Deep-Sea Research II 54 (2007) 290–297 Comparison of echolocation behaviour between coastal and riverine porpoises Tomonari Akamatsu a,Ã , Jonas Teilmann b , Lee A. Miller c , Jakob Tougaard b , Rune Dietz b , Ding Wang d , Kexiong Wang d , Ursula Siebert e , Yasuhiko Naito f a National Research Institute of Fisheries Engineering, Fisheries Research Agency, Hasaki, Kamisu, Ibaraki 314-0408, Japan b National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark c Institute of Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark d Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, PR China e Forschungs- und Technologiezentrum Westkuste, University of Kiel, Hafentorn, D-25761 Busum, Germany f National Institute of Polar Research, 1-9-10, Kaga, Itabashi, Tokyo 173-8515, Japan Accepted 15 November 2006 Abstract Echolocation behaviour of a harbor porpoise and six finless porpoises was recorded in open-water systems using acoustic data loggers (A-tag). In total 1359 click trains were recorded during 4.6 h for the harbor porpoise and 46,240 click trains were recorded during 82.3 h for the finless porpoises. The harbor and finless porpoises produced sonar click trains every 12.3 and 6.4 s on average, respectively. During the inter-click-train interval, the porpoises were silent or produced clicks below 148 dB re. 1 mPa, the detection threshold of the tag. Ninety percent of the inter-click-train intervals were 20 s or less in both species. This means that porpoises frequently produce intense click trains. Click-train intervals lasting over 50 s constituted 1% of the total intervals in finless porpoises and 4% in the harbor porpoise. Both species swam without intense clicks for less than 10 m in most cases, but occasionally remained silent or used undetected low-intensity clicks for more than 1 min. During these periods, the porpoises would be susceptible to entanglement in fishing nets. r 2007 Elsevier Ltd. All rights reserved. Keywords: Bioacoustics; Echolocation; Marine mammals; By catch, Finless porpoise; Harbor porpoise 1. Introduction In recent years, passive acoustic monitoring of odontocete biosonar signals using stationary data loggers has provided valuable data concerning the abundance of animals in their natural habitats. The reliability of detection can be quite high as shown for finless porpoises (Neophocaena phocaenoides). For example, 80% of visually detected finless porpoises were also detected acoustically with false alarm rates as low as 1% (Akamatsu et al., 2001; Wang et al., 2005). In cetacean management, abundance surveys and monitoring of density changes are essential to assess the status of a population and determine the impact of by-catch. For the reliable estimation of abun- dance, the probability of finding the target animal on the survey cruise line (the so called g(0)) and the ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.11.006 Ã Corresponding author. Tel.: +81 479 445929; fax: +81 479 441875. E-mail address: akamatsu@affrc.go.jp (T. Akamatsu).