The Pacific Islands Fisheries Science Center (PIFSC) Cetacean Research Program conducted Mariana Archipelago Cetacean Survey (MACS) from July 9 to August 1, 2018, aboard the NOAA R/V Oscar Elton Sette. The survey included ship-based visual search and towed hydrophone array passive acoustic monitoring for cetaceans, as well as the deployment of DASBRs and collection of oceanographic data using CTD casts. The study area extended from the Mariana Archipelago to the West Mariana Ridge, roughly 280 km to the west, and from 100 km south of Guam at 13°N to the northern end of Pagan around 18°N (Figure 1).

Drifting Acoustic Spar Buoy Recorders were deployed at random locations, generally near the island chain between the islands of Guam and Guguan and were left to drift while the ship-based visual and passive acoustic surveys continued. DASBRs were retrieved when the ship was in their general vicinity or when they were nearing the West Mariana Ridge. The survey area was divided into a 50 × 50 km grid, and a CTD cast to 1,000 m depth was conducted using a Sea-Bird Scientific SBE9 (#91660) sensor package within most grid cells across the study area (Figure 1 and Table 1).

The PIFSC DASBR is based on the design of Griffiths and Barlow (2015), but modified to withstand the rough seas and higher water temperatures common to the tropical central and western Pacific (Figure 2). The DASBR includes a spar-buoy surface float with trailing line and trailer buoy for recovery, 150 m of 1/2-inch (1.3 cm) line, and an anchor at the bottom. The spar-buoy contains an Iridium satellite tracker (NAL 9602-AB) that provides GPS location at programmed time intervals (typically every 2 h). A multi-channel (up to 4) SoundTrap ST4300-HF (Ocean Instruments, Auckland, New Zealand) is attached to the section of line 15 m above the anchor, with two hydrophones (HTI-92 & HTI-96; High Tech, Inc., Long Beach, MS, United States) spaced 10 m apart, one above and one below the ST4300-HF (Figure 2).

The SoundTrap ST4300-HF sampled at 576 kHz on two channels, with a custom low pass filter at 150 kHz to improve detection of NBHF Kogia spp. clicks (standard units filter at 90 kHz). Acoustic recordings were collected using a duty cycle with a 2-min sampling period starting at 5-min intervals. A Time-Depth-Recorder (TDR) (LAT1400-500m; Lotek, St. John’s, Canada) was attached to the vertical line below the lower hydrophone to collect accurate hydrophone depth measurements (with 0.5 m resolution).

We identified frequency-modulated beaked whale echolocation signals and Kogia spp. NBHF clicks within the acoustic data using the click detector module (IIR Butterworth 2 kHz high pass filter) within PAMGuard software v. 2.00.14c (Gillespie et al., 2009) and coded them with custom specifications based on peak frequency (Keating and Barlow, 2013). Secondarily, we manually extracted and grouped FM pulses for acoustic encounter analysis based on bearing angles and click trains. FM pulse bearing angles from PAMGuard were relative to the vertical hydrophone array (0° = surface, 90° = depth of array, 180° = directly below array). Lastly, we manually classified detections of beaked whale FM pulses to the species level based on their spectral characteristics (e.g., peak frequency, −10 dB bandwidth, duration, inter-pulse-interval, encounter level spectrums) (Baumann-Pickering et al., 2013, 2014; Keating et al., 2016), and detections of NBHF clicks were identified as Kogia spp. based on similarity of click characters to those described by Merkens et al. (2018) and Marten (2000). We did not attempt to calculate the detection range for any species. However, Barlow et al. (2021) performed multiple analysis of detection range for Cuvier’s beaked whale FM pulses received on DASBRs and found a likely maximum detection range of ∼5 km.

