Estimating the effect of mid-frequency active sonar on the population health of Blainville's beaked whales (Mesoplodon densirostris) in the Tongue of the Ocean

Abstract

Passive acoustic methods were used to study the effect of mid-frequency active sonar (MFAS) on a population of Blainville’s beaked whales (Mesoplodon densirostris, Md) at the U.S. Navy Atlantic Undersea Test and Evaluation Centre (AUTEC), Bahamas. AUTEC contains an array of bottom-mounted hydrophones that can detect Md echolocation clicks. Methods to estimate abundance, the risk of behavioural disruption, and the population level effect of repeated MFAS exposure are presented. A passive acoustic abundance estimation method, a parametric equation that predicts the probability of foraging dive disruption as a function of MFAS received level and an Md bioenergetics model were developed. The effect of changes in energy flow on the demographic characteristics of an Md population were explored. Passive acoustic data from AUTEC were used to estimate the behavioural disturbance resulting from sonar operations; combined with the bioenergetic model, this suggested that the effect of sonar operations could cause an increase in a female’s age at maturity, a longer inter-calf-interval, calf survival rate and probability of giving birth that could in turn result in a declining population. Model results were also compared to those from an expert elicitation study. A power analysis for predicting abundance via passive acoustic and visual monitoring were used to inform recommendations for a long-term monitoring plan that includes passive acoustic monitoring of Md abundance combined with a photo-identification study at both AUTEC and a reference site (Abaco) to improve demographic rate estimates.