Introduction & Objectives

The U.S. Navy’s Marine Species Monitoring Program has sponsored autonomous real-time passive acoustic monitoring for large whales in the mid-Atlantic region since 2019, first utilizing Slocum G3 gliders and later shifting to moored buoys. The primary intent of this work is to contribute to the broader effort to monitoring for the presence of North Atlantic right whales although the system is capable of detecting several species of baleen whales.

Technical Approach

Both the glider and moored buoy utilize the digital acoustic monitoring instrument or DMON, a passive acoustic instrument that is capable of recording and processing audio in real time. Additionally, a low-frequency detection and classification system (LFDCS; Baumgartner and Mussoline 2011) identifies four species marine mammal calls (fin, humpback, sei and north Atlantic right whales) and produce a pitch track that can be transmitted via satellite in near real time to a server where detections are verified by analysts on a daily basis. Notifications can be sent via text message to the analysts to notify them of possible priority species detections such as right whales. Results of the analysis are posted on the project website (Robots4Whales), distributed to interested parties by automated email messages, and made available for display in NOAA’s Passive Acoustic Cetacean Mapper and the Whale Alert App. North Atlantic right whale detections are also contributed to WhaleMap.

Progress & Results

Slocum Gliders (2019-2020)

In both 2019 and 2020, two autonomous Slocum G3 gliders equipped with DMON instruments and near real-time reporting capabilities were deployed and operated in the mid-Atlantic Bight to the north and south of Cape Hatteras, North Carolina, respectively. The gliders were programmed to survey pre-determined cross-shelf transects by traveling between specified waypoints from roughly the 20 m isobath eastward to the shelf break as local currents allowed, but also could be remotely piloted in the event mechanical or environmental factors required intervention for course deviation. Pressure, temperature, conductivity (to derive salinity measurements), chlorophyll fluorescence, and turbidity metrics along with the pitch tracks were transmitted in near real time.

2019 deployment

Both gliders were deployed within one day of each other in late January 2019, and although winter storms tended to move the glider operating to the north of Cape Hatteras off the pre-determined survey track, the instrument generally was able to stay in the study area and traverse across the shelf successfully. The glider deployed to the south of Cape Hatteras needed remote pilot intervention to avoid being caught in the powerful Gulf Stream currents. The maneuvers were successful, but the glider repeatedly made contact with the sea floor, which needed intervention to fix and continue with the modified survey plan.

Humpback whales were the most frequently detected species on the northern glider, although fin whale detections were also frequent, and there was only a single sei whale detection. There was a single day with several reliable NARW calls, but a subsequent aerial survey aboard a United States Coast Guard C-130 was unsuccessful at locating any NARWs. The flight did not occur until four days after the glider detection, and important lessons were learned during the exercise that can be applied to future rapid-response flights triggered by NARW detections. The southern glider had “possible detections” of all four species (including two occurrences each of fin and sei whales), but detections of humpbacks were the only detections classified with high confidence.

2020 deployment

Both gliders were deployed in late January and early February 2020 respectively. Of the four baleen whale species monitored, humpback whales were the most commonly recorded on the northern glider. Fin whale detections were also fairly common, while sei whales were detected on a single occasion. There were three days with possible NARW calls, but no detections that satisfy the analysis protocol for confirmed detection. The southern glider had no detections or possible detections of sei or North Atlantic right whales on any day. Fin whales were detected on a single occasion, with several other “possible detections”, but humpbacks were the only species detected with high confidence. Both gliders were recovered in mid-March 2020, earlier than anticipated, due to general disruptions and travel restrictions caused by the beginning of the COVID-19 pandemic

Moored Buoy (2020-present)

An autonomous real-time reporting passive acoustic detection buoy was deployed by Woods Hole Oceanographic Institute off the coast of Cape Hatteras, North Carolina in December 2020 (ncch1220). Technical issues with the Iridium communications equipment prevented data from being transmitted from March into April, and the buoy had to be retrieved for servicing at the end of May. It was redeployed in late October 2021 (ncch1021).

The second deployment of this buoy broke free from its mooring in July 2022, presumably due to the constant flow pressure from the Gulf Stream. The DMON and mooring were unable to be retrieved by divers but a new buoy system was deployed in September 2022 north of the previous location to avoid the Gulf Stream and major shipping traffic (ncch0922). 

The Cape Hatteras monitoring location will be retired in December 2023 (potentially to continue with support from NOAA) and a new buoy was deployed in October 2023 off the coast of the eastern shore of Virginia, just north of the Chesapeake Bay shipping lanes (vacc1023).  This location was chosen to support the Mid-Atlantic Nearshore & Mid-shelf Baleen Whale Monitoring project by providing acoustic detections for potential small vessel work such as photo identification and tagging.