Biological significance of sperm whale responses to sonar: comparison with anti-predator responses

C Curé,Pjo Miller,Ld Sivle,Fpa Lam,Ph Kvadsheim,S Isojunno,F Visser,Pj Wensveen

doi:10.3354/esr00748

Abstract

A key issue when investigating effects of anthropogenic noise on cetacean behavior is to identify the biological significance of the responses. Predator presence can be considered a natural high-level disturbance stimulus to which prey animals have evolved adaptive response strategies to reduce their risk of predation by altering behavior away from fitness-enhancing activities such as foraging. By contrasting the type and magnitude (duration, severity, consistency) of behavioral responses to anthropogenic noise and playback of killer whale (KW) sounds that simulated predator presence, this study aimed to provide a relative index of the disturbance level as an indication of the biological significance of responses to the anthropogenic stimulus. Using multi-sensor tags as well as visual observations of surface behavior of adult male sperm whales, we assessed a comprehensive range of behavioral metrics that could reduce individuals' fitness if altered for a biologically relevant duration. Combining previously published results and new analyses, we showed that the responses to 1-2 kHz upsweep naval sonar and to KW playback were very similar, including horizontal avoidance, interruption of foraging or resting activities and an increase in social sound production. However, only KW playbacks elicited grouping behaviors, indicating that this social response component was specific to predator detection. Animals responded to a lesser extent to 6-7 kHz upsweep naval sonar, indicating weaker disturbance effects. Our study demonstrates the benefit of using anti-predator responses as a reference of disturbance when evaluating the relative impacts of anthropogenic stimuli, which can be of particular interest in studies of threatened species such as sperm whales. © The authors 2016.

Highlights

Marine mammals, and cetacean species, rely on acoustics as a primary sensory modality to communicate, reproduce, find prey, orientate and obtain information from their environment (Richardson et al 1995, Tyack2008)
To investigate how sperm whale behavioral responses to naval sonar compare to the disturbance effect induced by the detection of a predator, we focused on 3 primary behavioral aspects: horizontal avoidance responses, alteration of foraging and resting behaviors, and induction of social responses
Each of the 3 categories of behavioral responses were exhibited in response to at least one of the sound exposure types (i.e. Low frequency active sonar (LFAS), Mid frequency active sonar (MFAS), LFASDS, killer whale (KW) playbacks, no-sonar CTRL and noise CTRL playback)

Summary

Introduction

Cetacean species, rely on acoustics as a primary sensory modality to communicate, reproduce, find prey, orientate and obtain information from their environment (e.g. detection of predators) (Richardson et al 1995, Tyack2008). Underwater anthropogenic noise can impact cetaceans in different ways, including physiological effects (e.g. increasing level of stress or hearing impairment) and behavioral changes (e.g. avoidance responses) (Nowacek et al 2007, Southall et al 2007). The biological importance of anthropogenic impacts should be investigated by examining long-term effects on critical biological population metrics such as growth, survival, reproduction and distribution (Wartzok et al 2003, Southall et al 2007). These metrics require a large and representative sample from the target population, which is often difficult to obtain and can lead to uncertainty in the results. Most behavioral response studies have concentrated on establishing links between short-term behavioral changes that can have a possible relevance to individual fitness and long-term population consequences (New et al 2014)

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