Body length and growth pattern of free-ranging Indo-Pacific bottlenose dolphins off Mikura Island estimated using an underwater 3D camera

Indo-Pacific bottlenose dolphins (Tursiops aduncus) around Mikura Island are important both commercially (swim-with-dolphin programs) and scientifically (long-term underwater behavioral studies). However, this population experienced a substantial population decline (31% of identified dolphins) between 2008 and 2011, which prompted us to monitor population health using body length. A decrease in the growth rate of neonates and calves is a warning sign of unhealthy conditions in the population. This study examined the total length of free-ranging Indo-Pacific bottlenose dolphins off Mikura Island, using a low-cost commercially available 3D underwater camera system. Length-at-age data from 129 measurements of 108 identified dolphins were successfully obtained and were best described by the Richards growth model compared to the Gompertz and von Bertalanffy models. Body length did not differ significantly between females and males, with an estimated population asymptotic length of 246.9 cm (95% confidence interval: 241.7-252.7 cm). Calves were approximately 100 cm in length at birth and reached 178.2 cm at 1 year of age and 208.6 cm at 3 years when many calves became independent from their mothers. Length-at-age estimates of the Mikura Island population are similar to and greater than those reported in southwestern Australia and Shark Bay, respectively. Our simple non-invasive underwater technique demonstrated to be effective in quantifying the growth pattern in a free-ranging dolphin population without using dead or stranded specimens, which provides essential information for monitoring of dolphin populations and sustainable swim-with-dolphin programs.

Automated summary

This study used a low-cost 3D underwater camera system to measure the body length of Indo-Pacific bottlenose dolphins around Mikura Island and analyzed their growth pattern using the Richards growth model. The results showed that the body length was similar to or greater than that of other dolphin populations in different areas. The non-invasive technique used in this study provides important information for monitoring dolphin populations and sustainable swim-with-dolphin programs.