Effects of boat traffic on the behavior of gray whales, Eschrichtius robustus, in Bahia Magdalena, Baja California Sur, Mexico: a bioacoustic assessment

Abstract

There is widespread concern about the effects that man-made noises have on marine mammals. In this study, potential acoustic and behavioral changes produced by boat traffic on gray whales (Eschrichtius robustus) were assessed. Time of day, seasonal day, tide, sea state, maximum number of whales per scan, and total number of moving boats per scan were correlated (Spearman rank correlation r=0.356, n=52 a .01). A significant correlation was observed between maximum number of whales per scan and total number of moving boats per scan (r=0.4568). Estimates of behavioral reactions included changes in: swimming speed, swimming direction, and successive breathing intervals (blows). The blow intervals for all gray whales were not distributed normally. There was a significant difference (Kruskal-Wallis p<.OOOI) in blow intervals among classes (mothers, calves, mother-calve pairs, and single adults). Blow intervals of all classes pooled were significantly different in the presence or absence of boats (Kolmogorov-Smirnov p=.0006). Acoustically, ten signals were identified for gray whales in this lagoon. Qualitative and quantitative measurements of each signal included signal duration (sec.), peak frequency (Hz), and bandwidth (Hz). Changes in the measured characteristics of the signals or differences in calling rates were monitored as indicators of disturbance to gray whale behavior. Most signals ceased in the presence of whale-watching boats. As the area's first acoustic assessment of boat-noise disturbance, this work provides baseline information to establish guidelines for the extensive lagoon-based whale-watching industry.