Humpback whale singing activity off the Goan coast in the Eastern Arabian Sea

<p>For over two decades, passive acoustic monitoring (PAM) methods have been successfully employed around the world for studying aquatic megafauna. PAM-driven studies in Indian waters have so far been relatively very scarce. Furthermore, cetacean populations inhabiting the north western Indian Ocean are far less studied than those in many other regions around the world. This work likely constitutes the first systematic study of the vocal repertoire of humpback whales (<i>Megaptera novaeangliae</i>) at a near-shore site along the western coast of India. Analysis of the observed vocalizations provides an insight into the behaviour of the species. This is significant as it assists in developing a better understanding of the habitat use of the non-migratory Arabian Sea humpback whale population. In contrast, other breeding populations such as those around the North Atlantic, South Pacific and Australia have been relatively well studied. Underwater passive acoustic data were collected during March 2017 using an autonomous logger at a shallow-water site off the eastern edge of Grande Island off the coast of Goa. Humpback whale vocalizations were found to occur over multiple days in the recordings. Time–frequency contours of individual units of vocalization were extracted with the aid of an automatic detection technique and the characteristics of the units were measured. Further, successive units were analysed for formation of phrases and themes. Reconstruction of putative songs from the identified units and themes was not possible due to the limitations imposed by the nature of data collection. Detailed analyses of units, phrases and themes are presented.</p

Characterization of Yellow Seahorse <i>Hippocampus kuda</i> feeding click sound signals in a laboratory environment: an application of probability density function and power spectral density analyses

<p>Do the sounds generated by different-sized fish of different sexes differ from each other in temporal, spectral or intensity patterns? Such differences would enable the development of passive acoustic techniques to locate seahorses in open water bodies. On the basis of this perspective, we characterize and present in this study Yellow Seahorse <i>Hippocampus kuda</i> (Syngnathidae) feeding click sounds, recorded in a controlled laboratory environment. The characterization involved analysis of the “probability density function” (PDF) and the “power spectral density” (PSD) of the seahorse feeding click sound signals of different sizes and sexes. The PDF describes the general distribution of the magnitude of a random process, and such analysis, using recorded seahorse clicks, points towards a multimodal statistical distribution, which is the existence of more than one fitting component for the majority of seahorse click signals above the tank ambient noise level. This fact has been appraised towards the involvement of more than one process in the seahorse click signal generation mechanism. However, lack of prior knowledge about the individual PDF components of the data leads towards the mismatch between the data of PDFs. The PDF gives no information on the time and frequency content of the processes as provided by the PSD. Under such conditions, curve fitting of the “power law” expressions to the PSD functions estimates “slope” and “intercept” parameters of the seahorse click signal. Striking differences in the feeding click characteristics of various sizes of the <i>H. kuda</i> and between the sexes were discernable based on the clustering pattern among the estimated “slope” and “intercept” parameters. Further studies using the seahorse body sizes and weight (wet) with respect to the peak frequencies estimated from the PSD distribution show results equivalent to those from previous studies.</p