To avoid oversampling the presence of each species, we aggregated adjacent files containing echolocation signals produced by the same species into “acoustic encounters.” Beaked whales generally do not produce echolocation FM pulses near the surface or during recovery dives (between foraging dives) (Barlow et al., 2013). Recovery dives typically last no less than 15 min, and we use these periods of silence to define the end of acoustic encounters, which for beaked whales included adjacent 2-min data periods with gaps in detections of less than 15 min. Beaked whale FM pulse detections occurring more than 15 min since the last detection are considered a new encounter, with no attempt to determine if the new encounter was from the same group of beaked whales detected previously. In this way, a beaked whale acoustic encounter represents an individual foraging dive by a group of beaked whales. Periods of Kogia spp. NBHF clicking in adjacent 2-min files were also aggregated into acoustic encounters. Kogia spp. have been observed echolocating near the surface (Merkens et al., 2018; Malinka et al., 2021) and their vocal behavior during foraging dives is not understood; therefore, we used a break in detections of one or more 2-min detection files to indicate a new acoustic encounter. Due to the duty cycled data collection, it is not possible to examine the specific start and stop time of each acoustic encounter as echolocation signals may have begun or ended during a period with no recording, resulting in acoustic encounter duration lasting 0–6 additional minutes.

Acoustic encounters of beaked whales and Kogia spp. were further aggregated for comparison to environmental covariates (Table 2). For this analysis, we associated each acoustic encounter of beaked whales and Kogia spp. with the GPS location closest in time (either forward or backward) to the start of the encounter. This provided a binomial classification for each GPS point from the DASBR of either present or absent for each species, hereafter referred to as “GPS associated encounters.” In some instances, a single GPS point could contain the presence of more than one beaked whale species and/or Kogia spp. producing echolocation signals.

Out of 21,731 2-min recording files, 3% (561) contained acoustic detections of four species of beaked whales (Blainville’s, Cuvier’s, Longmans, BWC) as well as Kogia spp. (Table 3). There were two FM pulse detections with insufficient signal quality to differentiate between Blainville’s or Cuvier’s beaked whales.

Most acoustic encounters spanned more than one 2-min recording period, resulting in 152 total acoustic encounters with beaked whales and Kogia spp. over the full recording duration (Table 3), with 76% of the encounters classified as Blainville’s or Cuvier’s beaked whales. The median encounter duration for Longman’s beaked whales (90 min) was substantially longer than the other species (6–15 min). The vast majority (89%) of acoustic encounters occurred on the five DASBR tracks between Saipan and Pagan (15.5°N to 18°N latitude). There were no encounters with Longman’s beaked whales south of 15.5°N (Saipan) and only seven encounters of (4) Cuvier’s and (3) Blainville’s beaked whales in this region. Although relatively few, encounters with BWC and Kogia spp. were more evenly distributed throughout the monitored area.

The DASBR drift tracks contained 980 GPS points representing the drift track at 2-h intervals, with 139 (14.2%) GPS associated encounters with beaked whales and Kogia spp. (Table 3). Twelve GPS points included more than one species.

Fourteen CTD casts were conducted across the survey area (Figure 1). Evaluation of the temperature and salinity data from the CTDs revealed differences in the physical characteristics of the water. The stations sampled to the north of 15.5°N have a strong salinity maximum at ∼100–200 m and a deep salinity minimum near 600 m, consistent with North Pacific Tropical Waters (Suga et al., 2000; Suntsov and Domokos, 2013). A surface salinity front can be seen near 15°N (Figure 3A). Waters to the south of this front have a surface salinity minimum and a deep salinity maximum weaker than that found to the north. Temperature and salinity values from the CTD and modeled HYCOM data from the same location on the same date were highly correlated (r = 0.998 and r = 0.730, respectively) (Figure 3).

The six ERF analyses performed with varying success; the highest TSS and AUC (0.68 and 0.93) values for the ERF model included only Cuvier’s beaked whale (Table 4).

The bulk of what is known about beaked whale and Kogia spp. occurrence in the Mariana Archipelago comes from long-term fixed-site passive acoustic monitoring in the southern portion of the archipelago. Blainville’s, Cuvier’s, and BWC beaked whales have all been recorded year-round at fixed long-term monitoring sites near Saipan and Tinian (Baumann-Pickering et al., 2014; Simonis et al., 2020). Hill et al. (2020) compared the overall relative occurrence (based on proportion of recording days with each species present) of these species at the same monitoring locations as well as at a site near Pagan. They reported that Blainville’s were the most commonly detected species at all three locations, with varying proportions of Cuvier’s and BWC depending on the monitoring location. These studies did not specifically examine the occurrence of Longman’s beaked whales and did not report echolocation signals other than those identified as one of these species.

