Caracterização das emissões sonoras do boto-cinza Sotalia guianensis (Van Benédén, 1864) (Cetacea: Delphinidae) e a investigação do ambiente acústico na Baía de Benevente, ES

Abstract

The Delphinidae exhibits great plasticity of acoustic signals and adapts their sound emission according to circumstances. Currently, ocean noise pollution is a threat to cetaceans and this issue has not been well studied in relation to the estuarine dolphin (Sotalia guianensis). Therefore, their acoustic signals represent an important biological aspect to be understood. In this context, this study aimed to characterize the sound repertoire and investigate the acoustic environment of estuarine dolphin in Benevente Bay, ES, Brazil. The recordings were performed using hydrophone Cetacean Research C54XRS coupled to digital recorder Fostex FR-2 LE recording at 96kHz/24bits. Data collected between December 2011 and July 2012 totaled 27 hours and 55 minutes of effort recording. We evaluated 69 whistles, 42 burst sounds and 33 clicks’ train. Among the whistles contour the most common type was ascending (N = 37, 53%), followed by ascending-descending (N = 15, 22%) and multiple (N = 13, 19%) types. The fundamental frequency of whistles ranged between 3.51 kHz and 37.56 kHz. The frequency parameters analyzed were: start, end, minimum, maximum, range and frequencies at 1/4, 1/2 and 3/4 of the duration. The duration and inflection points were also analyzed. The average duration of whistles was 0.298 second (SD = 0.147). About the burst pulse sounds the sign "bray call" (N = 36) was more common and longer than the "buzz sound" (N = 6). These two types occurred immediately after or near echolocation clicks. On these, 33 trains were analyzed and presented 36.45 (SD = 43.47) clicks and 11.404 (SD = 21.226) seconds, in average length. The interval between clicks or “Inter-click interval” (ICI) lasted 0.308 (SD = 0.301) seconds in average. It was also found two distinct temporal patterns for ICIs: 81% (N = 946) intervals lasted between 0.001 and 0.400 seconds and 19% (N = 224) lasted between 0.401 and 1.246 seconds. Most of the frequency parameters’ average from whistles were higher than those observed in studies with S. guianensis at south of the study area and lower than populations at north. This could be related to different frequency limits and/or the assumption that the frequency increases from south to north. The burst sounds observed were previously cited for S. guianensis and other odontocetes. Besides presenting a social function, these sounds may be related to obtaining prey. The distribution of the ICIs’ values in distinct temporal patterns was also reported for S. guianensis and other species of dolphins, which may represent the different functions of echolocation clicks. The estuarine dolphin occurred in the presence of a low-frequency anthropogenic noise. On this occasion there was a change in the acoustic behavior that possibly expresses an attempt to compensate for the effect of masking to maintain effective communication in an acoustically polluted environment.