Where Dolphins Sleep: Resting Areas in the Red Sea

Abstract

Periods of physiological quiescence are ubiquitous in animals. Resting is a vital, vulnerable and delicate phase of reduced vigilance to external stimuli that, in all animals, includes sleep components. In dolphins, resting is characterised by low activity and mobility, and sleep is exclusively unihemispheric slow wave sleep (USWS), an arrangement compatible with the voluntary respiratory function. Physiological needs and ecological conditions affect the way individuals arrange their behaviour during the photoperiod in order to accommodate the desirable but incompatible resting, foraging, mating, travelling needs and opportunities, to optimise benefits and minimise fitness costs. The duration and quality of rest strictly depend on the surrounding environmental conditions and the phase is susceptible to interruptions and disruptions. If animals are chronically deprived of rest and sleep, the cumulative effects of the deprivation can impact individual physiology and cognitive abilities, to the extent that the viability of individuals and their populations may be compromised. Dolphin-based tourism operations affect resting and sleeping patterns in a number of species and can lead to short-term behavioural responses, as well as long-term detrimental consequences, on wild dolphin populations. Among the Red Sea species, the spinner and Indo-Pacific bottlenose dolphins display diurnal resting patterns inside, or in proximity to, coastal reefs. This generates a situation of high conservation concern as these species become not only more accessible for the tourism industry, but also more heavily exposed to it during a critical phase. In these circumstances, a precautionary approach is required. The spinner dolphin resting behaviour is well described and provides an interesting and comprehensive case study on the management of human interactions on resting dolphins. The island-associated ecotype of the spinner dolphin (Stenella longirostris) feeds exclusively at night and retreats to bays and lagoons to rest during the daytime. Resting areas have been reported in Hawaii, Brazil, Fiji, and in the Red Sea off Egypt, Sudan and Saudi Arabia. The spatial-temporal constraints on resting and the scarce behavioural plasticity make the spinner dolphin particularly vulnerable to rest disruptions. Indeed, the long-established tourism industry was held responsible for population decline and changes in habitat use in the Hawaiian dolphin population. In the Red Sea, the scientific investigation of impacts is still preliminary, but the establishment and success of the Samadai Reef specially managed area in Egypt shows that science-informed, precautionary and pragmatic management of dolphin-based activities is possible even in data poor contexts. Rest being a vital life function and given the dependence of dolphins on specific selected sites, resting areas surge to a status of highly critical habitats. Adequate investigation of impacts and management of anthropogenic activities inside resting areas and in their proximity are therefore priorities and key actions for the conservation of wild dolphin populations.