A low-cost approach in acoustic monitoring of dolphin presence

Passive acoustics is widely used to monitor dolphin presence in marine environment. The current study proposes a self-made/low-cost approach to assemble a complete and compact underwater acoustic recorder employing inexpensive and widely used components. The system is composed of two main elements: the computing unit (Raspberry Pi Zero 2W single board computer) and the sound acquiring sensor, i.e., a hydrophone based on a single-face piezo capsule disk (hydrophone cost ≈$10.00). The performance of the proposed hydrophone was preliminarily tested versus the commercial-available AS-1 hydrophone (commercial value = $395.00), during 30-minute acquisition sessions performed in a dolphin pool. The comparison was performed quantifying the capability of the two systems to provide a high-quality signal to assess echolocation clicks (EC), whistles (W), and burst pulse sounds (P) emitted by dolphins. Results showed that the worsening of performances introduced by the current low-cost approach is very limited since: 1) the average number of ECs detected over sixty selected 5-second portions of the acquired signal is not significantly different (146.6±66.7 for home-made vs. 151.1±73 for AS-1, p >0.05); 2) the total number of detected ECs is comparable (8797 vs. 9065, ≈3% difference); 3) the number of detected whistles is the same for the two devices (11); and 4) the error made by the low-cost approach in the detection of burst pulse sounds is lower than 7%. Looking to the future, this effective and low-cost technology will make possible to monitor dolphin presence over larger areas using a network of low-cost acoustic recorders.