Abstract
Male fin whales sing using 20 Hz pulses produced in regular patterns of inter-note intervals, but little is known about fin whale swimming behavior while they are singing. Even less is known about fin whales in Hawaiian waters because they have rarely been sighted during surveys and passive acoustic monitoring has been limited to sparse hydrophone systems that do not have localization capabilities. We hypothesized that fin whale kinematics may be related to their singing behavior, or external variables such as time and sea state. To investigate this hypothesis, we analyzed 115 tracks containing 50,034 unique notes generated from passive acoustic recordings on an array of 14 hydrophones from 2011 to 2017 at the U.S. Navy Pacific Missile Range Facility off Kauai, Hawaii. Fin whales swam at an average speed of 1.1 m/s over relatively direct paths. We incorporated the whales' speed and turning angle into hidden Markov models to identify different behavioral states based on the whales' movements. We found that fin whale kinematic behavioral state was related to the vocalization rate (also known as cue rate) and time of day. When cue rate was higher, fin whales were more likely to swim slower and turn more than when cue rate was lower. During the night, fin whales were also more likely to swim slower and turn more than during the day. In addition, we examined whether the presence of singing fin whales was related to time and sea state using generalized additive models. Fin whale track presence was affected by day of the year and song season, and possibly also wind speed and wave height. Although the track kinematics from the fin whale tracks presented here are limited to a subset of whales that are acoustically active, they provide some of the only detailed movements of fin whales in the region and can be compared against fin whale swim speeds in other regions. Understanding how fin whale swimming behavior varies based on their vocalization patterns, time, and environmental factors will help us to contextualize potential changes in whale behavior during Navy training and testing on the range.
Highlights
Fin whale (Balaenoptera physalus) song has been recorded in every ocean basin, but little is known about fin whale swimming behavior while they are singing
We modeled the fin whale step length and turning angle as hidden Markov model (HMM) that were functions of variables that we hypothesized might influence fin whale swimming behavior
To test the effect of time, we modeled transition probabilities between kinematic behavioral states as a function of hour of the day, day measured since October 1 of that song season and resetting each song season, and song season which is defined as the time from October to May of each year when singing fin whales are recorded on Pacific Missile Range Facility (PMRF)
Summary
Fin whale (Balaenoptera physalus) song has been recorded in every ocean basin, but little is known about fin whale swimming behavior while they are singing. This song is made up of two different note types (referred to here as A and B notes) that are repeated in regular inter-note intervals (INIs) (e.g., Watkins, 1981; Thompson et al, 1992; Delarue et al, 2013; Širovicet al., 2017; Helble et al, 2020). Male fin whales have been observed to sing and this song primarily occurs during winter months (Croll et al, 2002; Širovicet al., 2013). Singing fin whale kinematics can contextualize the behavioral significance of different song patterns and can illuminate habitat use, possibly giving clues about seasonal migration patterns
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