A number of species of resident and non-resident odontocetes use the waters of the Kaulakahi Channel between Kaua‘i and Ni‘ihau, overlapping with the Pacific Missile Range Facility (PMRF). Submarine Command Courses (SCCs) held at PMRF provide an opportunity to assess exposure and measure odontocete reactions to mid-frequency active sonar (MFAS) being used in realistic training scenarios. The primary goal of this assessment was to estimate MFAS received levels (RLs) for satellite tagged odontocetes and determine whether any large-scale movements occurred in response to hull-mounted surface ship MFAS exposure. Two prior reports have used odontocete satellite tag data and information on MFAS use to estimate RLs for animals tagged in 2011 through 2015.
This study continues and extends the earlier efforts using data from three species of odontocetes satellite tagged prior to SCCs in August 2016, February 2017, and August 2018. Methods in the current analyses were consistent with prior methods to allow for comparison, using filtered satellite tag data, the locations of ships transmitting MFAS, and the times of sonar transmissions from PMRF range hydrophones. However, several key improvements were made in data collection and the ability to quantify received noise exposure and describe various sources of error. One (in 2016) or two (in 2017 and 2018) land-based Argos receiving stations were used to supplement satellite tag data received through the Argos satellite system. RLs were estimated at the nominal location of tagged individuals and variability in RL estimates were assessed using information on the accuracy of locations, for locations received within one hour of MFAS transmissions. For each exposure, multiple metrics (mean, SD, minimum, maximum) of estimated RLs (measured as dB re: 1μPa RMS) were calculated, both near the surface (10 m depth) and at depths meant to represent typical foraging depths for each species. When available, information on diving and surfacing behavior of tagged individuals before, during and after MFAS use was also compared to assess potential reactions to MFAS.
From the 2016-2018 SCCs data for estimating RLs and examining potential responses were available for three short-finned pilot whales, Globicephala macrorhynchus (one from the resident population and two from a non-resident population), two rough-toothed dolphins, Steno bredanensis (both from the resident population), and two melon-headed whales, Peponocephala electra (both from an offshore population). The three short-finned pilot whales (in two different groups) were exposed to MFAS at estimated distances of 27.2 km to 145.5 km, with estimated mean RLs ranging from 122.6 to 145.0 dB at 10-m depth. Estimated RLs at 500 m depth for these individuals were typically 10-20 dB lower. The two pilot whales in the same group were traveling together before, during and after the August 2016 SCC, and were thought to be from a non-resident population. Prior to the start of surface ship MFAS use the two individuals had moved off PMRF. After the start of MFAS use these individuals moved away from the source for approximately 24 hours (to approximately 127 km from the source), then moved back towards PMRF to approximately 44 km from the source, moving from an area with estimated RLs of approximately 122 dB to 145 dB. Changes in diving behavior over the three sonar exposure periods (before, during, after the SCC) were documented for both individuals, but the patterns were not consistent between them, suggesting that some factor other than MFAS exposure may have been influencing the diving behavior of one or both individuals. The other individual, tagged prior to the August 2018 SCC, was a member of the resident population. At the start of surface ship MFAS use this individual was 32.9 km from the MFAS source in an area with an estimated median RL of 133.3 dB, and over the next six hours moved to 52.4 km from the MFAS source into an area with an estimated median RL of 130.6 dB.
Two rough-toothed dolphins were tagged in the same group prior to the February 2016 SCC, but had separated prior to the start of the SCC. Both individuals moved south of PMRF prior to the start of surface ship MFAS use. At the start of MFAS exposures the individuals were in areas with estimated median RLs of 143.4 and 147.3 dB. Some movements away from the MFAS source were documented for both individuals, although by the end of the SCC the individuals were in areas with estimated median RLs of 146.3 and 151.4 dB, respectively. Nighttime dive depths and durations did differ significantly among the three sonar exposure periods for both individuals, but not in a consistent way, suggesting that some factor other than MFAS exposure may have been influencing the diving behavior of one or both individuals.
Two melon-headed whales tagged in the same group prior to the August 2017 SCC appeared to remain associated throughout the SCC. Both individuals moved off PMRF prior to the start of surface-ship MFAS use, and were almost 100 km from the MFAS source at the start of exposure, continuing to move away during the sonar exposure period. Only a few Argos locations were obtained within one hour of MFAS use for these individuals, so we undertook a preliminary exploration of a continuous-time animal movement model (a Bayesian switching state space model, SSSM) to generate locations at regular (one-hour) time intervals during the period of MFAS exposure. Using the modeled location data, estimated median RLs were in the range of approximately 75 to 105 dB. While we are confident in the magnitude of exposure for these individuals (i.e., that they were exposed at low RLs), further refinements and assessments of continuous-time movement models are needed for this type of application. Overall these analyses provide additional case studies of exposure and responses of three species of odontocetes to hull-mounted surface ship MFAS use, including individuals both from resident (rough-toothed dolphins, short-finned pilot whale) and non-resident populations (melon-headed whales, short-finned pilot whales). Six of the seven individuals had moved off PMRF prior to the start of surface ship MFAS use; the one individual which had not moved off PMRF prior was a short-finned pilot whale from the resident population. It is unknown whether or not such movements off PMRF may have been in response to surface or sub-surface Navy activities on the range prior to the start of surface-ship MFAS use. For the two pilot whales from the nonresident population, initial movements away from the MFAS source may have been a largescale movement response to exposure. Movements back towards areas with higher RLs starting approximately 24 hours after the beginning of MFAS exposure may have indicated an increased tolerance of MFAS exposure. Alternatively, the exposure levels could have been too low to elicit a large-scale movement response and movements documented were in response to some other factor (e.g., prey patterns).
Citation:
Baird, R.W., E.E. Henderson, S.W. Martin, and B.L. Southall. 2019. Assessing Odontocete Exposure and Response to Mid-Frequency Active Sonar During Submarine Command Courses at the Pacific Missile Range Facility: 2016-2018. Prepared for Commander, Pacific Fleet, under Contract No. N62470-15-D-8006 Task Order KB16 issued to HDR Inc., Honolulu, HI
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