Migratory behavior of eastern North Pacific gray whales tracked using a hydrophone array

Abstract

Eastern North Pacific gray whales make one of the longest annual migrations of any mammal, traveling from their summer feeding areas in the Bering and Chukchi Seas to their wintering areas in the lagoons of Baja California, Mexico. Although a significant body of knowledge on gray whale biology and behavior exists, little is known about their vocal behavior while migrating. In this study, we used a sparse hydrophone array deployed offshore of central California to investigate how gray whales behave and use sound while migrating. We detected, localized, and tracked whales for one full migration season, a first for gray whales. We verified and localized 10,644 gray whale M3 calls and grouped them into 280 tracks. Results confirm that gray whales are acoustically active while migrating and their swimming and acoustic behavior changes on daily and seasonal time scales. The seasonal timing of the calls verifies the gray whale migration timing determined using other methods such as counts conducted by visual observers. The total number of calls and the percentage of calls that were part of a track changed significantly over both seasonal and daily time scales. An average calling rate of 5.7 calls/whale/day was observed, which is significantly greater than previously reported migration calling rates. We measured a mean speed of 1.6 m/s and quantified heading, direction, and water depth where tracks were located. Mean speed and water depth remained constant between night and day, but these quantities had greater variation at night. Gray whales produce M3 calls with a root mean square source level of 156.9 dB re 1 μPa at 1 m. Quantities describing call characteristics were variable and dependent on site-specific propagation characteristics.

Gray Whale (Eschrictius robustus) Call Types Recorded During Migration off the West Coast of Vancouver Island

“…Gray whales call frequently during migration, which is in agreement with a recent study by Guazzo et al (2017) on northward migrating whales in Californian waters, and demonstrate greater vocal activity than that suggested by earlier work (Rasmussen and Head, 1965;Dahlheim, 1987;Crane and Lashkari, 1996). The average rate of calling concurs with recent reports for migration (Guazzo et al, 2017), and bottom mounted hydrophone recordings made in the breeding lagoons (5,982 calls from 3 weeks, López-Urbán et al, 2016) and compares consistently to the corrected auto-detection data ( Table 3).…”

“…These recordings are from a relatively uniform and stereotyped behavior state dominated by linear, constant swimming. Migration is a continuous, protracted trail of whales, whose movements are consistent in direction and average speed (Perryman et al, 2002;Mate and Urbán-Ramirez, 2003;Guazzo et al, 2017). The timing and location of the deployments, and likely detection range of the recorder, largely excludes vocal behaviors associated with foraging and weaning (see Burnham, 2015).…”

“…Recordings were made constantly, covering the peak migration sightings off the west coast of Vancouver Island (Duffus, Pers. Obs, 1984-2017 and correlated to shore counts from California counting and PAM survey stations (ACS Sighting data, Guazzo et al, 2017).…”

“…For many whale species, calls have been correlated with either a behavioral, social or geographic context, with changes in call repertoire and call rate over a species range. Whereas, vocalizing behaviors of gray whales (Eschrichtius robustus) have been well-documented in their calving/breeding lagoons (Dahlheim et al, 1984;Dahlheim, 1987;Ollervides, 2001;Charles, 2011;López-Urbán et al, 2016), studies have only recently added to the body of knowledge from the rest of their range (see for example Moore and Ljungblad, 1984;Stafford et al, 2007;Youngson and Darling, 2016;Guazzo et al, 2017;Rannankari et al, 2018). The use of bottom-mounted passive acoustic monitoring (PAM) systems allows long duration recordings with sensitivity into the lowest frequencies, capturing their full vocal range.…”

“…On the northward migration, previous studies have found class 3 moans most frequent, in contrast to the class 1 knocking calls that dominate in breeding areas. Moans comprise up to 87% of the total calls recorded during migration (Cummings et al, 1968;Dahlheim, 1987;Crane and Lashkari, 1996;Guazzo et al, 2017), and are presumed to aid long distance herd cohesion, with broader spectrum modulated calls like class 1 and 4 used for subor within-group communication. At a time of negative energy balance, such as, northward migration, we assume acoustic behaviors would be limited as a conservation measure, but recent recordings taken in California show that not to be the case (Guazzo et al, 2017).…”