Although such long-term monitoring provides a detailed examination of the temporal variability in the occurrence of beaked whale species through time, inference about geographic distribution is limited to the monitoring location only. Our data similarly reveal the highest encounter rates for Blainville’s beaked whale across the monitoring region (40% of beaked whale encounters), though with only slightly lower encounter rates for Cuvier’s beaked whales (36%) when sampling across the broader region (Table 3). Our encounter rate for BWC (10%) is similar to the relative presence of this species reported by Hill et al. (2020). Together with a single towed array acoustic detection of Longman’s beaked whales during the same survey that supported our DASBR deployments (discussed in Hill et al., 2020), our data show Longman’s beaked whale to be present in this region despite its absence in previous visual or acoustic studies in the Mariana Archipelago. There were considerably fewer encounters of Longman’s beaked whales than any other beaked whale species in this study. However, Longman’s encounters were unique in that they were the only species detected strictly in the waters north of 15.5°N, and their acoustic encounter duration was substantially longer than other species, representing 30% of the total duration of beaked whale detections (Table 3). Our data support the widespread distribution of all four of these beaked whale species throughout the archipelago, including previously unsampled waters as far west as the West Mariana Ridge.

Historical stranding records indicate that Deraniyagala’s beaked whales may also be present in the broader western Pacific and Philippine Sea region (Lacsamana et al., 2015). The sounds of this species have been described previously (Baumann-Pickering et al., 2010), and our detection and classification approach would have allowed for their identification if they were present and vocalizing near our recorders. The absence of Deraniyagala’s beaked whales within our dataset and from fixed site monitoring within the archipelago suggests that this species is rare in this region.

There are few visual sightings of Kogia species during visual surveys in the Mariana Archipelago, all of which were pygmy sperm whales (Kogia sima) in nearshore waters off Guam and Saipan (Hill et al., 2020). In contrast, Kogia spp. were detected on five DASBR drifts in deep, open waters between the Mariana Archipelago to the West Mariana Ridge, suggesting a pelagic distribution similar to that observed in other regions. Although their occurrence in deep waters near the Marianas is not surprising given what is known of global distribution patterns for the species, our DASBR detections are the first known data indicating that Kogia spp. are widely distributed throughout the Mariana region.

The echolocation FM pulses produced by Blainville’s and Cuvier’s beaked whales correspond to time spent foraging at depth (Madsen et al., 2005; Zimmer et al., 2005; Barlow et al., 2013). Additionally, the synchronous diving behavior of these beaked whales (Aguilar de Soto et al., 2020), followed by time spent near the surface between foraging bouts (Barlow et al., 2013), represents a natural break in our acoustic encounters, where individual encounters likely represent a single foraging dive by the group of whales. Although DASBR acoustic data are duty cycled precluding precise measurement of encounter duration, we can compare encounter durations among species. Blainville’s, Cuvier’s, and BWC beaked whales averaged encounter times less than 16 min in sharp contrast to the relatively long (91 min) average encounter duration for Longman’s beaked whales (Table 3). Visual sighting data for Blainville’s and Cuvier’s beaked whales indicate that both of these species are not particularly social, dive in sync, and generally occur in small groups of only 2–3 animals (Baird, 2005, 2019; Bradford et al., 2017; Aguilar de Soto et al., 2020; Hill et al., 2020; Alcázar-Treviño et al., 2021). Visual group size estimates for Longman’s beaked whales during surveys in the Hawaiian Islands varied from 15 to 60 animals (Bradford et al., 2017, 2021) and have been observed breaking into smaller subgroups and diving in a staggered rotation (Yano et al., 2018). This continual, staggered diving rotation could explain the extremely long acoustic encounter durations for Longman’s beaked whales.

The broad geographic sampling achieved with our DASBR tracks revealed a clear geographic demarcation in the detection of beaked whales and Kogia spp. showing substantially more encounters of all species on those drift tracks to the north of Saipan compared to those in the south. This north-south division aligns with a distinct shift in oceanography; all three full study area ERF models are described by temperature and mixed layer covariates that partially define the gradient between the North Pacific Tropical Waters in the north and equatorial waters to the south. The break between water masses is generally defined by a salinity front (Suga et al., 2000; Suntsov and Domokos, 2013), and although only models for Cuvier’s contained salinity covariates with a mean decrease accuracy greater than 5% of whale encounters (Table 4), all full study area models did include salinity metrics as significant predictors of whale occurrence (Figure 4).

