Introduction & Objectives

The focus of Navy monitoring efforts under this project includes establishing a baseline of occurrence patterns, understanding the potential for exposure to mid-frequency active sonar (MFAS) or other sounds associated with Navy testing and training activity, and assessing any resulting behavioral responses to those exposures at both the individual level (short-term) and population level (long-term). 

In Hawai‘i, the Navy range complex includes the Pacific Missile Range Facility (PMRF), an instrumented underwater range off Kaua‘i that covers over 1100 km2. Since PMRF is an area with frequent MFAS use, and since Navy scientists can access the underwater hydrophones to record marine mammal and anthropogenic activity, it is an ideal location to conduct opportunistic behavioral response studies of a variety of species exposed to realworld scenarios.

Vessel-based field studies of odontocetes first began in the Kaulakahi Channel between Kaua‘i and Ni‘ihau in 2003 as part of a long-term, multi-species assessment of odontocetes in the main Hawaiian Islands. Since 2008, 14 additional vessel-based field projects off Kaua‘i have been conducted for this effort. These studies have been focused on odontocete occurrence, potential for exposure to MFAS and associated received levels (RLs), and behavioral responses to MFAS for cetaceans that use all or part of the waters on PMRF. 

Technical Approach

These studies have combined acoustic and boat-based methods. Most of the field efforts examining key U.S. Pacific Fleet monitoring questions of marine mammal exposure and response have strategically occurred prior to the start of Submarine Command Courses (SCC), which are typically conducted on PMRF twice a year. Satellite tags that have been deployed on individuals of a number of odontocete species have remained attached and transmitting as designed through all or part of several SCCs. This has also allowed for novel assessments of increasing resolution of both exposure and response to MFAS for these species in this region, directly addressing key U.S. Pacific Fleet monitoring objectives. 

Progress & Results

All tag data obtained in 2014 through 2020 were utilized in analyses of received level and movement and dive behavior. 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). 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.

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.

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.