This report summarizes the development and application of substantially upgraded analytical methods to quantify the movement and diving behavior of satellite tagged odontocetes before, during, and after Submarine Command Course (SCC) training events at the Pacific Missile Range Facility (PMRF) off Kaua‘i, Hawai‘i, including their predicted exposure and potential response to mid-frequency active sonar (MFAS) during these events. Eleven short-finned pilot whales and seven rough-toothed dolphins were tagged between February 2011 and February 2019. These data had been previously analyzed using simpler, temporally coarser, two-dimensional MFAS received level (RL) estimations. Argos positions typically only occur once every few hours, limiting the number of RL estimates per animal using these earlier methods. In addition, the RLs were estimated for broad “shallow” and “deep” diving depth categories defined for each species, without integrating empirical measurements of diving behavior into the analysis. For this reanalysis effort, satellite tag data were re-processed through the Kalman smoothing algorithm, which compared to the previously used least-squares location data, provides greater temporal resolution in tracks and more robust estimates of positional uncertainty. Available positional data, including Fastloc GPS locations for some recent tags (including one new short-finned pilot whale and two common bottlenose dolphins tagged in 2020), were interpolated every 5-min using the R-package crawl, and using dive data obtained from individual tags, dive depths were estimated at each of those 5-min locations. Further, 95% confidence interval error ellipses were calculated around each 5-min position, with multiple radials running from source locations through error ellipses in order to model transmission loss (TL) values (and thus estimate RLs) from the source to the far edge of the radial and to the seafloor. RL values were then derived for many points in the three-dimensional space within the error ellipse most likely to contain the animal at that time and around the estimated depth value in order to arrive at the most accurate possible modeled RL estimate (with associated variance estimates). A sub-set of data analyzed following the 100-imputed-track method utilized by Schick et al. (2019) yielded comparable results. The error ellipse method was thus applied here.
In addition to these more detailed RL analyses, the movement and dive behavior of the tagged odontocetes were examined relative to both phases of the SCC. Past analyses have focused only on Phase B, which includes the use of hull-mounted MFAS, as well as other sources of MFAS including helodipping sonar and active sonobuoys. The initial part of the SCC, Phase A, does not include any of these sources or any surface-combatant vessels, but does include other surface and subsurface vessels and other active sonar sources that could potentially cause behavioral responses. Therefore, odontocete behavior herein was examined in five phases: (1) before Phase A; (2) during Phase A; (3) between phases A and B; (4) during Phase B; and (5) after phase B (when all of those periods were applicable for the tagged individual). Movement and dive behavior were also examined in the context of normal patterns, including diel and lunar cycles, in order to put the context of any potential response within the context of baseline behavioral variability. This framework can only be developed because of the high sample size and long-term effort that has been conducted off Kaua‘i and the other Hawaiian Islands, and is an essential facet of behavioral response analyses. In this study, while there were statistical differences in dive behavior of all three species across the periods of the SCC, there were no apparent consistent patterns indicating broad, sustained responses to MFAS (e.g., large-scale habitat abandonment). In fact, there were often inter-individual differences in how dive behavior changed across periods, with some instances of behavioral changes more apparently related to the lunar cycle than to training activity.
A quantitative analysis of fine-scale individual responses to evaluate potential behavioral changes as a function of range or RL (as conducted in some dedicated behavioral response studies) was not conducted here. However, there are a few instances where a behavioral response appeared to have occurred at a relatively coarse scale based on the timing of events and apparent movement, as individuals moved towards the region of activity on PMRF and then changed the direction of their travel away during a period of MFAS. However, there were also sharp changes in direction of travel during baseline and non-MFAS periods. A more detailed quantitative analysis may be able to tease apart these differences to determine what may constitute a behavioral response. Statistical methods utilizing comparable satellite-telemetered data such as those collected here are being developed in other studies and would be applicable to these data in the near future.
Although no overt, broadly evident behavioral responses seem to have occurred, there were several instances of animals occurring and apparently continuing to remain quite close to active ships. We estimate that these individuals received some of the higher levels of MFAS exposure yet documented for any marine mammal species. Although vastly improved over previous RL estimates there is still positional uncertainty in these data; while the highest median levels in these close exposure cases were approximately 175 dB re 1µPa, the median levels plus two standard deviations were around 195 dB re 1µPa, and maximum modeled levels exceeded 200 dB re 1µPa. While the probability of reaching those maximum levels was quite low, these results do indicate that in some cases odontocetes at PMRF may be experiencing very high RLs that could approach temporary or potentially even permanent threshold shift levels, provided animals experienced these conditions for some more extended exposure. There are further considerations regarding physiological stress responses even in the absence of behavioral response. All but two of the individuals included in this study have been determined to be part of the resident populations for these species. There is thus likely some degree of habituation that has occurred for these animals to the presence of MFAS, which may lead to a lack of strong behavioral responses. However, additional behavioral samples for most species and the potential for physiological or auditory impacts beyond behavior speaks to the need to continue these detailed tagging, behavior, and received level analyses for all odontocete species at PMRF. Such data will inform not only the short-term MFAS exposure and response questions relevant to U.S. Pacific Fleet monitoring objectives but are also relevant to addressing the long-term population level consequence objectives of understanding potential impacts to these animals.
Citation:
Henderson, E.E., C.R. Martin, R.W. Baird, M.A. Kratofil, S.W. Martin, B.L. Southall. 2021. FY20 Summary Report on the Received Level Analysis of Satellite Tagged Odontocetes at the Pacific Missile Range Facility. Naval Information Warfare Center Pacific.
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