The IUCN Red List of Threatened Species
Sousa teuszii, Atlantic humpback dolphin, Humpback dolphin, West Africa, conservation status, Bycatch, Threat assessment
Critically Endangered: Justification: As with other species in the genus, Sousa teuszii has very precise habitat requirements, limiting its resilience and ability to escape environmental and anthropogenic stressors (Davidson et al. 2011). Remnant populations are faced with a suite of “largely intractable threats” (Ayissi et al. 2014) that either affect them directly, or contribute to the rapid and widening deterioration of their inshore habitats (Perrin and Van Waerebeek 2007, Van Waerebeek et al. 2004, Weir et al. 2011). Although there are no empirically derived abundance estimates for either the species as a whole or any of the putative populations, the available information indicate that these dolphins occur in very low and declining numbers throughout most or all of their range. Most populations for which any data are available are extremely small and several appear to be isolated. Available published estimates indicate that the total species’ population size plausibly falls well below 3, 000 individuals, suggesting that the number of mature individuals is less than 1, 500 (following Taylor et al. 2007). Declines have been observed or are suspected for every known population, and continued declines are considered inevitable given the ongoing expansion of all identified threats throughout the species’ known range. Bycatch in fisheries, the principal cause of the declines, has been identified or suspected everywhere the species has been studied. Directed killing has also been identified or suspected in several areas, and major threats to habitats (including ports) are increasingly prevalent (Perrin and Van Waerebeek 2007; Van Waerebeek et al. 2004, 2015; Collins 2015). Ecosystem impacts that compromise vital rates may also have deleterious effects, including increasing the susceptibility of populations’ to environmental stochasticity (Moore 2015, Weir et al. 2011). Appropriate management interventions that limit habitat loss and mortality from bycatch and hunting are limited or entirely lacking across the range, and in the absence of targeted and sustained conservation management efforts, long-term prospects appear grim. The available data for other species in the genus can be used to infer that S. teuszii has a low reproductive rate and thus a low intrinsic potential for population increase (Jefferson and Rosenbaum 2014, Moore 2015, Taylor et al. 2007). Given the small apparent population sizes, any mortality over and above natural rates is likely to lead to appreciable declines in abundance. Moore (2015) estimated that given an inferred generation time of 25 years (as estimated for S. plumbea and S. chinensis), an average annual adult mortality rate of 5.3% would lead to an 80% decline over 75 years (three generations). Limited data for some areas (e.g., Congo) clearly demonstrates that non-natural mortality (particularly captures) is high and when considered alongside the scale of anthropogenic pressures, a population reduction of more than 80% over three generations is highly likely. The available information, much of it characterized by high levels of uncertainty, suggests that the Atlantic Humpback Dolphin merits classification as Critically Endangered (CR) under criteria A3cd+4cd. For the A criterion, a reduction of more than 80% in the total population over three S. teuszii generations (~75 years) is suspected, with declines likely to have begun with the rapid expansion of West African coastal fisheries during the 1980s, and bycatch likely to increase as new areas are targeted and fishery pressures increase. The reduction has not ceased, nor have its causes – nor is there any reason to think they will in the foreseeable future. The inference and suspicion of the large decline in population size are based on the declining quality of the species’ habitat (subcriterion c) and its vulnerability to mortality in artisanal fisheries (subcriterion d).