NOAA Technical Memorandum NMFS-NWFSC-89

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Sound Exposure and

Southern Resident Killer Whales

(Orcinus orca):

A Review of Current Knowledge and Data Gaps

February 2008
NOAA Technical Memorandum

NMFS Series

The Northwest Fisheries Science Center of the National 

Marine Fisheries Service, NOAA, uses the NOAA Technical Memorandum NMFS series to issue informal scientific 

and technical publications when complete formal review 

and editorial processing are not appropriate or feasible due 

to time constraints. Documents published in this series may 

be referenced in the scientific and technical literature.

The NMFS-NWFSC Technical Memorandum series of the 

Northwest Fisheries Science Center continues the NMFSF/NWC series established in 1970 by the Northwest & 

Alaska Fisheries Science Center, which has since been 

split into the Northwest Fisheries Science Center and the 

Alaska Fisheries Science Center. The NMFS-AFSC Technical Memorandum series is now being used by the Alaska 

Fisheries Science Center.

Reference throughout this document to trade names does 

not imply endorsement by the National Marine Fisheries 

Service, NOAA.

This document should be cited as follows:

Holt, M.M. 2008. Sound exposure and Southern Resident 

killer whales (Orcinus orca): A review of current knowledge and data gaps. U.S. Dept. Commer., NOAA Tech. 

Memo. NMFS-NWFSC-89, 59 p.
NOAA Technical Memorandum NMFS-NWFSC-89

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Sound Exposure and

Southern Resident Killer Whales

(Orcinus orca):

A Review of Current Knowledge and Data Gaps

Marla M. Holt

Northwest Fisheries Science Center

Conservation Biology Division

2725 Montlake Boulevard East

Seattle, Washington 98112

February 2008
ii

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Table of Contents 

List of Figures...............................................................................................................................................v 

List of Tables ..............................................................................................................................................vii 

Executive Summary .....................................................................................................................................ix 

Acknowledgments........................................................................................................................................xi 

Abbreviations and Acronyms ....................................................................................................................xiii 

Introduction...................................................................................................................................................1 

Basic Concepts .........................................................................................................................................1 

Frequency Bandwidth...............................................................................................................................2 

Southern Resident Killer Whales..................................................................................................................4 

Killer Whale Sound Production and Function ..............................................................................................6 

Characterizing Sound and Propagation.........................................................................................................8 

Sound Propagation Variables ...................................................................................................................8 

Ambient Noise..........................................................................................................................................8 

Whale Watching Sound Propagation......................................................................................................12 

Other Anthropogenic Sound Sources .....................................................................................................14 

Modeling Sound Propagation .................................................................................................................15 

Auditory Capabilities and Auditory Effects of Sound Exposure ................................................................18 

Audiograms and Basic Auditory Function .............................................................................................18 

Hearing Sensitivity .................................................................................................................................19 

Other Capabilities...................................................................................................................................21 

Auditory Masking, Critical Ratios, and Critical Bandwidths.................................................................22 

Spatial Overlap .......................................................................................................................................23 

Hearing Loss Due to Sound Exposure....................................................................................................25 

Behavioral Changes in the Presence of Unwanted Sound ..........................................................................29 

Behavioral Patterns.................................................................................................................................29 

Active Sonar and other Anthropogenic Sounds......................................................................................30 

Vocal Response to Background Noise ...................................................................................................31 

Strandings and other Nonauditory Effects of Sound Exposure ..................................................................32 

Zones of Influence ......................................................................................................................................34 

iii
Zone of Audibility ..................................................................................................................................34 

Zone of Responsiveness .........................................................................................................................36 

Zone of Masking and Effects on the Active Space of Sound Emissions................................................37 

Zone of Hearing Loss or Injury ..............................................................................................................42 

Likelihood of Acoustic Impacts on the SRKW Population ........................................................................45 

Risk Assessment.....................................................................................................................................45 

Extent of Masking Effects......................................................................................................................46 

Interaction with Nonacoustic Variables..................................................................................................47 

Recommendations for Future Work............................................................................................................49 

Masking Effect Assessment....................................................................................................................49 

Behavioral Response ..............................................................................................................................50 

Likelihood of Auditory Injury ................................................................................................................50 

Conclusions.................................................................................................................................................52 

References...................................................................................................................................................53 

iv
List of Figures 

Figure 1. Critical habitat designation for Southern Resident killer whales .................................................5 

Figure 2. Cumulative distributions of 2-second average SPLs..................................................................10 

Figure 3. Spectrum levels in Haro Strait in July 2005 and November 2005..............................................11 

Figure 4. Received spectral levels of a container ship traveling at 21 knots as it passed 442 m from 

the recording equipment .............................................................................................................................14

Figure 5. Spectrum source levels of all vessels at cruise speeds recorded on the broadband 

hydrophone system .....................................................................................................................................15 

Figure 6. Behavioral and physiological audiograms based on averaged thresholds for two female 

killer whales................................................................................................................................................20 

Figure 7. Equal loudness contours of human subjects depicting the level of a comparison tone 

required to match the perceived loudness of a 1,000 Hz tone presented at different levels .......................22 

Figure 8. Behavioral audiograms plus electrophysiological thresholds below 2 kHz for the killer 

whale, bottlenose dolphin, and beluga whale .............................................................................................28 

Figure 9. Killer whale behavioral audiogram and one-third octave ambient levels in a sea state of 0......35 

Figure 10. Predicted maximum horizontal detection ranges at 50 kHz for a killer whale at the surface 

echolocating on a Chinook salmon at 65 m depth for various noise conditions.........................................42 

v
vi
List of Tables 

Table 1. Source levels for whale watching vessels at different speeds, calculated received levels, 

and measured received level of a container ship.........................................................................................39 

Table 2. Predicted maximum horizontal detection ranges at 50 kHz for a killer whale at the surface 

echolocating on a Chinook salmon at 65 m depth, reduction in range relative to ambient Haro Strait 

measurement, and percent of reduction in range ........................................................................................41 

vii
viii
Executive Summary 

This document reviews what is currently known about potential acoustic impacts on 

endangered Southern Resident killer whales (SRKWs). Killer whales (Orcinus orca) use sound 

for echolocation, social communication, and passive listening. Ambient noise, including that 

from natural and anthropogenic sources, has the potential to interfere with the reception and use 

of these important biological sounds. Significant sources of anthropogenic sounds that 

contribute to ambient background noise in critical habitats of SRKWs include sonar, acoustic 

harassment devices, vessel traffic, and construction noise. 

Most measurements of ambient sounds made in SRKW habitat are greatly influenced by 

vessel traffic that, at close ranges, raises noise levels significantly above ambient levels. In order 

to address potential acoustic impacts, particularly from anthropogenic sources, this document 

reviews parameters of sound that are pertinent to the auditory capabilities of killer whales and 

various studies on noise effects in killer whales and other dolphins. The latter includes auditory 

ramifications such as auditory masking or hearing loss and behavioral effects such as disruption 

of foraging events or avoidance of an area. With this information, the document then 

incorporates information on the soundscape of SRKW habitat and defines zones of audibility, 

responsiveness, masking, and hearing loss and addresses the likelihood of acoustic impacts on 

the SRKW population. 

