Accuracy of Using Visual Identification of White Sharks to Estimate Residency Patterns

Gennari E (2012) Accuracy of Using Visual Identification of White Sharks to Estimate Residency Patterns. PLoS ONE 7(4): e34753. doi:10.1371/journal.pone.0034753 Accuracy of Using Visual Identification of White Sharks to Estimate Residency Patterns David G. Delaney 0 Ryan Johnson 0 Martha n N. Bester 0 Enrico Gennari 0 Adam Stow, Macquarie University, Australia 0 1 Oceans Research, Mossel Bay, South Africa, 2 Mammal Research Institute, University of Pretoria , Pretoria , South Africa , 3 South African Institute for Aquatic Biodiversity , Grahamstown , South Africa 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. - Funding: This study was funded in-part by World Wildlife Fund South Africa, the South African Department of Environmental Affairs (and Tourism), and the International Fund for Animal Welfare. Also a vessel was provided by Andre Hartman. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Visual sightings and photo-identification (hereafter this coupled methodology is simply referred to as photo-ID) is a widely utilized approach [1] as it can produce long-term datasets on a variety of topics such as population size [24], population demographics of a given population [57], absolute trends in population numbers [8], and residency [911]. While photo-ID can collect data on a wide range of topics, the approach is both economically viable and noninvasive [12]. Therefore, photo-ID allows for non-invasive markrecapture studies, which is critical for threatened species [8] about which we lack sufficient fisheries data [13]. Photo-ID has been used as a monitoring tool on a variety of marine species including marine mammals [1420] and cartilaginous fish such as whale sharks Rhincodon typus [2123], nurse sharks Ginglymostoma cirratum [24], sand tiger sharks Carcharias taurus [25,26], manta rays Manta alfredi [1,27] and white sharks Carcharodon carcharias [24,9,10,13]. Given the wide use of photoID for studying marine animals, photo-ID is arguably one of the most widely used approaches to estimate population size and residency of marine organisms (i.e. the amount of time an organism spends in a given area). This is clearly the case for white sharks (13, 9, 10, 13). A number of scientists have used photographs of white sharks (e.g. their dorsal fins, scars) either during predatory events or at a chumming vessel (e.g. cage diving boat, research vessel) to estimate patterns of residency. For example, Klimley and Anderson [9] used visual identification of sharks at an aggregation site during predatory encounters with pinnipeds at the Farallon Islands, California to measure residency. Ferreira and Ferreira [10] used chum to attract sharks to a vessel and then collected tag-visual re-sighting data from Dyer Island and Struis Bay, South Africa. Visual sightings at a commercial cage diving vessel at the Neptune Islands, South Australia were used to describe the potential that the cage diving activity caused conditioning, and suggested that white sharks are temporal visitors to small scale aggregation sites moving through the sites quickly (Robbins R., pers. comm.). Around the world, researchers are beginning to compliment photo-ID with acoustic telemetry. Bruce et al. [28] used a VR2 acoustic telemetry array to assess residency of 22 tagged sharks at Dangerous Reef and the North and South Neptune Islands in Australia. Strong et al. [29] used both visual identification of individuals visiting a chumming vessel and acoustic tracking for estimating the abundance and residency of a population of white sharks in the lower Spencer Gulf, South Australia. More recently, Laroche et al. [30] used a radio acoustic positioning system in conjunction with photo-ID at Seal Island, False Bay in South Africa to assess the impact of chumming and this approach also estimated the presence/absence of white sharks. This research indicated that visual sightings did not accurately reflect visitation patterns of three individuals [30]. Adopting multiple methodologies may allow researchers to evaluate which approach maximizes precision and accuracy for a given objective (e.g. estimating residency of a species). In this paper we determined the probability of detection of white sharks from three different sampling approaches and examined if the data produced shows comparable results for residency patterns of white sharks. Then we examined how the sampling intensity affected the accuracy of residency data. Finally, using the results of our experiment, we discuss the advantages and disadvantages of these three approaches and propose how to effectively choose the appropriate method to collect accurate data on white sharks. Study area Mossel Bay is a small open bay situated along South Africas southern coast that is a known aggregation site for white sharks [31]. The bay is approximately 26 km wide and varies in its protection from the open sea. The topography of the bay is dominated by sand bottom, with distinct patches of coastal reef present. Within the inner bay, some 800 m offshore, a small island emerges from the sand bottom and hosts a small population of Cape fur seals (Arctocephalus pusillus pusill (...truncated)