cues (e.g., mode of approach) and other sensory modalities (e.g., vision) associated with these boat-dolphin interactions may be more important contributors to changing dolphin acoustic behavior.