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Correspondence should be addressed to: D. A. Andradi-Brown, Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK email: [email protected] †
These authors have contributed equally and are joint last authors.
INTRODUCTION
Many studies have assessed the effectiveness of marine protected areas, including different management forms such as no-take zones and partial protection, generally finding that marine protected areas are effective in maintaining fish density and biomass (Sciberras et al., 2013). However, assessments rarely consider whether varying fish behaviours across the study area may bias the results of their survey techniques (Kulbicki, 1998, Feary et al., 2011). Fish behaviour is known to be impacted by previous exposure to humans (Januchowski-Hartley et al., 2015), yet locations with direct human interactions with the marine environment tend to be those reef managers are most interested in assessing. Even on a local scale, the exposure of fish communities to these effects can be highly variable along natural gradients such as depth. With much recent interest in the threats faced by mesophotic coral ecosystems (MCEs; reefs 30–150 m depth) (Andradi-Brown et al., 2016a), and whether they act as refuges from fishing (Bejarano et al., 2014; Lindfield et al., 2016), gaining a better understanding of fish behavioural survey biases across depth gradients is crucial.
While bias in some form is an unavoidable symptom of all survey methods, stakeholders rely heavily on data pertaining to fish populations and their responses to management interventions to inform decision-making. On tropical coral reefs this tends to involve baseline fish community data collected using underwater visual census (UVC) by surveyors in the water (English et al., 1997; Sale, 1997; Mapstone & Ayling, 1998). In many cases, to conduct UVC open-circuit (OC) scuba divers swim along a fixed-length transect recording individuals of all (or target) fish species, additionally estimating lengths in some cases (English et al., 1997). As a result of concerns about repeatability between surveyors because of observer bias (Thompson & Mapstone, 1997), video surveys have begun to replace in-water observations for many surveys (Mallet & Pelletier, 2014). While the use of video removes many errors associated with in-water data collection, the differing response behaviours of reef fish to diver presence raises...