First estimates of population size and home range of Caribbean reef and nurse sharks using photo-identification and BRUVS

Abstract

The assessment of parameters population size and individual home range is important for effective conservation management of sharks. This study uses the novel application of photo identification (photo-ID) to BRUVS footage as a non-invasive alternative to tagging in order to generate individual capture histories. These were used in mark-recapture models to estimate effective population sizes and to determine home ranges. In the Cayman Islands a total of 499 shark sightings of six coastal shark species were recorded on BRUVS from 2015 - 2018, but re-sighting rates were only sufficient for the determination of population parameters for two species - Caribbean reef shark (Carcharhinus perezi) and nurse shark (Ginglymostoma cirratum). The calculated super-population sizes for Caribbean reef shark (180 ± 37 SE) and nurse shark (336 ± 61 SE) were greater than the estimates for each species based on a closed-population model (Caribbean reef shark: 128 ± 40 SE, nurse shark: 249 ± 48 SE), though both measures indicated that there were about twice as many nurse sharks (1.3 - 1.8 sharks/km2) as Caribbean reef sharks (0.7 – 1 shark/km2) within the study area. The demographic compositions included numerous immature individuals, indicating that breeding of both species takes place within the study area of 188 km2. Most recognizable individuals of both species showed linear home ranges of <20 km, but a few individuals were observed to have moved longer distances (Caribbean reef shark: 125.37 km, nurse shark: 156.07 km). The data indicate that the home ranges and long-distance movements of individual sharks observed within the islands’ marine protected areas (MPAs) often extend to areas beyond the MPA’s boundary, potentially exposing them to fishing activities. This study provides the first estimates of population size for Caribbean reef and nurse sharks in the Cayman Islands and the first estimate of a Caribbean reef shark population globally.

Individual residency behaviours and seasonal long-distance movements in acoustically tagged Caribbean reef sharks in the Cayman Islands

“…While a suggested year-around presence of Caribbean reef sharks in the Cayman Islands was found, this study also demonstrated that individual Caribbean reef sharks exhibit occasional long-distance (> 100 km) movements and across deep (up to 2,000 m) water. These long-distance movements by some sharks between the different Cayman Islands were also evident from parallel work using photo-identification of individuals recorded on BRUVS [22]. Large scale movements (> 100 km) have been observed for other reef-associated species including grey reef shark (250 km [86], 134 km: [53]), blacktip reef shark (138 km: [88]) and bull shark (Carcharhinus leucas) (650 km: [49], 2,000 km [89]) and some studies suggested that Caribbean reef sharks might also be more dispersive than previously thought [11,23].…”

“…Evidence from previous acoustic telemetry studies suggests that Caribbean reef sharks are highly resident and exhibit strong site-fidelity to particular areas [2,[18][19][20][21]. Other recent data suggest that this species may be more mobile and dispersive than previously thought [2,20,22], with individuals undertaking excursions far beyond the typical, relatively small linear home range (< 20 km) and including occasional long-distance movements (>50 km) across open water [11,23,24] up to 700 m deep [25]. Furthermore, this species may play an important role in habitat linkages through both diel and ontogenetic habitat shifts, moving from shallower to deeper water between night and day and with increasing individual size [26][27][28].…”

“…The present study was undertaken in the Cayman Islands on the Caribbean reef shark, one of the most abundant species in the coastal waters of the wider Caribbean, including Cayman [22,24]. Prior to the beginning of shark research in Cayman in 2009 [24], local knowledge of the species consisted only of catch records [29][30][31] documenting the decline of shark populations of a former shark fishery (up to the 1960s).…”

The effectiveness of the semi‐automated Identifin software to accurately fast‐track dorsal fin photographic identifications of sharks

“…As a result, knowledge of the biology and demography of sharks is often limited. To overcome some of these challenges, the use of photographic identification (photo‐ID) can provide non‐invasive and affordable solutions that enable individual recognition based on distinctive natural markings captured on photographs or video cameras (Anderson et al, 2011; Kohler et al, 2023; Marshall & Pierce, 2012; Pierce et al, 2018).…”

“…Using either a combination of identification features or only one feature, the use of photo‐ID can provide insights into how populations change at the level of individuals: photo‐ID has been used to study the spatial movement patterns of grey nurse sharks, Carcharias taurus (Barker & Williamson, 2010); Caribbean reef sharks, Carcharhinus perezi ; nurse sharks, Ginglymostoma cirratum (Kohler et al, 2023); sevengill sharks, Notorynchus cepedianus Lewis et al, 2020); and whale sharks, Rhincodon typus (Holmberg et al, 2009). Photo‐ID also enabled estimates of population demography using capture‐mark‐recapture (CMR) analyses (also known as photographic mark‐recapture; Meekan et al, 2006; Dudgeon et al, 2008; Towner et al, 2013; Andreotti et al, 2016; Gore et al, 2016).…”