Changes in the Movement and Calling Behavior of Minke Whales (Balaenoptera acutorostrata) in Response to Navy Training

Abstract

Many marine mammals rely on sound for foraging, maintaining group cohesion, navigation, finding mates, and avoiding predators. These behaviors are potentially disrupted by anthropogenic noise. Behavioral responses to sonar have been observed in a number of baleen whale species but relatively little is known about the responses of minke whales (Balaenoptera acutorostrata). Previous analyses demonstrated a spatial redistribution of localizations derived from passive acoustic detections in response to sonar activity, but the lack of a mechanism for associating localizations prevented discriminating between movement and cessation of calling as possible explanations for this redistribution. Here we extend previous analyses by including an association mechanism, allowing us to differentiate between movement responses and calling responses, and to provide direct evidence of horizontal avoidance responses by individual minke whales to sonar during U.S. Navy training activities. We fitted hidden Markov models to 627 tracks that were reconstructed from 3 years of minke whale (B. acutorostrata) vocalizations recorded before, during, and after naval training events at the U.S. Navy’s Pacific Missile Range Facility, Kauai, Hawaii. The fitted models were used to identify different movement behaviors and to investigate the effect of sonar activity on these behaviors. Movement was faster and more directed during sonar exposure than in baseline phases. The mean direction of movement differed during sonar exposure, and was consistent with movement away from sonar-producing ships. Animals were also more likely to cease calling during sonar. There was substantial individual variation in response. Our findings add large-sample support to previous demonstrations of horizontal avoidance responses by individual minke whales to sonar in controlled exposure experiments, and demonstrate the complex nature of behavioral responses to sonar activity: some, but not all, whales exhibited behavioral changes, which took the form of horizontal avoidance or ceasing to call.

Singing Fin Whale Swimming Behavior in the Central North Pacific

“…Bryde's whales, which were tracked from August to October 2014, exhibited the most uniform movement on PMRF, and swam along the fastest and most directed paths of all the baleen whales measured (mean speed = 1.6 m/s, directivity index >0.95 for 16/17 tracks, Helble et al, 2016). During times before MFA sonar exposure (i.e., during periods of baseline behavior), minke whales tended to travel slower than these fin whales and turned almost randomly (mean speed of 0.8 m/s and angular concentrations of 0.18, Durbach et al, 2021). Singing humpback whales on the range behaved the most similarly to these singing fin whales in that they also traveled along relatively straight tracks (mean directivity index = 0.8-1) with mean speeds of ∼1.0 m/s (Henderson et al, 2018).…”

“…If PMRF was a migratory corridor for fin whales traveling between two destinations seasonally (suggested by Thompson and Friedl, 1982), we would expect them to shift their primary swimming direction between seasons (similar to gray whales off the U.S. west coast, Guazzo et al, 2017). Other whale species in Hawaiian waters including humpback whales, Bryde's whales, and minke whales have also been observed to preferentially travel west (Baker and Herman, 1981;Helble et al, 2016;Palacios et al, 2019;Durbach et al, 2021). The average surface current direction in Hawaiian waters is westward (Bondur et al, 2008), so perhaps singing fin whales and these other species are drifting west with the current.…”

“…From these 115 smoothed tracks, we extracted speed and turning angle for each 5 min interval. These methods were similar to those used for minke whales by Durbach et al (2021). The primary purpose of the crawl fitting was to have tracks that were evenly sampled in time, which is required for HMMs.…”

“…Many different marine mammal species spend time around this range and swimming behavior has been quantified for some of these species using passive acoustic localization and tracking techniques. Minke whales (Balaenoptera acutorostrata) significantly changed their swimming behavior before, during, and after Navy mid-frequency active (MFA) sonar exposure (Durbach et al, 2021). Bryde's whales (Balaenoptera edeni) traveled along straight tracks parallel to conspecifics (Helble et al, 2016).…”

Assessing the Role of Sampling Uncertainty When Predicting Behavioral Responses of Tagged Cetaceans Exposed to Naval Sonar

“…The data from CEEs are complemented by other sources of information from captive settings (e.g., Houser et al, 2013) or observational studies (Falcone et al, 2017;Harris et al, 2019b;Durbach et al, 2021).…”