Underwater Ambient Noise in a Baleen Whale Migratory Habitat Off the Azores

Miriam Romagosa,Marc O Lammers,Mónica A Silva,Tiago A Marques,Irma Cascão,Eva Giacomello,Nathan D Merchant

doi:10.3389/fmars.2017.00109

Abstract

Assessment of underwater noise is of particular interest given the increase in noise-generating human activities and the potential negative effects on marine mammals which depend on sound for many vital processes. The Azores archipelago is an important migratory and feeding habitat for blue (Balaenoptera musculus), fin (Balaenoptera physalus) and sei whales (Balaenoptera borealis) en route to summering grounds in northern Atlantic waters. High levels of low frequency noise in this area could displace whales or interfere with foraging behavior, impacting energy intake during a critical stage of their annual cycle. In this study, bottom-mounted Ecological Acoustic Recorders were deployed at three Azorean seamounts (Condor, Acores and Gigante) to measure temporal variations in background noise levels and ship noise in the 18-1,000 Hz frequency band, used by baleen whales to emit and receive sounds. Monthly average noise levels ranged from 90.3 dB re 1 μPa (Acores seamount) to 103.1 dB re 1 μPa (Condor seamount) and local ship noise was present up to 13% of the recording time in Condor. At this location, average contribution of local boat noise to background noise levels is almost 10 dB higher than wind contribution, which might temporally affect detection ranges for baleen whale calls and difficult communication at long ranges. Given the low time percentatge with noise levels above 120 dB re 1μPa found here (3.3 % at Condor), we woud expect limited behavioural responses to ships from baleen whales. Sound pressure levels measured in the Azores are lower than those reported for the Mediterranean basin and the Strait of Gibraltar. However, the currently unknown effects of baleen whale vocalization masking and the increasing presence of boats at the monitored sites underline the need for continuous monitoring to understand any long-term impacts on whales.

Highlights

Marine acoustic pollution has become an issue of special concern over recent decades
The only anthropogenic noise source found in the recordings was ship noise, which had the maximum energy above 100 Hz for boats with higher noise levels, or in the bandwidth of 10–100 Hz for boats with lower noise levels
The best compromise considering a minimum of 90% of visually confirmed boats in the spectrogram detected by the Adaptive Threshold Level (ATL), and a maximum of 5% of false positives was obtained using a threshold ceiling C = 4 dB for duty cycle of 3,600 s every 12,600 s, and C = 8 dB for the 90 and 30 s duty cycle (Table 2)

Summary

Introduction

Marine acoustic pollution has become an issue of special concern over recent decades. Underwater Ambient Noise in Azores noise increasing at an average rate of 2.5–3 dB per decade at 30– 50 Hz since the 1960s (Andrew et al, 2002; McDonald et al, 2006; Chapman and Price, 2011). This rise has been mainly due to shipping and together with seismic surveys has become one the principal sources of ambient noise below ∼1 kHz. When a ship passes nearby, it increases temporarily and substantially noise levels at that location at much greater frequencies since propagation removes the high frequency portion of the spectrum (Wenz, 1962; Hildebrand, 2009)

Methods

Results

Conclusion