Abstract:
Determining the residency of an aquatic species is important but challenging and it remains unclear what is the best
sampling methodology. Photo-identification has been used extensively to estimate patterns of animals’ residency and is
arguably the most common approach, but it may not be the most effective approach in marine environments. To examine
this, in 2005, we deployed acoustic transmitters on 22 white sharks (Carcharodon carcharias) in Mossel Bay, South Africa to
quantify the probability of detecting these tagged sharks by photo-identification and different deployment strategies of
acoustic telemetry equipment. Using the data collected by the different sampling approaches (detections from an acoustic
listening station deployed under a chumming vessel versus those from visual sightings and photo-identification), we
quantified the methodologies’ probability of detection and determined if the sampling approaches, also including an
acoustic telemetry array, produce comparable results for patterns of residency. Photo-identification had the lowest
probability of detection and underestimated residency. The underestimation is driven by various factors primarily that
acoustic telemetry monitors a large area and this reduces the occurrence of false negatives. Therefore, we propose that
researchers need to use acoustic telemetry and also continue to develop new sampling approaches as photo-identification
techniques are inadequate to determine residency. Using the methods presented in this paper will allow researchers to
further refine sampling approaches that enable them to collect more accurate data that will result in better research and
more informed management efforts and policy decisions.
