ORIGINAL PAPER Factors affecting the detection distances of reef fish: implications for visual counts Yves-Marie Bozec • Michel Kulbicki • Francis Laloe ¨ • Ge ´rard Mou-Tham • Didier Gascuel Received: 29 January 2010 / Accepted: 4 January 2011 / Published online: 30 January 2011 Ó Springer-Verlag 2011 Abstract Detection patterns of coral reef fish were assessed from the meta-analysis of distance sampling sur- veys performed by visual census in New Caledonia and French Polynesia, from 1986 to 1999. From approximately 100,000 observations relating to 593 species, the frequency distributions of fish detection distances perpendicular to the transect line were compared according to species charac- teristics and sampling conditions. The shape and extension of these detection profiles varied markedly with fish size, shyness, and crypticity, indicating strong differences of detectability across species. Detection of very small and cryptic fish decreased strongly 1 m away from the line. Conversely, sightings of shy and large species were exces- sively low in the first meters due to diver avoidance prior to detection. The larger the fish, the greater the fleeing distance. Distance data underscore how inconsistent detectability biases across species and sites can affect the accuracy of visual censuses when assessing coral reef fish populations. Introduction Underwater visual census (UVC) techniques are widely applied for surveying reef fish populations in temperate and tropical waters. Given adequately trained observers, UVC provides quick estimates of species richness, abundance, and length frequency distributions with simple and inex- pensive logistical requirements (English et al. 1997). Usual survey methods rely either on strip transects (Brock 1954), which count all fish detected along a path of fixed width, or on stationary point counts (Bohnsack and Bannerot 1986) from which a diver records all fish detected within a cir- cular area of fixed radius. Fixed-width strip transects are the most commonly used UVC method on coral reefs (Mapstone and Ayling 1998; Sale 1997) and have been adopted for current monitoring programs (e.g., Hodgson et al. 2004; Hill and Wilkinson 2004). UVC techniques have a number of biases that affect the accuracy of density estimates (Russell et al. 1978; Brock 1982; Harmelin-Vivien et al. 1985; Sale 1997) through inconsistent fish detection (Thresher and Gunn 1986; Kulbicki 1998; MacNeil et al. 2008). In particular, visual counts largely underestimate the abundance of cryptic and Communicated by D. Goulet. Electronic supplementary material The online version of this article (doi:10.1007/s00227-011-1623-9) contains supplementary material, which is available to authorized users. Y.-M. Bozec Marine Spatial Ecology Lab, School of BioSciences, University of Exeter, Prince of Wales Road, Exeter EX4 4PS, UK M. Kulbicki IRD, Universite ´ de Perpignan, 66860 Perpignan Cedex, France F. Laloe ¨ UMR GRED, IRD/UPV-Montpellier 3, IRD, BP 64501, 34394 Montpellier Cedex 5, France G. Mou-Tham IRD, BP A5, 98848 Noume ´a Cedex, New Caledonia, France D. Gascuel Universite ´ Europe ´enne de Bretagne, UMR INRA/Agrocampus Ouest ‘‘Ecologie et Sante ´ des Ecosyste `mes’’, CS 84215, 35042 Rennes Cedex, France Y.-M. Bozec (&) School of Biological Sciences, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia e-mail: ymbozec@gmail.com 123 Mar Biol (2011) 158:969–981 DOI 10.1007/s00227-011-1623-9