Lastly, recommendations are made for future work in order to address gaps in 

information that, if available, would increase confidence in predicting the likelihood of acoustic 

impacts on SRKWs. 

ix
x
Acknowledgments 

Sue Moore, Brad Hanson, Dawn Noren, Candice Emmons, and Mike Ford provided 

valuable feedback on writing this report. This work was funded by the National Research 

Council Postdoctoral Research Associateships Program and the Northwest Fisheries Science 

Center. 

xi
xii
Abbreviations and Acronyms 

ABR auditory brainstem response 

AHD acoustic harassment device 

CR critical ratio 

dB decibel 

DL difference limen 

DI directivity index 

EL echo level 

Hz hertz 

kHz kilohertz 

μPa micropascal 

ms milliseconds 

MTTS masked temporary threshold shift 

NL noise level 

Pa pascal 

PAL passive acoustic listener 

PCB polychlorinated biphenyl 

PTS permanent threshold shift 

RMS root-mean-square 

RL received level 

SL source level 

SEL sound exposure level 

SPL sound pressure level 

SRKW Southern Resident killer whale 

TL transmission loss 

TS target strength 

TTS temporary threshold shift 

VTOSS Vessel Traffic Operations Support System 

xiii
xiv
Introduction 

Southern Resident killer whales (SRKWs) are an endangered population of 

approximately 85 individuals that spend the summer in inland waters surrounding British 

Columbia and Washington State. These killer whales (Orcinus orca) are fish eaters that 

typically feed on Pacific salmon (Oncorhynchus spp.). They actively use sound for echolocation 

and vocal communication. Additionally, they can glean information about the environment such 

as the presence of prey from passive listening. Both natural and anthropogenic sounds have the 

potential to impact the use of biologically important acoustic signals by SRKWs. Concern about 

anthropogenic sound exposure, such as those produced by vessels or military sonar, has provided 

the impetus to study and describe the acoustic environment that Southern Residents inhabit and 

the effects of sound exposure on their auditory system, behavior, and physiology. 

The purpose of this paper is to review what is currently known about killer whale 

auditory capabilities and the use of sound by killer whales, the characteristics of sound in their 

environment, and effects of sound exposure in killer whales and other dolphins in order to 

address potential acoustic impacts on the SRKW population. As is the case for all marine 

mammal groups, it is extremely difficult to address acoustic effects that might have indirect or 

small but consistent consequences at the population level, as opposed to those with immediate 

and sometimes extreme outcomes. The life history patterns and habitats of marine mammals in 

general make this assessment extremely challenging (NRC 2005). In most cases, there is 

insufficient empirical data on which to draw in order to address acoustic impacts at the 

population level. A review of the current data and their limitations allows the opportunity to 

assess data gaps, which are summarized at the end of this document. 

Basic Concepts 

Given the various ways that sound energy is quantified, it is necessary to review some 

basic acoustical concepts. Sound is essentially generated when a vibrating object sets molecules 

in a medium adjacent to that object into motion. Sound amplitude or what is perceived as 

loudness is directly related to the amount of pressure generated by the vibrating object. In a 

compressible medium, the motion of molecules produces positive pressure where there is 

condensation and negative pressure where there is rarefaction of molecules. The intervals of 

condensation and rarefaction typically occur in a cyclical fashion. In a plane progressive wave 

of sound (when the acoustic pressure is the same in all planes perpendicular to the direction of 

propagation), the instantaneous pressure, p, generated in a compressible fluid can be described 

by 

p = ρcu (1) 

where ρ equals the fluid density, c equals the speed of sound, and u equals the particle velocity. 

Acoustic pressure is typically measured as the root-mean-square (RMS) pressure average over 

the duration of the sound. For impulsive sound such as pile driving strikes or biosonar clicks, 

peak sound pressure (the range from zero to the greatest pressure of the signal) or peak-to-peak 
sound pressure (the range of the most positive to the most negative pressure of the signal) are 

often reported instead, since it is difficult to define an appropriate duration over which to average 

the signal’s pressure (Madsen 2005). Pressure is typically reported in units of pascals (Pa) or 

micropascals (μPa). In a plane progressive wave, sound intensity is described by the sound 

power per unit area and is a product of the sound pressure and particle velocity by 

I = pu (2) 

and substituting u from first equation, intensity of the sound, I, is related to p by 

I = p(p/ρc) = p2

/ρc (3) 

where p is the RMS pressure average over the duration of the sound. Intensity is typically 

reported in units of watts per square meter. Sound levels are most often described in units of 

decibel (dB), which is traditionally defined as a power or intensity ratio. Sound intensity level in 

decibels is as follows: 

dB = 10 log10 (I1/I2) (4)

where I1 is the intensity of the sound of interest and I2 is a reference intensity. In the case of a 

plane wave, sound pressure which is typically what is measured by a microphone or hydrophone 

may also be used to measure the sound’s magnitude in dB. Because sound intensity is 

proportional to pressure squared, sound pressure level (SPL) in dB is given by 

dB = 10 log10 (p1

2

)/( p2

2

) = 20 log10 (p1/ p2) (5)

where p1 is the pressure of the sound of interest and p2 is typically the standard reference pressure 

for a given medium. In water the reference is usually 1 μPa. SPLs in this document are 

referenced to the underwater convention (re 1 μPa) based on RMS measurements unless 

otherwise noted. This reference pressure is different from the standard used to measure sound 

pressure levels in air. Thus a dB (re 1 μPa) underwater is not equivalent to a dB (re 20 μPa) 

measured in air. Pulsed sounds such as explosions, seismic air gun pulses, or pile driving 

impacts are often measured in terms of their energy and not just pressure or intensity. Energy 

measures include time as a dimension and are also used to quantify sound exposure when both 

amplitude and duration of exposure is important. Energy is proportional to the time integral of 

the pressure squared and in dB sound exposure levels (SELs) has the units of dB re 1 μPa2

s. 

Frequency Bandwidth 

Amplitude, intensity, or energy measurements in dB are always dependent on the 

measurements integrated across a frequency bandwidth. Broadband SPL measurements (overall 

SPL) will be different from those based on one octave, one-third octave, narrower band, and 

spectral density level measurements. For example, sound pressure spectral density gives the 

mean squared pressure of a sound within a given frequency bandwidth divided by the 

measurement bandwidth and the units are pressure square per hertz. The decibel unit based on 

spectral density level is dB re 1 μPa2

 /Hz (or re 1 μPa /Hz1/2). On the other hand, one-third 

octave levels in dB are based on the mean square pressure level for each one-third octave band. 

2
An octave is a factor of two in frequency and sound levels are often reported in one-third 

octave bands because the effective filter bandwidth of the auditory system in humans and some 

animals is approximated by one-third octave (Richardson et al. 1995, but see the Auditory 

Capabilities section and Auditory Masking subsection below). The term noise in this document 

usually refers to the general definition of unwanted sound. For a further review of these 

concepts, see Urick (1983), Richardson et al. (1995), and the Web site http://www.dosits.org/ 

science/intro.htm, hosted by the University of Rhode Island. 

3
Southern Resident Killer Whales 

Killer whales are the largest cetacean in the dolphin family, Delphinidae. The three 

identified ecotypes of killer whales in the northeastern Pacific Ocean are residents, transients, 

and offshores. While there is considerable overlap in their geographic range, these ecotypes are 

genetically distinct and do not appear to interbreed. The differences between ecotypes also 

extend to their morphology, foraging ecology, behavior, and acoustic repertoire. For example, 

residents are generally fish eaters while transients are generally mammal eaters (Ford et al. 2000) 

and less is known about the diet of offshores. Residents tend to live in larger, more stable groups 

consisting of multigenerational, matrilineal-related kin, while transients live in smaller, less 

stable groups usually consisting of females and a few offspring (Ford et al. 2000). Residents 

tend to be more vocal, particularly when foraging and socializing, while transients are 

acoustically cryptic presumably because their prey can hear within the frequency range of their 

sound emissions (Barrett-Lennard et al. 1996, Deecke et al. 2002, Deecke et al. 2004). 

Along the U.S. and Canadian west coast, there are currently four communities of resident 

killer whales that have been identified: Northern, Southern, Southern Alaska, and Western 

Alaska Residents (Krahn et al. 2004). SRKWs consist of three pods—J, K, and L pod—found 

during the late spring to early autumn in the inland waters of Washington State and British 

Columbia. Members are individually identified based on photo identification records of natural 

markings. Like all marine mammals, they are long-lived and slow to mature. 

