Reproductive dynamics of an inshore delphinid reflect demographic consequences of large-scale coastal constructions

Abstract

In fast-developing regions, rapid coastal urbanization has led to considerable transformation of both landscapes and seascapes, with various adverse implications for species dependent on inshore habitats. As reliable estimates of vital demographic rates reflect the population-level responses to stressors, rigorous quantification of these parameters is central to assessing the consequences of human-induced environmental change. As a case-in-point, we quantified the reproductive parameters and calf survivorship of an obligatory inshore delphinid, the Indo-Pacific humpback dolphin (Sousa chinensis), in the anthropogenic seascape of Hong Kong prior and during two consecutive large-scale coastal infrastructure projects. Our 8-year photo-ID mark-recapture dataset was examined with both frequentist and Bayesian techniques. The reproductive output of female dolphins worsened significantly following the onset of construction activities, eventually declining alarmingly close to zero. Annual recruitment rate followed a similar trajectory. Concurrently, calf survival rates (pooled across the study period) were low, with <60% of newborns surviving the first 3-month and <46% surviving their first year of life. Both mean crude birth rate (~0.03) and annual recruitment rate (~0.02) were among the lowest reported for coastal delphinids to date. Cumulatively, all of this slows population recruitment and increases population susceptibility to stochastic events. We conclude that natural factors alone are not likely to cause such critically low demographic rates, underscoring the vulnerability of obligatory inshore species to growing anthropogenic pressure and habitat loss in fast-changing coastal seas. We emphasize the need for proactive (instead of reactive) conservation based on reliable sentinel model system and wise use of the precautionary principle.