US Navy

Marine Species Monitoring

Atlantic Behavioral Response Study

Introduction & Objectives

The Atlantic Behavioral Response Study (Atlantic-BRS) builds on previous Navy-supported research and monitoring efforts to measure the behavioral responses of deep-diving odontocetes to several types of mid-frequency active sonar (MFAS). This research program was initiated in 2017 off Cape Hatteras, North Carolina and has evolved through a series of phases focusing on different sonar sources of direct relevance to U.S. Navy operations and compliance. It was strategically staged in an area where extensive previous monitoring efforts have documented the occurrence and baseline behavior of several key species using various well-established sampling methods (e.g., surveys, photo ID, animal-borne tags, and passive acoustics). The research team, led by the Duke University Marine Laboratory, has demonstrated the ability to consistently find and tag large numbers of both goose-beaked whales (Ziphius cavirostris), a species of high Navy interest in terms of potential MFAS effects, and short-finned pilot whales (Globicephala macrorhynchus). The research site is positioned relatively near (<100 nm), but not within, areas of operational MFAS sources used during training and testing operations of SQS-53C-equipped combat vessels based out of Norfolk, VA.

The Atlantic-BRS program has produced dozens of scientific papers and technical presentations to technical, Navy, and regulatory audiences around the world. It is generating novel data that has expanded basic biological understanding of these key species. It has also substantially increased limited direct information on their behavioral responses to Navy tactical sonar, producing many known exposure-response measurement that are used directly in environmental compliance assessments for ongoing Navy training and testing operations. This project has developed novel approaches to expand the temporal and spatial scales over which potential responses are measured from previous BRS efforts by strategically integrating satellite-linked time-depth recording tags (SLTDRs) with short-term, high-resolution acoustic digital archival tags (DTAGs) in experimental conditions where key aspects of exposure context are known. Concentrated field effort has yielded the largest sample size of tagged goose-beaked whale behavior and behavioral responses to MFAS exposures in controlled exposure experiments (CEEs) obtained to date anywhere in the world. This large sample size is also enabling key insights into ecological and environmentally important contextual variables of behavioral responses observed, or lack thereof.

The primary objective of the Atlantic-BRS is to conduct controlled exposure experiments (CEEs) with tagged beaked and pilot whales exposed to Navy MFAS signals. Given the high degree of success with goose-beaked whales and their very high priority to the Navy, this species been the priority species, and in recent years the sole experimental focus, for this program. The primary objective is to conduct CEEs with goose-beaked whale using tactical sonars from operational Navy vessels conducting ongoing training operations. This has been accomplished through close coordination between the research team and colleagues from U.S. Fleet Forces Command. Experimental simulations of such sources are used when Navy vessels are unavailable. Earlier phases of the program focused on experimental trials with pulsed active sonar (PAS) signals (~1-2s duration). This resulted in >100 exposure events during 23 CEE trials with both simulated and operational sources. Recent efforts have transitioned to CEE trials using identical field approaches, but with continuous active sonar (CAS) MFAS signals. The CAS signals have similar frequency content to the PAS signals but are of much longer (~50s) duration and are continuously repeated. The first-ever experiment with U.S. Navy CAS signals and any marine mammal, as well as the first ever CEE with any form of CAS and goose-beaked whales was conducted with three tagged individuals in 2024.

The data obtained from these field efforts are providing an unprecedented level of direct, experimental, and comparative insights into the behavioral effects of an increasing diversity of operational navy tactical sonars on goose-beaked whales, a species of very high priority to the Navy.

Technical Approach

Deliberate measures have been taken to maintain consistency with the methods employed in previous BRS projects so that results can be integrated across sites. The experimental design for the Atlantic-BRS was derived from methods employed in the Navy-funded (ONR, LMR programs) southern California behavioral response study (SOCAL-BRS), with adaptations related to the novel use of both SLTDRs and DTAGs. Identical experimental methods are employed for operational and experimentally simulated MFAS exposures, as well as control sequences that allow evaluation of the potential effects of field operations. Such consistency allows comparisons to be drawn across studies to support meta-analyses and ongoing integration of results into exposure-response probabilistic functions for application in Navy environmental compliance assessments.