Both male and female resident killer whales of the area do not become sexually mature 

until an average age of 15 years and females produce an average of 5.5 surviving offspring 

(Olesiuk et al. 1990). Annual population censuses indicate that SRKW numbers experienced a 

population decline in the mid to late 1990s. This distinct population segment of killer whales has 

been listed under the Endangered Species Act of 1973 (ESA) as endangered (NMFS 2005b) with 

three factors believed to be related to their decline: food availability, contaminant loads, and 

vessel and noise interactions (Krahn et al. 2004). 

As part of the ESA listing, NOAA’s National Marine Fisheries Service (NMFS) is 

responsible for designating critical habitat for SRKWs. As shown in Figure 1, this includes the 

following areas but excludes spaces around U.S. military sites: 1) core summer area of U.S. 

waters up to the border of Canada and surrounding the San Juan Islands, 2) Puget Sound 

excluding Hood Canal, and 3) U.S. waters within the Strait of Juan de Fuca (NMFS 2006). The 

following two sections will review information on the use of sound by killer whales in general 

and the acoustic scene of the areas defined as critical habitat for SRKWs in particular. 

4
 

 Figure 1. Critical habitat designation for Southern Resident killer whales (NMFS 2006). 

5
Killer Whale Sound Production and Function 

Killer whales produce a wide variety of clicks, whistles, and pulsed calls (Schevill and 

Watkins 1966, Ford 1989, Thomsen et al. 2001). Clicks are echolocation signals that are 

produced individually or in click trains. Individual clicks produced by Northern Resident killer 

whales are relatively broadband, short (0.1–25 milliseconds [ms]), and range in frequency from 8 

to 80 kHz with an average center frequency of 50 kHz and an average bandwidth of 40 kHz (Au 

et al. 2004). These broadband biosonar signals are predominantly used for sensing objects such 

as prey in the environment and are produced by whales foraging on salmon at peak-to-peak 

source levels ranging from 195 to 225 dB re 1 μPa at 1 m (Au et al. 2004). There may be a 

considerable amount of variation in source level and frequency content of killer whale biosonar 

emissions depending on prey type. For example, Norwegian killer whales feeding on herring 

produced echolocation clicks that were lower in mean center frequency and source level, 

although there was some overlap in the range of these biosonar parameters (Simon et al. 2007). 

SRKWs also feed on salmon and it is likely that their biosonar emissions are similar to those of 

Northern Residents. 

Whistles are tonal, nonpulsed signals that are relatively longer in duration (0.06–18 

seconds [s]) and lower in frequency (0.5–10.2 kHz, Thomsen et al. 2001). Whistles are most 

often heard during close-range social activities but not as often during foraging and traveling 

(Thomsen et al. 2002). Whistles produced by Northern Residents typically have source levels 

ranging from 133 to 147 dB re 1 μPa at 1 m (Miller 2006). 

Pulsed calls are the most commonly observed type of sounds emitted by killer whales and 

are categorized as discrete (stereotyped), variable, or aberrant (Ford 1989). These calls produced 

by both Northern and Southern residents are relatively long (600–2,000 ms), appear 

harmonically rich, and range in frequency between 1 and 10 kHz; but, those with high frequency 

components may contain harmonics up to 30 kHz (Ford 1989, Miller 2002). Variable calls are 

produced at source levels ranging from 133 to 165 dB while stereotyped calls are produced at 

source levels ranging from 135 to 168 dB re 1 μPa at 1 m (Miller 2006). These calls are most 

often used when killer whales are foraging and traveling and likely function to maintain social 

cohesion among pod members in the absence of other sensory information (Ford 1989, Miller 

2002). 

Killer whale communities have different discrete call repertoires, both among and 

between ecotypes (Ford 1991, Ford and Ellis 1999). Discrete calls are highly stereotyped and 

repetitive, and have pod-specific qualities that are stable over time (Ford 1989, Foote 2005). For 

example, each of the three pods among the Southern Resident population uses one or two 

discrete call types more than 50% of the time (Ford 1989, Foote 2005). 

Given the biological significance of sound for biosonar and social functions in killer 

whales, it is important to address how ambient noise, from a variety of sources acting alone or in 

6
combination with other threats, might impact the population structure of SRKWs. In order to 

assess noise impacts on this population, it is necessary to consider several parameters of sound 

that are pertinent to the auditory capabilities of these whales. Such factors include amplitude, 

duration, and spectral characteristics of the source, as well as how it propagates in the 

underwater environment. 

7
Characterizing Sound and Propagation 

Ambient noise is essentially the background din. Several sources of sound contribute to 

ambient noise levels in the ocean and coastal marine environments (for review, see Richardson et 

al. 1995). Natural sounds include those produced from abiotic and biotic sources. Abiotic 

sounds arise from activities related to weather (i.e., wind, waves, and rain), seismic activity, 

underwater slides, and currents. Some of these sources can substantially increase ambient noise 

levels. For example, heavy precipitation can raise levels from a few to 20 dB re 1μPa2/Hz or 

more between 1 and 20 kHz (Wenz 1962, Nystuen et al. 1993). Biotic sources of noise include 

those produced by marine mammals, fish, and snapping shrimp (Alpheus and Synalpheus spp.). 

For example, ambient noise in some areas is dominated by snapping shrimp in frequencies that 

overlap with those of echolocating dolphins (Au et al. 1985). 

Anthropogenic sounds that contribute to overall ambient noise include active sources 

(e.g., air guns, sonars) and those that are by-products of various human activities in the ocean. 

The latter include oil drilling, construction, and vessel traffic. Active sources include those used 

for military tactics, seismic surveys, fisheries, and oceanographic research. In the areas proposed 

as SRKW critical habitat, almost all of these sources of underwater sound are present. 

Sound Propagation Variables 

Sound propagated in sea water is subject to a number of variables that degrade the 

signal’s amplitude over distance and thus affect potential received levels. The transmission loss 

of sound occurs due to two primary factors: spreading and attenuation. If a source is allowed to 

spread in all directions equally, then a spherical spreading law may be used to describe 

transmission loss. This might be an appropriate assumption if a source emanates in deep water 

in the open ocean. However, even oceanographic features of the water column can create less 

than spherical spreading conditions. This is because the speed of sound varies based on 

temperature, salinity, and pressure (depth). 

Sound will also be attenuated as it travels over a distance due to absorption of sound by 

sea water and scattering due to particles. Sound is also reflected at the sea surface and from the 

bottom, creating instances of constructive or destructive interference. The details of spreading 

and attenuation of underwater sound are reviewed in Urick (1983). In shallow water 

environments such as those inhabited by SRKWs, the parameters that affect sound propagation 

can vary considerably in space and time. The acoustic environment in shallow water, therefore, 

is often described as complex. 

Ambient Noise 

Several investigations have reported ambient noise measurements in the areas designated 

as SRKW critical habitat and adjacent Canadian waters known to be important SRKW habitat. 

8
Many of these studies have focused on describing sound levels generated by vessel propulsion. 

Noise from vessel propulsion is usually generated by propeller cavitation (producing broadband 

noise), propeller “singing” (producing tonal sounds and harmonics related to the propeller blade 

rate), and auxiliary machinery (such as pumps and rotating shafts) with the amount of noise 

related to vessel size, speed, and mode of operation (Richardson et al. 1995). Data from the 

following peer-reviewed articles and government reports* are examined in this section: Veirs and 

Veirs (2005), Nystuen (2006), Erbe (2002), Hildebrand et al. (2006), Laughlin (2005), and Jones 

and Wolfson (2006). 

