Northern bottlenose whales in a pristine environment respond strongly to close and distant navy sonar signals

Abstract

Impact assessments for sonar operations typically use received sound levels to predict behavioural disturbance in marine mammals. However, there are indications that cetaceans may learn to associate exposures from distant sound sources with lower perceived risk. To investigate the roles of source distance and received level in an area without frequent sonar activity, we conducted multi-scale controlled exposure experiments ( n = 3) with 12 northern bottlenose whales near Jan Mayen, Norway. Animals were tagged with high-resolution archival tags ( n = 1 per experiment) or medium-resolution satellite tags ( n = 9 in total) and subsequently exposed to sonar. We also deployed bottom-moored recorders to acoustically monitor for whales in the exposed area. Tagged whales initiated avoidance of the sound source over a wide range of distances (0.8-28 km), with responses characteristic of beaked whales. Both onset and intensity of response were better predicted by received sound pressure level (SPL) than by source distance. Avoidance threshold SPLs estimated for each whale ranged from 117-126 dB re 1 µPa, comparable to those of other tagged beaked whales. In this pristine underwater acoustic environment, we found no indication that the source distances tested in our experiments modulated the behavioural effects of sonar, as has been suggested for locations where whales are frequently exposed to sonar.

Keywords: Hyperoodon ampullatus; cetacean; disturbance; marine mammal; military sonar.

Figure 1.

(a) Large-scale view. (b) Jan Mayen study area with all deployment locations (DTAG, focal whale; SAT, satellite-tagged whale). (c) Map of experiment 2016-1 with tracks of the tagged northern bottlenose whales before, during and up to 24 h after exposure. For satellite tags, colour-coding indicates the most likely sequence of states: state 1, tortuous movement; state 2, low-speed directional movement; and state 3, high-speed directional movement. (d) Detailed view of the track of the focal animal during exposure. Maps were created using the equidistant conic projection and GEBCO_2014 bathymetry data (www.gebco.net).

Figure 2.

DTAG time-series data from a northern bottlenose whale that underwent controlled exposure to naval sonar during experiment 2016-1. The arrow indicates the middle of the first 15 min averaging window that reached the threshold criterion (i.e. the change-point) of the MD metric for avoidance movement.

Figure 3.

Dive summary (black) and regular depth (grey) profiles measured by satellite tags on northern bottlenose whales around the time of experiments (a) 2016-1 and (b) 2015-2. Panels are shown in order of mean distance during the exposure. Hourly state predictions based upon modelling of horizontal movement are indicated at the bottom of the panel, with the same colour-coding as in figure 1c. Note that most depth data were missing for ID134669. Also note the similarity in dive patterns between ID161588 and ID161590 and between ID161592 and ID161593, suggesting these whales were associated.

Figure 4.

Avoidance response threshold SPLs (large symbols) for responses during sonar experiments versus source distance, for northern bottlenose whales carrying a DTAG (n = 4; less than 5 km and at 17 km) or a satellite tag (n = 8). Response thresholds were determined to be independent (filled square), from associated animals ID161588 and ID161590 (filled inverted triangle), associated animals ID161592, ID161593 and ha16_170a (filled triangle) or were from the 2013 experiment (filled diamond [12]). For satellite tags, the marker position indicates the maximum SPL and mean distance during exposure, vertical error bars a 90% confidence interval and horizontal error bars a min/max range. The data are detailed in electronic supplementary material, table S2. Small symbols indicate data for individual sonar pulses, with arrows showing their temporal progression. Whales can only respond if a signal is audible; therefore, the electrophysiological hearing threshold (dotted line) of another Ziphiid (Blainville's beaked whale [26]) is also shown.