Population ecology of Blainville's beaked whales (Mesoplodon densirostris)

Abstract

Quantifying population demographics is necessary to analyse the status of wildlife populations and to support effective conservation and management. Such a need exists for beaked whales which are vulnerable to anthropogenic noise, including navy sonar. Here, population demographics were estimated for Blainville’s beaked whales (Mesoplodon densirostris) in The Bahamas and the potential population-level effects of sonar investigated. Mark-recapture models were fitted to photo-identification data collected at the US Navy’s Atlantic Test and Evaluation Centre (AUTEC) where sonars were used regularly and 170 km away at Abaco where sonar use was limited, with the exception of a navy exercise correlated with a stranding of beaked whales in 2000. Life history data collected from 1997-2011 revealed that onset of sexual maturity occurred at age nine for both males and females and minimum longevities were 23 years. The annual turnover of individuals at Abaco was supported by the estimation of a larger parent population. However, adult females showed high site fidelity and survival, while adult males’ occupancy patterns were different, making survival of males difficult to separate from permanent emigration. Average annual abundance was lower at AUTEC when compared to a same-sized area at Abaco. Despite a similar number of adult females at both sites, a higher female:calf ratio was found at AUTEC, suggesting lower recruitment through births may have contributed to lower abundance. Population demographics in Abaco changed after the 2000 stranding; abundance and temporary emigration increased then returned to pre-2000 levels remaining stable thereafter. Two stranded whales re-floated in 2000 were later re-sighted having survived exposure to sonar and the physiological stresses related to stranding. This work provides evidence of a possible population-level effect of sonar use at a navy range and during a multi-ship exercise, emphasising the valuable role that longitudinal studies play in monitoring impacts of anthropogenic activities.