Veirs and Veirs (2005) report average sound pressure levels (SPL dB re 1μPa measured 

from 0.1 to 15 kHz) and power spectra (converted to noise spectral density levels in dB re 

1μPa2/Hz) of ambient sound recorded on the west side of San Juan Island in Haro Strait (lat 

48°33′25′′ N, long 123°10′23′′ W). All measurements were reported as received levels between 

0.1 and 15 kHz, thus propagation loss was not determined from source to receiver. The 

frequency range of these measurements is pertinent for the lower frequency range of killer whale 

hearing relevant to the reception of communication calls (see the Auditory Capabilities section 

below). Measurements were made on hydrophones that were cross-calibrated with a calibrated 

hydrophone and projector rented from the Naval Undersea Warfare Center in Newport, Rhode 

Island. 

Veirs and Veirs (2005) reported average SPLs over hourly, daily, and monthly time 

periods from April 2004 to November 2005. Half-hourly averaged SPLs ranged from 95 to 130 

dB with an overall half-hour SPL average of 115 dB. Broadband ambient sound levels were 

highly influenced by large vessels, such as commercial ships, that increased SPLs between 20 

and 25 dB over a 10–30 minute period and to a lesser extent by smaller vessels, such as motor 

boats, that increased SPLs by 15–20 dB (Veirs and Veirs 2005). During the summer (July and 

August), these smaller vessels contributed more to the overall ambient levels during the day, 

raising hourly SPL averages by 2–4 dB compared to nonsummer daytime hours. As a 

consequence, there was a more pronounced diurnal pattern in the summer but it only amounted 

to about a 2 dB difference in 12-hour averaged SPLs. 

Nighttime SPLs did not exhibit much seasonal variation, since larger ships operate during 

all months of the year. Monthly averaged SPLs ranged from 114.5 to 117.5 dB and were 

generally lowest from November through April and highest from June through August. 

Cumulative distributions of SPLs were also provided from archived measurements taken as 2-

second averages (Figure 2). These distributions illustrate that 2-second SPL averages are greater 

than or equal to 120 dB 50% of the time during summer days, 30% of the time during summer 

nights, and 20% of the time during the winter (Veirs and Veirs 2005). 

 

*

 Government research reports are based on work contracted through the Northwest Fisheries Science Center Marine 

Mammal Program and go through a limited peer review process. These reports are online at http://www.nwfsc.noaa 

.gov/research/divisions/cbd/marine_mammal/research.cfm or can be requested by contacting the report author(s). 

9
0

10

20

30

40

50

60

70

80

90

100

80 90 100 110 120 130 140

Cumulative 

% Cumulative % 

Summer Day

SPL (dB re 1μPa)

Summer Night

Winter Average

Summer day 

Summer night 

Winter average

SPL (dB re 1μPa) 

Figure 2. Cumulative distributions of 2-second average SPLs (0.1–15 kHz). Summer day includes SPL 

measurements during the hours of 0800–2000 and summer night includes SPL measurements 

from the other 12 of the 24 hour cycle (based on Figure 6 of Veirs and Veirs 2005). 

Figure 2. Cumulative distributions of 2-second average SPLs (0.1–15 kHz). Summer day includes SPL 

measurements during the hours of 0800–2000 and summer night includes SPL measurements 

from the other 12 of the 24 hour cycle (based on Figure 6 of Veirs and Veirs 2005). 

Ambient noise levels were further described by frequency and showed similar trends for 

ambient levels recorded underwater elsewhere (Wenz 1962). For both July and November 2005, 

the highest spectrum levels occurred for the lowest frequencies, with peaks of about 82 dB re 

1μPa2/Hz occurring between 400 and 500 Hz, then generally decreased at a rate of 5 dB for each 

doubling of frequency (i.e., per octave) from 500 Hz to 5 kHz and at a rate of 6 dB for each 

doubling of frequency from 5 to 20 kHz (Figure 3). Between 1 and 20 kHz, there was a more 

pronounced difference in spectral levels (up to 5 dB re 1μPa2/Hz) between daytime and 

nighttime hours in July compared to November. Below 1 kHz, very little seasonal variation in 

spectral levels was observed (Veirs and Veirs 2005). 

Ambient noise levels were further described by frequency and showed similar trends for 

ambient levels recorded underwater elsewhere (Wenz 1962). For both July and November 2005, 

the highest spectrum levels occurred for the lowest frequencies, with peaks of about 82 dB re 

1μPa2/Hz occurring between 400 and 500 Hz, then generally decreased at a rate of 5 dB for each 

doubling of frequency (i.e., per octave) from 500 Hz to 5 kHz and at a rate of 6 dB for each 

doubling of frequency from 5 to 20 kHz (Figure 3). Between 1 and 20 kHz, there was a more 

pronounced difference in spectral levels (up to 5 dB re 1μPa2/Hz) between daytime and 

nighttime hours in July compared to November. Below 1 kHz, very little seasonal variation in 

spectral levels was observed (Veirs and Veirs 2005). 

Ambient sounds were also recorded from acoustic moorings called passive aquatic 

listeners (PALs) during the late spring and summer of 2005 from two areas off Cape Flattery, 

Washington (Nystuen 2006). Mean sound spectra from these moorings showed that close ships 

dominated the sound field below 10 kHz while rain and drizzle were the dominant sound sources 

above 20 kHz during a week in April 2005. As the dominant sound source, relative spectral 

density levels of close ships decreased with increasing frequency between 1 and 10 kHz while 

sound from rain had a slighter decrease with increasing frequency for spectral density levels 

between 20 and 50 kHz (Nystuen 2006). Furthermore at this location, shipping noise dominated 

the sound field approximately 10% to 30% of the time but was dependent on weather; that is, 

when the weather was poor, less shipping noise was present. From April to July the shipping 

activity increased while rain activity, which dominated the sound field from 1 to 10% of the 

time, decreased (Nystuen 2006). 

Ambient sounds were also recorded from acoustic moorings called passive aquatic 

listeners (PALs) during the late spring and summer of 2005 from two areas off Cape Flattery, 

Washington (Nystuen 2006). Mean sound spectra from these moorings showed that close ships 

dominated the sound field below 10 kHz while rain and drizzle were the dominant sound sources 

above 20 kHz during a week in April 2005. As the dominant sound source, relative spectral 

density levels of close ships decreased with increasing frequency between 1 and 10 kHz while 

sound from rain had a slighter decrease with increasing frequency for spectral density levels 

between 20 and 50 kHz (Nystuen 2006). Furthermore at this location, shipping noise dominated 

the sound field approximately 10% to 30% of the time but was dependent on weather; that is, 

when the weather was poor, less shipping noise was present. From April to July the shipping 

activity increased while rain activity, which dominated the sound field from 1 to 10% of the 

time, decreased (Nystuen 2006). 

10
a) 

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Nightly

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b) 

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75

80

85

90

100 1000 10000 100000

Frequency (Hz)

dB re 1μPa/Hz 1/2

Noon

Daily

Monthly

Nightly

Midnight

dB re 1μPa/Hz 

100 1,000 10,000 100,000 

Frequency (Hz) 

Figure 3. Spectrum levels in Haro Strait in a) July 2005 and b) November 2005 (based on Figure 7 and 

Figure 8 in Veirs and Veirs 2005). 

11
Whale Watching Sound Propagation 

Erbe (2002) reported sound level measurements in the presence and absence of whale 

watching vessels in Brotchi Ledge outside of Victoria Harbor (Canada) and in Haro Strait on the 

west side of San Juan Island (United States). Sound levels of whale watching inflatable and 

noninflatable motorboats were based on recordings made between 0.1 and 20 kHz and lasting 

10–15 seconds. Vessel source levels were estimated using propagation models based on ray 

theory which included variables based on sound speed profiles, absorption loss by the sediment, 

and frequency-dependent absorption by ocean water (Erbe 2002). 

