Population ecology of spinner dolphins (Stenella longirostris) in an offshore resting habitat in the Red Sea

Abstract

Populations of coastal and semi-pelagic dolphins that frequently use well-defined sites and habitats provide a great opportunity to investigate their population ecology, which is considerably more challenging in wide-ranging offshore species. However, their inshore occurrence also exposes them to anthropogenic impacts such as dolphin-watch tourism, which pose conservation challenges. In the Egyptian Red Sea, spinner dolphins (Stenella longirostris) frequently visit a small offshore reef, Samadai reef, to rest during the day. The site is a popular swim-with-dolphins destination, regulated by a management plan since 2004. To investigate the population ecology of spinner dolphins at Samadai reef and assess the effectiveness of the current management measures, a 4-year study was undertaken between 2011 and 2014. Capture-recapture techniques applied to photo-identification data indicated a stable population of ~200 non-calf spinner dolphins, composed of long-term year-round residents of both sexes and transient individuals, with a high adult survival rate (0.99; SE=0.02). The probability of temporary emigration was low (0.15; SE=0.004), with females seemingly more philopatric than males. Photographic cross-matching with spinner dolphins seen in other resting areas indicates low-level interchange with neighbour reef-associated communities.

Female spinner dolphins appear to reach sexual maturity at 8-10 years, typically having a 2-year calving interval. The presence of tooth rake marks on females suggests coercive male sexual behaviour. There is one well-pronounced annual calving peak between June and August, which coincides with warmer water temperatures. Calf survival to 1-year of age was generally high (0.79; 95%CI 0.73-0.87), but was significantly lower for calves born to younger females than older females (0.61, 95%CI 0.43-0.87 and 0.80, 95%CI 0.71-0.90, respectively). The crude birth rate (0.10; SD=0.016), the annual recruitment rate (0.08; SD=0.007) and the fecundity rate (0.20; SD=0.018) indicate a healthy population.

The Red Sea spinner dolphins showed pigmentation features apparently unique to the region. Adults were found to be sexually dimorphic. Underwater photogrammetry showed that female and male individual growth began to diverge at the length of ~151 cm, and that physical maturity was attained at the length of ~165 cm for females and ~180 cm for males, at approximately 7 and 10 years of age, respectively.

The findings of this study confirmed the importance of Samadai reef as resting habitat for resident and transient spinner dolphins and indicated that the animals are not subject to obvious natural or anthropogenic stressors. The currently implemented management measures appear to be adequate and effective, although some modifications to the management practice are suggested. However, as long-term effects of human activities may take some time to emerge, further long-term monitoring and research is strongly recommended to ensure that expansion of tourism will not menace the viability of spinner dolphins at Samadai reef and more broadly in the Egyptian Red Sea. Resolution of apparent taxonomic uncertainties of the Red Sea spinner dolphins is recommended, as it is important in addressing conservation issues and informing management decisions at an appropriate geographic scale.