Introduction & Objectives

The purpose of this Project was to use autonomous underwater vehicles (AUVs) to conduct passive acoustic monitoring (PAM) in the Southern California Bight (SCB) and northern Baja California, Mexico, with the goal of characterizing the distribution of mysticete and odontocete species in these areas.

Study questions to be addressed:

 What species of cetaceans other than beaked whales are present in the SCB?

 What is the spatial distribution of these cetacean species, particularly blue and fin whales, both inside and outside of the Navy’s training ranges in the SCB, including on and off the continental shelf?

This study was designed to complement ongoing marine mammal monitoring studies in the region, including small-vessel tagging and photo identification surveys for fin whales (Balaenoptera physalus) and Cuvier's beaked whales (Ziphius cavirostris) (e.g., Schorr et al. 2020); PAM of blue (Balaenoptera musculus), fin, and all species of beaked whales using highfrequency acoustic recording packages (HARPs) (e.g., Rice et al. 2020); and visual surveys for marine mammals on California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises (e.g., Trickey et al. 2020).

Previous United States (U.S.) Navy-funded AUV studies have been conducted in Hawaii, the Marianas, and the Pacific Northwest, with the goal of characterizing cetacean distribution in these areas. This work builds on those efforts and applies this new research technology to the Southern California (SOCAL) region.

Technical Approach

Two Seagliders™ were deployed in the San Nicolas Basin on 7 February 2020. One, the “abyssal glider” (SG607), was piloted to survey over the abyssal plain offshore of the continental shelf break. The other, the “shelf glider” (SG639), was piloted to conduct several transects inshore of the shelf break in water depths shallower than 2,500 meters. Both gliders were programmed to survey in the vicinity of existing United States (U.S.) Navy-funded fixed acoustic sensors (high-frequency acoustic recording packages) with the goal of eventually comparing the acoustic datasets collected by both types of instruments. The abyssal glider completed its mission, recorded a total of 763 hours of acoustic data (4,721 sound files on a duty cycle recording 10-minute files every 15 minutes) and traveled 940 kilometers (km) (50 days) before being recovered on 31 March. The shelf glider malfunctioned and stopped recording well ahead of schedule on 10 February and was recovered on 14 February. The shelf glider surveyed successfully for 87 hours and traveled 95 km before failing. It recorded 55 hours of acoustic data in 351 files with the same duty cycle as the abyssal glider.

Blue (Balaenoptera musculus), fin (B. physalus), humpback (Megaptera novaeangliae), and sperm whale vocalizations were recorded by the abyssal glider, and fin and humpback vocalizations (no blue or sperm whales) by the shelf glider. Fin and humpback encounters were particularly abundant. No confirmed minke whale (Balaenoptera acutorostarata) vocalizations were recorded by either glider. The abyssal glider recorded blue whale B calls on three separate days, all while the glider was inshore of the shelf break west of Tanner Bank. Fin whales were detected on every day of both glider deployments, and on several days, they were present in 100% of the 10-minute recording periods. Humpback whales were detected on 48 of the 50 days of the abyssal glider’s deployment, both on and off the continental shelf, and on all five days of the shelf glider recordings (continental shelf only). The abyssal glider recorded sperm whales on 12 of the 50 recording days (n=165 confirmed detections), when the glider was located at the shelf break and over the abyssal plain.

The deep-water areas on the shelf slope and the abyssal plain are seldom surveyed for marine mammals, making the results of this study particularly valuable in improving our understanding of marine mammal occurrence and distribution in these areas, and in and around U.S. Navy training and testing areas in the region.  

Progress & Results

In this study, blue whales were recorded in February and March, and only when the glider was on the continental shelf, among the basins and banks found off SOCAL. This is consistent with recent tagging studies where blue whales were tracked off SOCAL during the winter months (December through February). Blue whale calls have been detected on HARPs summer through winter, with a peak in November, and only a few detections in early winter. Most of these detections were at Sites N and H (north of Tanner Bank and south of San Clemente). HARPS have recorded very few blue whale detections from February to May. Sightings of blue whales in winter are similarly rare, with only two sightings during CalCOFI cruises from 2012 to 2019, and the lowest estimated density of this species is during winter and spring. 

Fin whales were detected throughout the paths of both gliders, on every survey day. This is in contrast to results from visual surveys. The majority of fin whale sightings on recent CalCOFI cruises (2016-2019) occurred in summer, with only a single winter sighting. Density estimates from CalCOFI surveys estimate lowest fin whale densities in the winter months. The low number of visual sightings in winter is not necessarily surprising because weather conditions are often poor and make visual surveys more difficult. On the other hand, fin whale 20-Hz pulses are produced most commonly during the breeding season in the winter months. These differences make it challenging to compare visual and acoustic data. It would be valuable to perform an acoustic survey in summer and compare the results to CalCOFI data from this period when more visual observations are recorded. This would help put the winter/spring acoustic results from this study in better historical context.