Perhaps more interesting was the finer-scale assessment of habitat covariates within the northern water mass, where beaked whale encounters were substantially higher, and some of the influence of the oceanographic shift across the front were removed. Within this region, the most significant environmental covariates within each ERF model varied widely. Model predictive power was fairly high (TSS = 0.65–0.68, AUC = 0.90–0.93) for both the full study area and northern region Cuvier’s models, and these models shared 4 of 5 of the top predictors, substituting only mixed layer depth for salinity at 400 m in the northern model. The similarity of the two models is not surprising because all but four of the 50 GPS associated encounters of Cuvier’s beaked whales were recorded north of 15.5°N. Only the bottom depth covariate was shared between the all species model and the Cuvier’s model, suggesting our models may have captured a unique set of environmental predictors that collectively describe the specific ecological niche for these species within this water mass.

Differences in the number of whale encounters north and south of the salinity front could have been impacted by the availability of micronekton within each region. Abecassis et al. (2015) found that a high density of midwater micronekton was an important covariate for Blainville’s beaked whale distribution off the island of Hawai‘i, likely because it represents the prey for beaked whale food sources. Benoit-Bird et al. (2020) found that additional oceanographic features highly correlated with beaked whale habitat were associated to deep-sea squid habitat, highlighting the importance of these focused food sources. When Suntsov and Domokos (2013) sampled micronekton and macro zooplankton on three meridional transects between 10°N and 17°N in the Mariana Archipelago, they found relatively low concentrations within the oligotrophic NEC suggesting the prey availability for beaked whale and Kogia spp. may be more limited in this region.

Similar to previous studies of beaked whale habitat (Ferguson et al., 2006; Hazen et al., 2011; Abecassis et al., 2015), we found bathymetry (bottom depth) to be an important predictor of beaked whale distribution. In particular, within the northern models, bottom depth was the most important covariate for the all species and Cuvier’s model with increased encounters taking place over relatively shallow bathymetry (1,800–2,300 m). Stomach contents of Cuvier’s beaked whales consist primarily of cephalopod species along with non-food source materials (asphalt, coal-like material) suggesting occasional foraging at the sea floor, perhaps by pinning prey items to the ground or foraging for prey within the substrate (Santos et al., 2001, 2007; Adams et al., 2015). Blainville’s beaked whales and other Mesoplodon prey species differ from Cuvier’s beaked whales, with stomach contents containing more fish species and some cephalopods (Santos et al., 2007) showing with no indication of foraging at the seafloor. In addition to bathymetry, chlorophyll a and mixed layer/thermocline covariates were also important. The ERF models suggest that these whale species prefer shallow/seamount areas that have higher chlorophyll concentrations and a shallower mixed layer depth and cooler thermocline temperature. This preference for shallow/seamount areas was also found in the tagged study of Cuvier’s beaked whales off southern California waters (Barlow et al., 2020). Ocean currents interacting with the physical structure of a seamount can generate current enhancement, turbulence, and upwelling to bring nutrient rich waters up into the euphotic zone causing a seamount induced chlorophyll enhancement (Lavelle and Mohn, 2010). Seamounts in the Mariana Archipelago region predominantly show chlorophyll enhancement with persistent increased primary productivity, which is transferred to higher trophic levels (Leitner et al., 2020), potentially providing richer foraging grounds for whales compared with surrounding waters. Additionally, seamounts can trap and concentrate micronekton that undergo diel vertical migration, further contributing to increased concentrations of potential forage (Cascão et al., 2019).

Overall, we gained significant new insight into the relative occurrence of beaked whale and Kogia spp. in a historically data deficient area, as well as providing a more nuanced view of their distribution and habitat associations. We presented a method of improving data collection of cryptic cetacean species in remote environments and of applying environmental covariates to explain their distribution, along with collecting viable data for efforts to estimate abundance where new techniques are currently being developed (Barlow et al. submitted). Our methods are applicable to a wide range of cetacean species, and could greatly enhance the spatial coverage of future marine mammal surveys.