Automatic discrimination and detection of small calf Ganges river dolphin (Platanista gangetica) from other age groups based on bio-sonar inter-click interval characteristics

Targeting 15-20 Ganges river dolphins (adults, young adults and calves) in a 10 km stretch from Karnavas to Narora in the Ganges river system, for detailed understanding of their echolocation strategies and underwater behavior over a long-term span, the annual long-term in-situ monitoring using a high frequency 6-hydrophone array system (Array #1) has been conducted at the stationary observatory for 5 years since November 2008. From phase 5 monitoring, one more 6-hydrophone array system (Array #2) has been deployed approximately 100 meters upstream from the original array system (Array #1) to increase the coverage and the overall positioning accuracy. Recorded data from both array systems was automatically uploaded to the database every hour for further analysis. From the visual observations that were periodically made by WWF-India, followed by the phase 4 monitoring, the appearances of small calves (body size: 0.6 - 0.8m) was visually confirmed. Considering ICI characteristics differences between small calf and other age groups from our previous studies (Sugimatsu et al.) [1], “small calf” group was acoustically defined as “click train” whose mean value of ICI is approximately from 6msec to 12msec and ICI spread (SD) is less than±3msec. Taking advantage of revealed unique ICI characteristics of “small calf“, the method that can automatically discriminate and detect the click train of “small calf” from data uploaded to the database was developed and demonstrated using the data set recorded from the previous 5 monitoring phases.