Exposure and response of satellite-tagged Blainville's beaked whales to mid-frequency active sonar off Kaua'i, Hawai'i

Abstract

Background: Beaked whale response to Navy sonars is a global concern due to past stranding events coinciding with training activity. Often, controlled exposure experiments involve tagging cetaceans with short-term, high-resolution tags and exposing them to relatively short, single bouts of mid-frequency active sonar (MFAS). In contrast, longer-duration satellite-transmitting tags deployed around Navy ranges enables behavioral response studies of animals exposed to realistic Navy training activities over extended periods and spatial scales, with multiple exposures to different sources.

Methods: To study their behavior relative to extended periods of realistic Navy training, satellite-transmitting tags were deployed on four Blainville's beaked whales (Mesoplodon densirostris) on the Pacific Missile Range Facility (PMRF) off Kaua'i. Tags were deployed in 3 years, ahead of Submarine Command Courses (SCCs) with multiple sources of MFAS. Dive behavior of two tagged together were compared to acoustically detected group vocal periods (GVPs) on the range. Pre-exposure dive behavior metrics were compared to those during exposures. Horizontal movement behavior metrics were analyzed using Kruskal-Wallis non-parametric and Tukey-Kramer multiple comparison tests.

Results: Two whales remained together and highly synchronized in their dive and movement behavior until the onset of MFAS, at which time they appeared to separate. Twenty-three deep foraging dives were matched to GVPs, including three during MFAS. Of the dive behavior metrics, only the depth of one intermediate dive during an exposure was outside the 95th percentile of baseline behavior. Three of the four movement behavior metrics (75%) were atypical relative to baseline for at least one whale across SCC phases, but response varied by individual. However, throughout the SCCs, the whales remained within tens of kilometers of PMRF, near areas used before and after SCCs.

Conclusions: These data demonstrate some apparent short-term changes to dive behavior and horizontal movement in response to MFAS. However, these beaked whales did not demonstrate sustained avoidance responses, remaining in the area west of the range during MFAS and in two cases returning to the range after the SCC. Additional tagging and photo-identification studies are critical to understand Blainville's beaked whale habitat use and residency and to assess the potential impact of repeated exposures to MFAS.

Keywords: Behavioral response; Blainville’s beaked whales; Dive behavior; Mid-frequency active sonar; Movement behavior; Satellite tags.

Fig. 1

Tracks of tagged Blainville’s beaked whales (2014, green, n = 1; 2021, purple and orange, n = 2; 2022, pink, n = 1), with the outline of the PMRF range shown as a black dashed line for reference. The approximate locations of the Mote antennas are given as blue triangles

Fig. 2

Movements of MdTag020 from the time of tag deployment until the start of MFAS given in panel A; the period during the two bouts of MFAS are given in panels B and C, with a wide view of the range in panel B, and a close up view of the track with corresponding median RLs shown during the bouts of MFAS in panel C; and the time from the end of the exposure periods to the end of the tag deployment given in panel D. Two bouts of MFAS are shown in B and C; mean ship locations for blocks of sonar within each bout are shown in panel B, as well as closest helicopter-dipping and active sonobuoy MFAS positions. PMRF is outlined by the dashed black line

Fig. 3

Movements of MdTag021 from the time of tag deployment until the start of MFAS given in panel A; the period during the two bouts of MFAS are given in panels B and C, with a wide view of the range in panel B, and a close up view of the track with corresponding median RLs shown during the bouts of MFAS in panel C; and the time from the end of the exposure periods to the end of the tag deployment given in panel D. Two bouts of MFAS are shown in B and C; mean ship locations for blocks of sonar within each bout are shown in panel B, as well as closest helicopter-dipping and sonobuoy MFAS positions. PMRF is outlined by the dashed black line

Fig. 4

Movements of MdTag022 from the time of tag deployment until the start of MFAS given in panel A; the period during the two bouts of MFAS are given in panels B and C, with a wide view of the range in panel B, and a close up view of the track with corresponding median RLs shown during the bouts of MFAS in panel C; and the time from the end of the exposure periods to the end of the tag deployment given in panel D. m the time of tag deployment until the start of MFAS in panel A, during the two bouts of MFAS (wide view of range given in panel B, close up view of tracks with corresponding median RLs shown during the bouts of MFAS details given in panel C); and from the end of the exposure periods to the end of the tag deployment in panel D. Two bouts of MFAS are shown in B and C; mean ship locations for blocks of sonar within each bout are shown in panel B, as well as closest active sonobuoy MFAS positions. PMRF is outlined by the dashed black line

Fig. 5

Modeled dive profiles of MdTag020 (orange) and MdTag021 (purple) during Phase A for the 2-h period the dives became asynchronous. MdTag020 conducted an anomalously deep intermediate dive to 303.5 m at the start of the asynchrony, and both whales conducted an anomalously deep intermediate dive to 375.5–391 m. Time on the x-axis is in HST

Fig. 6

Dive record of MdTag020 (orange) and MdTag021 (purple) starting 17 August 2021 0:00 through 19 August 2021 12:00. The yellow bar at the surface indicates periods of missing data for MdTag020, the blue bar is missing data for MdTag021. Pink dots are 5 min bins with sonobuoy or helicopter dipping MFAS exposures, red dots are 5 min bins with hull-mounted MFAS exposures. Time on the x-axis is in HST

Fig. 7

Track bearing values between 5 min predicted track steps, with all of MdTag017 being baseline data, while the data for the other three whales is broken down by Before, Phase A, Interphase, Phase B, and After periods. The numbers from 200 to 800 represent the number of 5 min predicted locations per phase

Fig. 8

Track step length values between 5 min predicted track steps, with all of MdTag017 being baseline data, while the data for the other three whales is broken down by Before, Phase A, Interphase, Phase B, and After periods. Note that the y-axis has been constrained to 1000 m

Fig. 9

Track speed values between 5 min predicted track steps, with all of MdTag017 being baseline data, while the data for the other three whales is broken down by Before, Phase A, Interphase, Phase B, and After periods. Note that the y-axis has been constrained to 5 m/s

Fig. 10

Track turning angle values between 5 min predicted track steps, with all of MdTag017 being baseline data, while the data for the other three whales is broken down by Before, Phase A, Interphase, Phase B, and After periods. Note that the y-axis has been constrained from −3 to 3