The following article concerning passive acoustic detection of Maui's dolphins has recently been published. PDFs are available from will.rayment@otago.ac.nz Rayment W, Dawson S, Scali S and Slooten E. 2011. *Listening for a needle in a haystack: passive acoustic detection of dolphins at very low densities*. Endangered Species Research 14: 149-156. ABSTRACT: Passive acoustic surveys have potential for detecting trends in abundance and habitat use by rare cetaceans. We deployed commercially available acoustic data loggers (T-PODs) in 4 harbours on the west coast of New Zealand's North Island between 2005 and 2008 to investigate the distribution of Maui's dolphin /Cephalorhynchus hectori maui/ and assess whether current protection measures are sufficient. A set of decision rules was developed to minimise the potential for false positive detections. Over 3211 'T-POD days' of acoustic monitoring, 39 click trains which satisfied all of our decision rules were detected, indicating the presence of Maui's dolphins in Manukau and Kaipara Harbours. Data from the site with the most detections were fitted to 3 models, showing that the number of detections varied temporally (p < 0.001). The models were also used to show to what degree dolphins could have been present at monitored locations yet remain undetected. The study highlighted the challenges of passive acoustic monitoring of rare species, particularly of small delphinids in an environment which is both physically and acoustically challenging. Nonetheless, we demonstrated that T-PODs are effective in studies of Maui's dolphin distribution, that Maui's dolphins are found in North Island harbours and remain at risk from gillnet bycatch. We make a number of recommendations concerning acoustic monitoring studies of rare cetaceans, principally that a thorough understanding of the target signals and the acoustic environment being monitored is essential for maximising acoustic detection rates. The following articles using T-PODs with Hector's dolphins are also available as PDFs: Rayment W, Dawson S and Slooten E. 2009. *Trialling an automated passive acoustic detector (T-POD) with Hector's dolphins (/Cephalorhynchus hectori/)*. Journal of the Marine Biological Association of the UK 89: 1015-1022. Rayment W, Dawson S and Slooten E. 2010. *Use of T-PODs for acoustic monitoring of /Cephalorhynchus/ dolphins: a case study with Hector's dolphins in a marine protected area*. Endangered Species Research 10: 333-339. -- Will Rayment FRST Post-doctoral Research Fellow Marine Mammal Research Group Marine Science Department University of Otago PO Box 56 Dunedin New Zealand phone: +64 3 4798304 (w) +64 21 488961 (mob.)