Article title: Acoustic masking of blue whale calls by ship noise in Northern Chilean Patagonia

Technical Summary: Acoustic Masking of Blue Whale Calls by Ship Noise in Northern Chilean Patagonia

Problem Statement
Anthropogenic underwater noise poses a significant threat to baleen whales by reducing communication space through acoustic masking. Northern Chilean Patagonia (NCP) serves as a critical feeding ground for the Endangered Southeast Pacific blue whale (Balaenoptera musculus), yet this area experiences substantial overlap with marine traffic. While previous studies have documented the spatial overlap between whale habitats and vessel traffic in NCP, and have established the general mechanisms of noise masking, no prior research had quantified the specific impacts of vessel noise on the communication space of blue whales in this region. Furthermore, existing masking studies often rely on simulated animal movements rather than empirical data. This study addresses the gap in understanding the temporal and spatial variability of communication masking for Southeast Pacific blue whales under current shipping conditions and evaluates the potential efficacy of vessel speed reductions (8 kt and 10 kt limits) as a mitigation strategy.

Methodology
The study employed an acoustic masking modeling framework coupled with empirical data from four satellite-tagged blue whales (B12, B13, B14, and B15) and Automatic Identification System (AIS) vessel data collected between February 4 and June 16, 2019.

• Data Sources:

  • Whale Tracks: Real-time movement data from satellite tags were processed using a continuous-time correlated-random-walk model to account for observational error and oceanographic variables.
  • Vessel Data: AIS data provided vessel coordinates, class, length, and speed. Vessel tracks were decimated to 10-minute waypoints.
  • Environmental Data: Wind speed data were obtained from NCEP/NOAA to model natural ambient noise.

• Modeling Approach:

  • Communication Space Estimation: Based on the framework by Clark et al. (2009), the study calculated "sender communication space" (the maximum area where acoustic communication could occur) for two call types: long-range "song calls" (20–25 Hz) and short-range "D-calls" (38–88 Hz).
  • Propagation Model: A spreading-loss model intermediate between spherical and cylindrical spreading was used, incorporating bathymetry from GEBCO. Source levels were assumed at 174 dB for songs and 166 dB for D-calls.
  • Masking Calculation: Signal Excess (SE) was calculated for a grid of potential receivers surrounding the whale tracks. Communication Space Loss ($CS_{loss}$) and Masking ($M_s$) were derived by comparing communication space under natural wind noise conditions versus conditions including vessel noise.
  • Scenarios: The model simulated four conditions: (a) wind noise only; (b) wind + current vessel noise; (c) wind + vessel noise with a 10 kt speed limit; and (d) wind + vessel noise with an 8 kt speed limit.

Key Results

• Current Masking Levels: Under current vessel traffic conditions, ship noise reduced the communication space by an average of 25% for song calls and 47% for D-calls across the four tagged whales.
• Maximum Impact: Peak masking reached 68.4% for song calls and 97.3% for D-calls.
• Variability: Masking varied significantly over time and space, influenced by whale location, bathymetry, wind conditions, and vessel traffic density. D-calls consistently experienced higher masking than song calls due to their lower source levels and higher frequencies, which limit their propagation range.
• Vessel Size Impact: Vessels greater than 100 meters generated the highest median masking (29% for songs, 50% for D-calls). This was over four times higher than the masking generated by vessels 51–100 m for songs, and more than double for D-calls.
• Speed Limit Efficacy:
  • Implementing a 10 kt speed limit reduced median masking to 16–27% for song calls and 33–43% for D-calls.
  • Implementing an 8 kt speed limit further reduced median masking to 5–9% for song calls and 22–32% for D-calls, depending on the specific whale track.
  • For smaller vessel classes (<100 m), an 8 kt limit resulted in 0% masking for song calls.

Significance and Claims
The authors claim this study is the first to estimate communication masking using real blue whale movement tracks rather than simulated animats, providing a more realistic assessment of the temporal and spatial variability in the loss of communication space.

The paper concludes that:

1. Vessel speed reduction is an effective dual-purpose mitigation measure. Reducing speeds to 8 kt or 10 kt simultaneously decreases acoustic masking and lowers the risk of fatal ship strikes, particularly for the largest vessels which contribute most to both noise and mortality.
2. D-calls are disproportionately affected. The high masking of D-calls suggests that ship noise specifically impairs close-range social communication and foraging coordination, which are critical behaviors in this feeding ground.
3. Management Implications. Integrating speed management into conservation strategies for the NCP feeding ground could provide substantial ecological benefits. The study supports the implementation of speed limits (specifically 8 kt) as a viable strategy to mitigate the acoustic footprint of shipping in critical blue whale habitats.