In general, inflatables with larger engines had higher source levels, ones with stern drives 

were considerably quieter than those with outboards, and inflatables were slightly louder than 

motorboats with inboards or stern drives. The relationship between source level and vessel 

speed was nonmonotonic and attributed to vessel speed measurement error of the radar gun (Erbe 

2002). At speeds of approximately 50 km/hour, average broadband source levels measured 

between 0.1 and 20 kHz were 162 and 159 dB re 1 μPa at 1 m for inflatables and motorboats, 

respectively (Erbe 2002). Noise levels were higher by about 15–27 dB re 1 μPa (based on onetwelfth octave band level analysis) in the presence of about five vessels (mix of inflatables and 

motorboats operating within 400 m of the hydrophone) compared to those measured in their 

absence (at a sea state of one-half, Erbe 2002). 

Hildebrand et al. (2006) also reported source level measurements (as spectra in dB re 1 

μPa2/Hz at 1 m) for a variety of vessels of different sizes, propulsion systems, and operational 

speeds in Haro Strait. Measurements include those made from a calibrated broadband recording 

system (1–75 kHz). The extended frequency range of the vessel noise is pertinent for the higher 

frequency range of killer whale hearing relevant to the reception of echolocation signals (see the 

Auditory Capabilities section below). Received levels were measured at ranges that varied 

between 125 and 442 m for idle, normal cruise speed (17–31 knots), and power acceleration to 

full speed. Source level spectra for 10-second averages were calculated at 1 m assuming 

spherical spreading loss and using absorption coefficients from the equation of Ainslie and 

McColm (1998). 

For power up conditions, the 10-second interval was chosen to include the highest sound 

pressure level. Noise spectra were not sufficiently above background levels to make accurate 

measurements of vessel noise levels under controlled conditions while vessels were idle despite 

efforts to collect data under conditions where no other vessels in the local vicinity were present. 

Thus data collected under idle conditions were not considered to be accurate measurements of 

real vessel noise (Hildebrand et al. 2006). 

Noise source spectra from four whale watching vessels reported by Hildebrand et al. 

(2006) are summarized here. Boat A is a 28-foot fiberglass monohull with twin 90-horsepower, 

four-stroke outboard motors. Only the power up measurements produced spectral levels that 

were sufficiently above ambient noise levels. Under these traveling conditions, source spectral 

levels were approximately 118 dB re 1 μPa2

/Hz at 1 m between 4 and 15 kHz, then generally 

decreased at a rate of approximately 5 dB per octave up to 50 kHz. Another small whale 

watching vessel, Boat B with a 29-foot aluminum monohull and twin 225-horsepower outboard 

motors, produced source levels at a cruise speed of 24 knots that were significantly above 

12
ambient conditions between 100 and 300 Hz and between 6 and 75 kHz. Another medium-sized 

whale watching vessel, Boat C, is a 38-foot aluminum catamaran with jet drives. While cruising 

at a speed of 31 knots, Boat C produced source spectral levels that were generally lower than the 

smaller vessel with no measurable rotating equipment noise at the lower frequencies. The 

difference between idle and power acceleration noise was less than 10 dB. 

Source levels for a larger whale watching vessel, Boat D, a 50-foot monohull vessel with 

three inboard/outboard (stern) drives, produced the highest source spectral levels that generally 

ranged between 110 and 145 dB re 1 μPa2/Hz at 1 m between 60 and 1,000 Hz, with higher 

peaks at some frequencies in this range while idling. At the same speed, this vessel produced 

source levels that ranged between 115 and 110 dB re 1 μPa2/Hz at 1 m between 1 and 5 kHz, 

then generally decreased at a rate of 5 dB/octave between 5 and 75 kHz. When Boat D was 

operating at a cruise speed of 23 knots, source spectral levels generally ranged between 125 and 

145 dB re 1 μPa2/Hz at 1 m between 60 and 1,000 Hz, with higher peaks at some frequencies in 

this range as well. At the same speed, this vessel produced source spectral levels of about 125 

dB re 1 μPa2

/Hz at 1 m between 1 and 20 kHz, then generally decreased at a rate of 

approximately 10 dB/octave up to 75 kHz. 

When Boat D was powering to cruise, source spectral levels generally ranged between 

122 and 145 dB re 1 μPa2

/Hz at 1 m between 60 and 1,000 Hz, with the highest peak of 148 dB 

re 1 μPa2/Hz at 1 m between 100 and 200 Hz. At the same speed, Boat D produced source 

spectral levels that ranged between 120 and 128 dB re 1 μPa2/Hz at 1 m between 1 and 20 kHz, 

then generally decreased at a rate of approximately 10 dB/octave up to 60 kHz (Hildebrand et al. 

2006). 

Hildebrand et al. (2006) also opportunistically measured the source spectral levels of a 

290 m long Korean container ship, the MV Hanjin Marseilles. Source spectral levels ranged 

from 135 to 165 dB re 1 μPa2/Hz at 1 m between 60 and 1,000 Hz when measured from the low 

frequency hydrophone and showed a more or less linear decrease in level of approximately 10 

dB/octave across this frequency range. At 442 m from the recording equipment, the received 

spectrum levels of the container ship ranged between 42 and 110 dB re 1 μPa2

/Hz between 0.06 

and 75 kHz. Even at the highest frequencies, received levels were approximately 20 dB above 

ambient levels (that were measured on a different day) as shown in Figure 4 (Hildebrand et al. 

2006). 

A comparison of source spectral levels for all vessels operating at cruise speed is shown 

in Figure 5 (Hildebrand et al. 2006). Above 2 kHz, the largest whale watching vessel, Boat D, 

produced the highest source spectral levels, while Boat C produced the lowest source spectral 

levels. Boat C presumably produced the lowest noise levels at higher frequencies because of its 

jet drive system. The MV Hanjin Marseilles produced surprisingly significant levels of noise 

above 2 kHz that, with the exception of Boat D, were higher in level compared to other whale 

watching vessels. 

From the results of Hildebrand et al. (2006) and Erbe (2002), noise generated by whale 

watching vessels as well as other vessel types is dependent on a combination of size, engine type, 

and operating speed. 

13
Pressure spectrum level dB re 1uPa2/Hz 

Ambient 5/31 at 12:30 

MV Hanjin Marseilles at 442 m 

Ambient 5/31 at 1:00 

Frequency (Hz) 

Figure 4. Received spectral levels of a container ship (MV Hanjin Marseilles) traveling at 21 knots as it 

passed 442 m from the recording equipment. Ambient conditions the day after the Hanjin passed 

in the Haro Strait as well as average sea states of 1 and 3 in deep water reported by Urick (1983) 

are also shown for comparison (adapted from Hildebrand et al. 2006). 

Other Anthropogenic Sound Sources 

While most studies thus far reviewed have focused on vessel sounds, other sources of 

anthropogenic sounds in SRKW habitat include those associated with marine construction such 

as pile driving and dredging. Sound exposure from pile driving has received particular attention 

given the loud impulsive (transient with rapid rise time) nature of such construction activity. As 

with other types of sounds, exposure from pile driving will depend on a number of factors 

including the size and type of pile driving equipment, the intensity of the driving activity, its duty 

cycle, and the local environment. Sound level measurements were made of pile driving activity 

associated with the restoration of the Friday Harbor ferry terminal. Average peak levels ranged 

between 180 and 215 dB re 1 μPa between 0 and 10 kHz (average RMS levels: 166–196 dB re 1 

μPa, average SELs: 171–187 dB re 1 μPa2s) that varied depending on the condition (Lauglin 

2005). 

