Long-term sound and movement recording tags to study natural behavior and reaction to ship noise of seals

Abstract

The impact of anthropogenic noise on marine fauna is of increasing conservation concern with vessel noise being one of the major contributors. Animals that rely on shallow coastal habitats may be especially vulnerable to this form of pollution.Very limited information is available on how much noise from ship traffic individual animals experience, and how they may react to it due to a lack of suitable methods. To address this, we developed long-duration audio and 3D-movement tags (DTAGs) and deployed them on three harbor seals and two gray seals in the North Sea during 2015-2016.These tags recorded sound, accelerometry, magnetometry, and pressure continuously for up to 21 days. GPS positions were also sampled for one seal continuously throughout the recording period. A separate tag, combining a camera and an accelerometer logger, was deployed on two harbor seals to visualize specific behaviors that helped interpret accelerometer signals in the DTAG data.Combining data from depth, accelerometer, and audio sensors, we found that animals spent 6.6%-42.3% of the time hauled out (either on land or partly submerged), and 5.3%-12.4% of their at-sea time resting at the sea bottom, while the remaining time was used for traveling, resting at surface, and foraging. Animals were exposed to audible vessel noise 2.2%-20.5% of their time when in water, and we demonstrate that interruption of functional behaviors (e.g., resting) in some cases coincides with high-level vessel noise. Two-thirds of the ship noise events were traceable by the AIS vessel tracking system, while one-third comprised vessels without AIS.This preliminary study demonstrates how concomitant long-term continuous broadband on-animal sound and movement recordings may be an important tool in future quantification of disturbance effects of anthropogenic activities at sea and assessment of long-term population impacts on pinnipeds.

Keywords: DTAG; anthropogenic noise; behavioral response; biologging; exposure rates; gray seal; harbor seal; long‐duration acoustic dataloggers.

Figure 1

Sleeping gray seal with a DTAG3 on Helgoland May 2015. Photo: Sabine Schwarz

Figure 2

(a) Audio from the full DTAG‐3 deployment on gray seal gs15_139b, displayed as RMS sound pressure level in third‐octave bands (TOL, 1–32 kHz) averaged over 30 s. The tag did not collect data during the three uniform white intervals due to technical issues. (b–f) Data from a single day, where (b) indicates when ship noise is audible; (c) third‐octave levels (TOLs); (d) dive profile; (e) acceleration along the animal's x‐ (surge, forwards‐backwards), y‐(sway, side to side), and z‐(heave, up and down) axis; (f) jerk calculated as the differential of the three acceleration axes. Periods when the seal is resting at sea, corresponding to low jerk levels, are indicated in the graph. Time is in UTC.

Figure 3

Track of harbor seal hs16_265c, color‐coded with 30‐s averages of third‐octave levels (TOL) in the 1‐kHz‐centered band (see Materials and Methods). Gray dotted lines indicate Automatic Identification System (AIS) tracks of ships that at some point pass within 5 km of the seal (roughly corresponding to the expected range of audibility) and red dots mark the positions of wind farms

Figure 4

Behavior of gs15_139b before, during, and after the ship encounter on 24 May 2015 indicated in Figure 2f. (a–c) data over a 75‐min period centered on the vessel pass. (a) Received power spectrum density level; (b) dive profile; (c) acceleration profile; (d–f) zoomed‐in view of the same data during 9 min of the vessel pass. Notice the regular dive/resting pattern that is interrupted when ship noise increases

Figure 5

(a, c) Frames from the video recordings obtained with camera tags on two harbor seals (part of the seals are seen in the lower part of each picture); b and d) the associated acceleration profiles, where the arrows indicate the specific time of the snap‐shots. Both images were taken from periods where the seals were resting on the seafloor. The animals are rolling from side to side presumably with the wave cycles (most obvious in d as small oscillations in the y (sway) axis). See the associated video (Supporting information Audio S1) to image (a) in the Supporting Information

Figure 6

An example of a change in behavior of gray seal gs15_139b presumably caused by vessel disturbance. In the dive beginning at 18:55, the seal descends deeper than in previous dives, presumably to the bottom where the seal remains during the vessel pass. After this fast moving boat passes, the seal resumes its previous diving style. (a) Received power spectrum density level; (b) depth profile; (c) acceleration profile

Figure 7

Example of disturbance during haul out of a harbor seal (hs15_069a) on 18 March 2015. (a) Received power spectrum density level; (b) acceleration profile