Evidence of the Atlantic nurse shark (Ginglymostoma cirratum) population shrink at Rocas Atoll, Southwestern Atlantic

Abstract

Abstract The Atlantic nurse shark, Ginglymostoma cirratum, was recently listed as Vulnerable by IUCN Red List, however, as highlighted, little is known about its populations in the Southwestern Atlantic. Therefore, population studies are essential to propose status evaluations and new conservation measures. Natural marks present on the animals' bodies are used as an alternative non‐invasive method rather than the usual tagging system for individual identification, thus avoiding the need for animal handling. This study aimed to evaluate the population size trend of G. cirratum at the Rocas Atoll Marine Biological Reserve, the only atoll located in the South Atlantic. Data were collected through underwater filming, photo‐identification, and visual censuses. Three expeditions were carried out in 2018 and the abundance of sharks was sampled in the inner part of Rocas Atoll Marine Biological Reserve. Sharks were identified individually with the aid of the software Interactive Identification System – Contour (I3S). Based on the re‐sightings of identified animals, two capture–recapture estimators of population size were used (Petersen–Bailey; Jolly–Seber). The estimates were then compared to the only previous estimate made 20 years before, which employed the same number of expeditions and estimators. A total of 139 sharks were identified from 444 sightings in 63 h of diving effort. Population estimates varied from 200 to 205 sharks, which are significantly lower than the previous estimate (339–368 sharks) made 20 years ago, suggesting a demographic decline in Southwestern Atlantic populations. Population studies and movement of G. cirratum are essential to inform the implementation of additional protective measures, including the establishment of protected corridors between marine protected areas and the recovery of G. cirratum populations. It is crucial to consider that the distribution of individuals may extend beyond the borders of the non‐take marine protected area. Therefore, the observed decline in the local population could potential indicate a broader decline across the Southwestern Atlantic.