Abstract
Baited Remote Underwater Video Stations (BRUVS) are widely used for monitoring relative abundances of fishes, especially sharks, but only the maximum number of individuals seen at any one time (MaxN) is usually recorded. In both the Cayman Islands and the Amirante Islands, Seychelles, we used photo-ID to recognise individual sharks recorded on BRUVS videos. This revealed that for most species the actual numbers of separate individuals (IndN) visiting the BRUVS were significantly higher than MaxN, with, for example, ratios of IndN to MaxN being 1.17 and 1.24 for Caribbean reef, Carcharhinus perezi, and nurse, Ginglymostoma cirratum, sharks in the Cayman Islands, and 2.46 and 1.37 for blacktip reef, C. melanopterus, and grey reef, C. amblyrhynchos, sharks, respectively, in the Amirantes. Further, for most species, increasing the BRUVS deployment period beyond the 60 min normally used increased the observed IndN, with more than twice as many individuals in the Cayman Islands and >1.4 times as many individuals in the Amirantes being recorded after 120 min as after 60 min. For most species, MaxN and IndN rose exponentially with time, so data from different deployment periods cannot reliably be compared using catch-per-unit-effort (CPUE) calculated as catch-per-unit-time. In both study areas, the time of first arrival of individuals varied with species from <1 min to >2 h. Individually identifiable sharks were re-sighted after up to 429 days over 10 km away in the Cayman Islands and 814 days over 23 km away in the Amirantes, demonstrating that many individuals range over considerable distances. Analysis of Cayman re-sightings data yielded mean population estimates of 76 ± 23 (SE) and 199 ± 42 (SE) for C. perezi and G. cirratum, respectively. The results demonstrate that, for sharks, the application of both photo-identification and longer deployment periods to BRUVS can improve the precision of abundance estimates and provide knowledge of population size and ranging behaviour.
Highlights
Many species of shark have suffered a dramatic decline in abundance over the past years [1,2,3], principally as a result of by-catch and finning in response to the demand for shark fin by the Far-East restaurant trade [4,5]
Underwater Video Stations (BRUVS), underwater video cameras set to record the fishes attracted to bait, is the method most frequently used for surveying and monitoring sharks in particular [10,11,12]
The results of the present study demonstrate that the photo-identification of individual sharks can be applied to Baited Remote Underwater Video Stations (BRUVS) videos and that in most species this approach can result in a greater ability to distinguish between different abundances
Summary
Many species of shark have suffered a dramatic decline in abundance over the past years [1,2,3], principally as a result of by-catch and finning in response to the demand for shark fin by the Far-East restaurant trade [4,5]. As a result, monitoring the population abundance of sharks is much needed [9]. Underwater Video Stations (BRUVS), underwater video cameras set to record the fishes attracted to bait, is the method most frequently used for surveying and monitoring sharks in particular [10,11,12]. Cappo et al [14] have reviewed
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