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.
Highlights
Over the last few decades there has been increasing effort to study and understand the impacts of anthropogenic disturbance on marine mammals
Based on the output of the Hidden Markov Models (HMMs) fitted to the entire dataset, we identified one state that described movement patterns that were consistent with a potential avoidance response and that accounted for the majority of observations in the During phase
Previous analyses of a subset of the data presented demonstrated a spatial redistribution of minke whale localizations in response to navy training activities that involved sonar emission (Harris et al, 2019a)
Summary
Over the last few decades there has been increasing effort to study and understand the impacts of anthropogenic disturbance on marine mammals. In particular there has been a focus on the effects of underwater noise on individuals, both physiologically and behaviorally, and the potential for these effects to result in population-level consequences (e.g., National Research Council, 2005; Harris et al, 2018; Pirotta et al, 2018; Booth et al, 2020). One of the best studied sources of underwater noise, with respect to its effect on marine mammals, is naval sonar (see Harris et al, 2018, for review). More recently there has been increased regulatory requirements to quantify marine mammal behavioral responses to noise and consider the fitness consequences that, e.g., cessation of foraging, may have on individuals and populations (Pirotta et al, 2018)
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