SCUBA diver observations and placard tags to monitor grey reef sharks, Carcharhinus amblyrhynchos, at Sha'ab Rumi, The Sudan: assessment and future directions

Abstract

Establishing baseline data on the abundance of threatened shark species is critical for monitoring site- and region-specific population tends over time. This is of particular importance for monitoring sharks at remote locations or in regions where there are no reliable data on shark numbers, fishing effort and current population status. Through establishing a standardized recreational SCUBA diver observation programme, this study examined the number, size and sex-composition of grey reef sharks, Carcharhinus amblyrhynchos, on a remote coral reef system off the Red Sea coast of Sudan. In addition, placard tags were attached to individual sharks to examine coarse scale residency and movement patterns and to determine the effectiveness of this technique. Over a 4.5 month period (December 2007–April 2008), a mean (±SE) of 5.9 ± 0.3 grey reef sharks were observed per diving day with peak numbers of sharks associated with temperatures of 26–26.9°C and strong currents. Estimated mean (±SE) total length of observed sharks was 1.9 ± 0.03 m identifying that most animals were mature. Female sharks were dominant on the site and pregnant females were recorded. Placard tagged sharks (N = 4) were observed by recreational SCUBA divers throughout the study period (23.1%, 20.0%, 16.9% and 3.1% of total observation diving days) indicating sporadic site attachment. The placard tags remained intact and were free of fouling for a total of 175 days. The numbers of grey reef sharks seen on this Red Sea coral complex suggest a healthy, relatively unexploited population. This study demonstrates that the recreational diver community, which forms a large pool of skilled volunteers, can generate baseline data on shark numbers at regularly dived sites and provide insights into the ecology of the observed species. Modification of placard tags, including attachment to the dorsal fin and time corrodible release systems may provide an inexpensive and accepted tool for monitoring individual shark residency and movement patterns. Engaging the recreational SCUBA diver community in a standardized scientific monitoring programme has the potential to monitor trends in shark populations over large spatial and temporal scales.