The fiord is home to one of the southernmost populations of bottlenose dolphins. The Doubtful Sound bottlenoses have formed a very insular subgroup of only about 70 individuals, with none having been observed to leave or enter the Sound during a multi-year monitoring regime. Their social grouping is thus extremely close, which is also partly attributed to the difficult and unusual features of their habitat, which is much colder than the habitats of other bottlenose groups and is also overlaid by the freshwater layer. Recently (2000s), there has been growing concern that the population is in significant decline, with calf survival rates having halved, as well as being noticeably lower than in captivity or in other New Zealand environments. The reasons for this are unclear, though increased tourism and the fresh-water discharge from the Manapouri Power Station (see Hydroelectricity section below) are considered potential causes.