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

Abstract

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 Hippocampus kuda (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 H. kuda 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.