In some cases, mitigation measures are taken to reduce the overall level of construction 

noise introduced into the environment. For example, air bubble curtains, in which a perforated 

air hose is placed outside the pile, generates bubbles as air is passed through the hose. Bubble 

curtain designs include single, dual, multiple ring, and tree curtains. In ideal cases, the bubble 

curtain effectively absorbs and reduces the magnitude of the pressure waves generated from 

14
Boat A 

Boat B 

Boat C 

Boat D 

Ship 

Boat D 

Ship 

Boat B 

Boat A 

Boat C 

Figure 5. Spectrum source levels of all vessels at cruise speeds recorded on the broadband hydrophone 

system (adapted from Hildebrand et al. 2006). 

driving the pile into the ground (Wursig et al. 2000). However, many factors can influence the 

effectiveness of the bubble curtain such as water currents, bathymetry, and tide levels. The 

average reduction in sound levels from multiple ring bubble curtain deployment during pile 

driving at the Friday Harbor ferry terminal ranged between 1 and 3 dB and the maximum 

reduction was 16 dB (Laughlin 2005). The results depended on the individual piles driven, the 

extent of the bubble curtain, and external factors such as ambient noise, given that these tests 

were not conducted in controlled conditions (Laughlin 2005). 

Modeling Sound Propagation 

Because many areas frequented by SRKWs are complex shallow water acoustic 

environments, it is necessary to consider how transmission loss from source to receiver will 

affect the sound levels potentially reaching killer whales in the area. Jones and Wolfson (2005) 

modeled propagation of sound emanating from large vessels in the main shipping lanes in the 

Haro Strait, focusing on numerical estimates in the open channel. Predicted values of received 

sound levels were compared to field measurements made by PALs collected in spring and 

summer 2004 (Nystuen 2006). Information on the position, course, and speed of large vessels 

(≥ 20 m in length) was obtained from the Vessel Traffic Operations Support System (VTOSS) of 

the Canadian Coast Guard Services. Information for 27 May–30 June 2004 from VTOSS 

illustrated that 24% of the vessels larger than 20 m were bulk carriers, 15% were tugs, 13% were 

15
container ships, 10% were ferries, 9% were fishing vessels, and a smaller percentage were a 

variety of other vessels types. 

Correlation of specific acoustic recordings with data from the VTOSS database provided 

information about the sound propagation of ships based on type, location, speed, and orientation. 

For example, when a large cargo ship passed to the side of the PAL mooring at a range of 1 km, 

received levels were approximately 85 and 75 dB (re 1μPa) at 3.6 and 10.4 kHz, respectively. In 

contrast, when the same ship passed to the side of the PAL mooring at a range of 4–5 km, the 

spectral levels of the same frequencies were about 20 dB lower. 

In Jones and Wolfson (2006), modeling and analysis were focused at the frequency of 3.6 

kHz to represent the frequency ranges between 1 and 10 kHz. Acoustic propagation modeling 

was described using two-dimensional parabolic equation numerical methods in which only 

reverberation in the forward direction was considered while backscattering was neglected (Jones 

and Wolfson 2006). This model is especially vulnerable to uncertainties in bathymetry, sea 

surface roughness, bottom substrate, and variables affecting sound speed profiles. 

Characteristics of these parameters along with appropriate assumptions were incorporated 

in the model for analysis. Sound speed profiles obtained from conductivity, temperature, and 

depth measurements collected in May and June in 1990 to 2002 illustrated relatively little 

variation in sound speed with depth and locations in the Haro Strait (Jones and Wolfson 2006). 

Model simulations were performed for a single ship at 10 ship positions, each position separated 

by 1 km as it passed by the stationary mooring. The model was run with three different 

categories of sediment type (smoother sand/mud sediment, rough sand/mud sediment, rough 

rock/sand sediment) and three levels of wind speed/sea surface roughness (0, 5, and 10 m/s). 

In all cases, the model results were consistent with the measured results when an 

estimated ship source level of 175 dB at 3.6 kHz was used. However, the results for the different 

cases of sediment and sea surface conditions varied within 3 dB of each other, implying these 

parameters do not strongly influence model results (Jones and Wolfson 2005). Furthermore, 

variability of the model results did not increase with increased sea surface roughness. The 

investigators hypothesize that the short distance between ship and receiver results in bathymetric 

effects that dominate the results while bottom and surface conditions contribute little to 

propagation effects. 

Model results were also obtained for the average positions of ships traveling in the 

northern shipping lanes of the Haro Strait using VTOSS database information. The closest 

distance to the mooring in this situation was approximately 2 km while the farthest was 

approximately 7 km. Predicted received levels at the farthest point were lower by about 15 dB at 

3.6 kHz compared to the closest point based on this model. Given the complexity of the acoustic 

environment of the Haro Strait, the report recommends that acoustic modeling is best used as a 

complement to field measurements (Jones and Wolfson 2006). Other recommendations include 

correlating seasonal and regional measurements of background sound levels with records of 

killer whale locations and direct measurements of broadband source levels of large ships in the 

main shipping lanes. 

16
To summarize, most measurements of ambient sounds levels in SRKW habitat are 

strongly influenced by vessel traffic that can reach broadband received levels of up to 130 dB 

and source levels that average approximately 160 dB (Erbe 2002, Veirs and Veirs 2005). In 

addition various vessels, including those used for whale watching activities as well as larger 

ships, can raise noise levels significantly above ambient levels even at frequencies up to 75 kHz 

(Hildebrand et al. 2006). Furthermore, data model comparisons illustrate that propagation 

models may accurately predict levels of vessel noise potentially received by killer whales at a 

particular location. However, given the complex shallow water environment, models should be 

used only to complement and cannot substitute for field measurements. 

17
Auditory Capabilities and Auditory Effects 

of Sound Exposure 

The reports summarized in the previous section focus on describing sounds in SRKW 

core summer habitats. In order to assess noise impacts, it is necessary also to consider several 

parameters of sound that are pertinent to the auditory capabilities of the whales. Such factors 

include hearing sensitivity of sound based on amplitude, duration and temporal factors, 

frequency, and how exposure of sound affects baseline sensitivity and other auditory functions. 

This section reviews what is known about the hearing capabilities and auditory effects of sound 

exposure in killer whales. Because of the limitations associated with time and cost of conducting 

laboratory research on marine mammals, there is a dearth of information about some aspects of 

auditory capabilities in killer whales, particularly in terms of auditory effects of noise. A number 

of studies have investigated such effects on smaller delphinids and appropriate information will 

also be reviewed to supplement current knowledge in this area. 

Audiograms and Basic Auditory Function 

There are several experimental approaches that have been used to determine the auditory 

capabilities of marine mammals including killer whales. A basic assessment of hearing is to 

determine sensitivity as a function of frequency in order to plot an audiogram. One of the most 

direct approaches to measure this involves behaviorally training subjects to respond in a 

particular way when they hear a particular sound and plot performance as a function of the 

sound’s amplitude (behavioral psychophysics). Another approach involves electrophysiological 

measurements of hearing sensitivity, which requires measuring auditory brainstem responses 

(ABRs) as a function of the sound’s amplitude. One advantage of the latter approach is that it 

typically takes less time to measure a threshold, which can be desirable in some instances, for 

example, when hearing recovery functions are measured after noise exposure. 

Both of these approaches have experimental limitations that should be noted. For 

example, not all listening environments are the same and ambient noise levels of the testing area 

should be measured and noted to avoid “noise-limited” measurements of hearing sensitivity. 

Additionally, the temporal summation (or temporal integration) properties of the auditory system 

influence sensitivity to a sound. Sounds shorter than some critical value or time constant are 

generally less detectable than longer signals; that is, sensitivity typically decreases nonlinearly 

for signals shorter than the time constant. Time constants for tonal stimuli in mammals typically 

vary between 30 and 800 ms and appear to be relatively consistent between several taxa (Fay 

1988). 

Although time constants have not been determined for killer whales, Johnson (1968) 

measured the dependence of thresholds on signal duration in a bottlenose dolphin (Tursiops 

truncatus) and reported time constants that ranged from 220 ms at 4 kHz to 30 ms at 45 kHz. 

