Simulated seal scarer sounds scare porpoises, but not seals: species-specific responses to 12 kHz deterrence sounds.

Lonnie Mikkelsen,Line Hermannsen,Kristian Beedholm,Jakob Tougaard,Peter Teglberg Madsen

doi:10.1098/rsos.170286

Abstract

Acoustic harassment devices (AHD) or ‘seal scarers’ are used extensively, not only to deter seals from fisheries, but also as mitigation tools to deter marine mammals from potentially harmful sound sources, such as offshore pile driving. To test the effectiveness of AHDs, we conducted two studies with similar experimental set-ups on two key species: harbour porpoises and harbour seals. We exposed animals to 500 ms tone bursts at 12 kHz simulating that of an AHD (Lofitech), but with reduced output levels (source peak-to-peak level of 165 dB re 1 µPa). Animals were localized with a theodolite before, during and after sound exposures. In total, 12 sound exposures were conducted to porpoises and 13 exposures to seals. Porpoises were found to exhibit avoidance reactions out to ranges of 525 m from the sound source. Contrary to this, seal observations increased during sound exposure within 100 m of the loudspeaker. We thereby demonstrate that porpoises and seals respond very differently to AHD sounds. This has important implications for application of AHDs in multi-species habitats, as sound levels required to deter less sensitive species (seals) can lead to excessive and unwanted large deterrence ranges on more sensitive species (porpoises).

Highlights

Human activities in offshore areas are on the rise, and underwater noise follows suit with potentially negative consequences for marine life [1]
The clear incentive for preventing or reducing such exposures to pile driving and other loud impulsive sound sources has prompted the use of acoustic harassment devices (AHD) or ‘seal scarers’ as mitigation devices
Results showed a very large variation in sound pressure level, 20 dB or more. This variation was both within trials and between trials and was evident both in sound pressure levels (Leq125 ms) and sound exposure levels (SELSS, table 1)

Summary

Introduction

Human activities in offshore areas are on the rise, and underwater noise follows suit with potentially negative consequences for marine life [1]. [2,3]) and harbour seals (Phoca vitulina) [4]; both facing increasing human encroachment in their shallow water habitats If these animals are close enough to loud transient noise sources, such as pile driving, explosions and airguns, they may face the risk of temporary or permanent hearing loss (TTS and PTS, respectively) [5,6]. AHDs were, originally designed to deter seals from fishing gear and aquaculture installations to avoid depredation on fishes and damage to fishing gear The effectiveness of these devices for this purpose appears to vary with type of device, characteristics of the emitted sound and context (see review by Götz & Janik [7]). Responses to AHDs, quantified as the effective deterrence distance for different species, will probably vary considerably with differences in hearing thresholds among species, as well as differences in ecology, group behaviour and exposure history of individuals

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