Data collection on movement and diving behavior of focal whales occurs before, during, and after exposure. A key aspect of the multi-scale experimental design is the ability to obtain data remotely with SLTDRs for long periods before and after exposures. This is supplemented by additional data collection using fine-scale tags (DTAGs) and direct visual observation in the days before, during, and after CEEs. Pre-exposure baseline behavioral data collection involves data from tag sensors (days or weeks for SLTDRs; hours for DTAGs), supplemented by focal follows of tagged animals by observers in small boats and passive acoustic monitoring (PAM). Sampling from these sources is maintained during MFAS or control exposure sequences (30-60 min), and post exposure periods (hours-days).

The Atlantic-BRS team has developed effective approaches to conducting field research with goose-beaked whales off Cape Hatteras. These approaches include close planning with Navy colleagues in advance and up to the occurrence of MFAS CEEs. Sonar transmissions for operational MFAS CEEs with PAS and CAS sources use identical operational protocols. This includes full-power transmissions used in tactical training with a specified, consistent waveform and duration type with ships transiting in a direct course at 8 kt for 60 minutes. Starting positions and vessel course are determined using in situ propagation modeling given the position of a focal animal using models developed and provided by the Naval Postgraduate School (NPS), These models are updated daily using HYCOM environmental data. The course of the vessel results in an escalation in received level for focal individuals based on the ship's movement directed generally (but not directly) toward the whales. Given the relatively large number of tagged individuals exposed during CEEs (in some cases exceeding 10 individuals per event), there is considerable variation in range and received level among focal whales. Target received levels for focal beaked whales in MFAS PAS experiments have been reliably achieved from 120-140 dB RMS. Target levels for CAS exposures have been identified and verified at target individuals based on noise metrics that account for the total energy in these much longer signals (e.g., sound exposure level (SEL)). This experimental design allows for positioning of sonar sources to result in target received levels at focal individuals that are also being monitored directly using other methods (focal follow, photo ID), in a deliberately matched design that will enable the comparison of responses for these high priority species to PAS and CAS signals. This is an issue of scientific interest as well as direct regulatory implications for the Navy given the similar spectral content but very different temporal structure of these operational sonar signals.

Analyses focus on how tagged whales respond to MFAS exposure in terms of avoidance; changes in behavioral state; and changes in social grouping. These analyses have benefited from methods developed in other BRS studies and the MOCHA and DOUBLE MOCHA projects supported by ONR. Short- and longer-term consequences of disturbance are evaluated separately, but the multi-scale nature offers a unique opportunity to explore how these methods may complement one another and how high-resolution, short-term response data may inform methods used for longer-term monitoring.

Progress & Results

The overall research project commenced in 2017 and is ongoing in the third phase of operations, with field work conducted in summer and early autumn. A total of 107 tags have been deployed on goose-beaked whales and 46 tags have been deployed on pilot whales. Twenty-four conventional MFAS CEEs have been conducted, nine of which involved operational US Navy vessels with 53-C MFAS (8 PAS; 1 CAS) sources. The Navy vessels involved with these field trials include the USS MACFAUL, USS NITZE, USS RAMAGE, USS COLE, USS LABOON, USS DELBERT BLACK, USS FARRAGUT (twice), and USS THOMAS HUDNER. An additional 10 CEE trials involved simulated MFAS sources, and five more were control sequences. In total, these 24 CEEs have resulted in a total of 108 unique tagged animal exposure events for goose-beaked whales and 62 unique events for pilot whales.

Analysis of this large data set is complex, extensive, and ongoing. Existing analytical approaches are being applied, together with novel integration on a variety of temporal and spatial scales. In addition, a new paradigm of spatial analysis has been developed to address positional error associated from the SLTDRs. More than a dozen peer-reviewed papers have been directly produced from this research program, including key methodological and technical developments and aspects of baseline behavior and behavioral response. A dozen more have included data from this program through collaborations with other Navy-funded researchers. A manuscript on the response of goose-beaked whales to simulated MFAS is currently in review and a complementary paper contrasting potential responses to operational PAS is in preparation. This will be followed by a systematic comparison and integration of these two sets of results. Results arising from this project have been presented at scientific meetings worldwide including Effects of Sound on Marine Mammals, Effects of Noise on Aquatic Life, Society of Marine Mammalogy, Biologging, Acoustical Society of America, Discovery of Sound in the Sea, and other venues (e.g., regulatory agency briefings).