18
Johnson (1991) also measured time constants in a beluga whale (Delphinapterus leucas) for a 60 

Hz single tone pulse and reported a time constant of 20 ms. The lower frequency results are 

consistent with those measured in humans (Plomp and Bouman 1959). Assuming that the 

temporal summation properties of killer whales follows the typical delphinid pattern, thresholds 

measured with signals shorter than these time constants will not be directly comparable to those 

measured with longer signals. For example, thresholds measured by electrophysiological 

methods are orders of magnitude shorter (and often below these time constants) than those 

measured by behavioral methods. 

Hearing Sensitivity 

In a behavioral psychophysical setting, hearing sensitivity is usually defined as a 

threshold. The concept of a threshold is a statistical probability of an animal detecting the sound 

at a particular level some percentage of the time (often 50 or 75% correct depending on the 

procedure). The physical parameters of the stimulus are not the only variables that affect 

threshold measurements of a subject. Nonsensory cues such as reinforcement contingencies and 

expectancies can significantly affect the measured threshold. For example, in most sound 

detection tasks, the signal level is reduced beyond some detection point and signal-present and 

signal-absent trials are both often used to keep the animal “honest” in reporting the presence of a 

signal. 

If an animal experiences punishment for responding to a signal when no signal is present, 

then the animal might become more conservative in reporting that it heard the signal. That is, 

the individual might adopt a tendency to report “no” when uncertain. This would lead to a 

higher measured hearing threshold or perhaps an underestimate of hearing sensitivity. For these 

reasons, it is necessary to account for animal response biases during sensory tasks (for further 

discussion see Schusterman 1974, Renouf 1991). This type of response behavior is not only 

important to consider in terms of how animals perform in laboratory settings but also how 

responses of free-ranging animals might be affected by different motivational states. For 

example, if there is disturbance in a unique feeding habitat where food is seasonally limited, 

animals may tolerate exposure because they are motivated to feed and may not have suitable 

alternatives. 

Another consideration of lab-based results is that very few individuals are typically 

tested. As with any population, auditory capabilities are expected to show individual variation 

based on factors including genetics, age, sex, and exposure to pathogens, ototoxic drugs, and 

noise. For example, in humans and presumably other mammals, high-frequency hearing loss or 

presbycusis is common with age (Yost 2000). Thus it is expected that a larger number of older 

animals would be less sensitive to higher frequencies than younger animals. 

Two laboratory studies on the hearing sensitivity of captive killer whales have been 

conducted (Hall and Johnson 1972, Szymanski et al. 1999). One subadult male was tested in the 

earlier study and an audiogram was obtained for 8-second pure tones ranging in frequency 

between 500 Hz and 31 kHz. Greatest sensitivity or a lowest threshold of 30 dB re 1 μPa for this 

individual occurred at 15 kHz, the observed upper limit of hearing sensitivity was 32 kHz, 

thresholds below 10 kHz were probably noise limited, and this individual was suspected to have 

high-frequency hearing loss (Hall and Johnson 1972). Both behavioral and electrophysiological 

19
thresholds were obtained in two adult females in a later study (Szymanski et al. 1999). These 

individuals had much higher and more plausible upper frequency hearing limits of about 120 

kHz compared to other delphinids than the individual in the Hall and Johnson (1972) study. 

Both behavioral and electrophysiological audiograms appeared similar but behavioral 

thresholds were typical lower (Figure 6). However, electrophysiological thresholds were 

measured using much shorter signals (1 or 0.5 ms durations) than those for behavioral thresholds 

(2 s). Furthermore, the investigators reported electrophysiological thresholds based on dB peakto-peak measurements and behavioral thresholds based on dB RMS measurements. If 

comparable dB measurements were computed, then the average differences of thresholds 

between the two methods would be reduced. The frequency of best sensitivity averaged for both 

subjects was 20 kHz and range of best sensitivity (±10 dB from lowest threshold) was 18–42 

kHz (Szymanski et al. 1999). 

One caveat of this study was that behavioral thresholds might be underestimates of 

hearing sensitivity because the subjects were reinforced after missing a signal if they waited for a 

recall tone and false alarm rates were reportedly so low that it is possible that the animals were 

trained to conservatively report the signal. Despite these experimental caveats, the audiogram 

based on Szymanski et al. (1999) is the best available data to determine hearing sensitivity of 

killer whales in quiet conditions. 

20

30

40

50

60

70

80

90

100

110

120

Behavioral - dB RMS

Physiological - dB peak-to-peak

Threshold (dB re 1 μPa) Threshold (dB re: 1 μPa)

100 1000 10000 100000 1000000

Frequency

100 1,000 1 1 1, 0,000 00,000 000,000 

Frequency (Hz) 

Figure 6. Behavioral and physiological (ABR) audiograms based on averaged thresholds for two female 

killer whales as reported by Szymanski et al. 1999. (Reprinted from Figure 5c in Szymanski et al., 

copyright 1999, with permission from Acoustical Society of America.) 

20
Other Capabilities 

Other auditory capabilities that are pertinent to sound exposure impacts include 

directional hearing and sound localization, frequency and intensity discrimination, and loudness 

perception. For example, animals must be able to localize the source in order to avoid a noxious 

sound. Unfortunately, there are no studies that have measured these auditory capabilities in 

killer whales, but results from studies on other delphinids are available to supplement current 

knowledge. Bottlenose dolphins have very good localization abilities; they can resolve angular 

separation of clicks within a fraction of a degree and pure tones between 6 and 100 kHz with a 

few degrees (≤ 5o) in both the horizontal and vertical planes (Renaud and Popper 1975). This 

ability is most likely related to use of echolocation for sensing objects in the underwater 

environment. 

Frequency discrimination involves the ability to perceive two tones as being separate in 

frequency or a pure tone being of constant frequency versus frequency modulated. This is 

usually measured via the difference limen (DL), which is typically defined as the difference 

between a reference sound frequency and that of the sound frequency that is just perceived as 

different. Because this discrimination ability is dependent on the reference value, discrimination 

is proportional to the standard and defined as the just noticeable difference or relative DL as a 

percentage of the standard (Richardson et al. 1995, Yost 2000). Bottlenose dolphins seem to 

outperform most mammals at frequency discrimination. Behavioral tests indicate that relative 

DLs are 0.21–0.81% from 2 to 130 kHz (Thompson and Herman 1975). Intensity discrimination 

is also very good in bottlenose dolphins, who can detect level differences of 0.35 to 2 dB for pure 

tones and clicks (reviewed in Richardson et al. 1995). 

For the most part, audiograms provide information about hearing sensitivity in quiet 

conditions; however, loudness perception is another important consideration of hearing and 

acoustic exposure impacts, particularly for predicting behavioral reactions to sounds and onset of 

auditory injury in mammals. The perception of loudness in humans is usually conducted through 

loudness-matching experiments. In these tasks, subjects are asked to match the loudness of a 

tone at one frequency to that of another frequency, then the results are plotted as level in dB of 

the comparison tone in equal loudness level contours. In people, equal loudness contours 

parallel the audiogram for SPLs but flatten out as SPL increases (Figure 7). Thus loudness 

perception can be approximated but not fully predicted by an audiogram. 

In determining noise exposure for people, idealized equal loudness curves have been used 

to calculate weighting functions, which emphasize some frequencies (at best sensitivity) and 

deemphasize other frequencies. Essentially, the weighting functions are used to filter sound for 

calculating exposure risk because weighting functions improve dose-response functions in 

humans (e.g., correlation between noise exposure and annoyance response, Leatherwood et al. 

2002). No equal loudness curves are currently available for delphinids or other marine 

mammals, so audiograms have been used to develop interim weighting function despite the fact 

that the effectiveness of this approach has not been tested (Miller et al. 2005, Southall et al. 

2007). 

21
Comparison level (dB re 20 

μPa)

Original standard 

Comparison frequency (Hz) 

Figure 7. Equal loudness contours of human subjects depicting the level of a comparison tone required to 

match the perceived loudness of a 1,000 Hz tone presented at different levels (threshold, 20, 40, 

60, 80, and 100 phons or dB SPL). Each of the above curves represents an equal loudness 

contour based on ISO Standard 226: 2003 revision. The original 40 phon standard is labeled 

(source: http://en.wikipedia.org/wiki/Equal-loudness_contour). 

Auditory Masking, Critical Ratios, and Critical Bandwidths 

While audiograms provide information about hearing sensitivity in quiet conditions, freeranging animals usually need to detect biologically important sounds in noisy environments. 

There are two types of auditory effects of noise exposure that will be reviewed here. One effect 

is a simultaneous or masking effect while the other effect is a residual or hearing loss effect that 

can be temporary or permanent (Yost 2000). In most cases, background noise must sufficiently 

overlap in frequency, level, duration, and direction of the target signal for these auditory effects 

to be fully realized. 

Any sound that reduces the audibility of another sound of interest has the potential to 

mask that sound. In practice, a target sound needs to be a certain level above the ambient noise 

in order for the target to be detected. This concept has been defined as the critical ratio (CR), or 

the difference in dB of the background ambient (masking) noise spectrum level and the amount 

by which a signal must exceed the background level in order to be audible. For example, a CR 

of 15 dB means that a signal must be 15 dB above the spectrum level of the noise in order for it 

to be detected. CRs across a number of mammal groups tend to be independent of the masker 

spectrum level and tend to increase with increasing frequency (Fay 1988). A few studies have 

measured CRs in the bottlenose dolphin, false killer whale (Pseudorca crassidens), and beluga 

22
whale (reviewed in Richardson et al. 1995). The results from these studies indicated that 

delphinid subjects have CRs ranging from 17 to 20 dB below 1 kHz to about 40 dB at 

approximately 100 kHz. Killer whales have CRs that ranged from about 20 dB at 10 kHz to 40 

dB at 80 kHz (Bain et al. unpublished data, cited in Bain and Dahlheim 1994). 

The amount of frequency overlap between a target signal and masker is probably one of 

the more important considerations of auditory masking in noisy environments. For example, 

pure tones are masked by sounds at frequencies near the tone frequency. Sounds at frequencies 

outside the bandwidth of the auditory filter typically do not affect audibility unless the noise 

level is very high. The limit of the frequency spread of the noise in its ability to mask a signal at 

a particular frequency is called the critical bandwidth. There are a number of different ways that 

critical bandwidths have been estimated. An indirect way, called the equal-power method, 

assumes that the signal power must be equal or exceed the total noise power in the masking band 

in order for it to be audible (Fletcher 1940). According to this assumption, the masking 

bandwidth (in Hz) is estimated as 10(CR/10). 

More direct ways to estimate critical bandwidths include measuring masked thresholds as 

a function of the bandwidth of the noise masker (band-narrowing technique) or using notched 

(band-reject) noise maskers and varying the bandwidth of the notch. These direct methods 

typically result in critical bandwidths that are usually but not necessarily wider than those 

estimated using the equal-power model for many mammals (Sharf 1970, Au and Moore, 1990, 

Southall et al. 2003a). It is now generally accepted that the indirect method using the equalpower model has limited accuracy in estimating critical bandwidths within a given species (see 

Southall et al. 2003b, Yost and Shofner 2005). 

While there are no direct critical bandwidth measurements available for killer whales, a 

few studies have measured critical bandwidths in other delphinids using both the band-narrowing 

and notched-noise techniques. Using the band-narrowing technique, Au and Moore (1990) 

reported critical bandwidths of 17, 25, and 45 kHz for pure-tone frequencies of 30, 60, and 120 

kHz, respectively, in a bottlenose dolphin. Using the notched noise technique, both Lemonds et 

al. (2000) and Finneran et al. (2002a) estimated much narrower critical bandwidths (reported as 

equivalent rectangular bandwidths). In the former study, critical bandwidths were 16% at 40 

kHz and about 11% between 60 and 100 kHz in a bottlenose dolphin (Lemonds et al. 2000). In 

the latter study, critical bandwidths were 12 and 17% of the center frequency at 20 and 30 kHz, 

respectively, in two bottlenose dolphins and 9 and 15% at 20 and 30 kHz, respectively, in a 

beluga whale (Finneran et al. 2002a). These values roughly correspond to one-sixth of an octave 

wide and, like CRs, are frequency dependent with critical bandwidths tending to increase with 

increasing frequency (Yost 2000). 

Spatial Overlap 

Spatial overlap between a target sound and masker is another important consideration of 

auditory masking. The directional hearing system helps ameliorate the masking effect of noise, 

provided that the masker itself has a direction. That is, when a signal and masker are spatially 

separate, masked thresholds are lower or sensitivity is better compared to when signals and 

maskers are collocated. The auditory phenomenon, termed “spatial release from masking” or 

“spatial unmasking,” can significantly improve signal detection of both simple and complex 

23
sounds. For example, Bain and Dahlheim (1994) measured auditory thresholds for tones at 4, 8, 

and 20 kHz masked by low frequency noise with energy primarily between 500 Hz and 5 kHz in 

two captive killer whales. The signal always occurred directly in front of the subject (0, 0°) 

while the masker was positioned at 0 (collocated), 90, or 150° in the horizontal plane or 30° 

above or below the subject. Although it is unclear whether any sound energy from the noise 

masked the 8 and 20 kHz signal, masked thresholds were generally lower (ranging between 0 

and 40 dB) when the signal and masker were spatially separate compared to when they were 

collocated. Masked thresholds were lowest (on average by −7 to −24 dB) when the signal and 

masker were separated by 90 and 150° relative to when the signal and masker were collocated. 

These effects were largest for the higher frequencies, but in most cases there were large 

interindividual differences (Bain and Dahlheim 1994). 

In another investigation using a bottlenose dolphin as a subject, masked thresholds were 

measured with spatial separation of signal (at 30, 60, and 120 kHz) and masker (band pass of 30 

and 150 kHz) in both the horizontal and vertical planes (Au and Moore 1984). As in the killer 

whale investigation, masked threshold differences of the dolphin were generally largest 

(thresholds were lowest) for the highest frequencies at both azimuth and elevation (Au and 

Moore 1984). CRs are generally measured with the signal and masker emanating from the same 

point in space. Thus masking effects in more real world situations would not be as substantial 

when signals and maskers have different spatial configurations relative to each other as data from 

critical ratio investigations suggest. 

Temporal factors can also cause release from masking in some cases, particularly when 

two separate sources of noise (maskers) across different frequencies are amplitude modulated in 

a coherent way. When the signal overlaps in frequency with one of the two maskers, detection 

of the signal improves when the two maskers are amplitude modulated coherently compared to 

when they are modulated incoherently. Such an auditory phenomenon is called comodulation 

masking release. This effect may be related to the degree of amplitude modulation of the two 

maskers. For example, when only one masker is present which overlaps in frequency with the 

target signals, detection of the signal is easier for a masker that has a larger degree of amplitude 

modulation (and periods of lower energy), compared to when a masker has less amplitude 

modulation even when the overall sound energy level of the maskers are equivalent (Brumm and 

Slabberkoorn 2005). No marine mammal studies on comodulation masking release have been 

conducted, but the effect seems to occur across a wide range of taxa including humans and birds 

and is likely to occur in marine mammals as well. 

While most investigations of auditory masking have measured thresholds of pure tones 

masked by white noise, very few studies have determined the effects of noise on the reception of 

natural sounds. Bain and Dahlheim (1994) reported thresholds of pure tones, killer whale clicks, 

and a call masked by recorded vessel noise from oil-spill clean up operations in Alaska’s Prince 

William Sound after the 1989 Exxon Valdez oil spill. While masking of pure tones occurred for 

all frequencies below 20 kHz, very little masking of the call and clicks was observed; however, a 

27 kHz pure tone was added to the call to facilitate performance transfer and it is unclear 

whether or not this improved detection (Bain and Dahlheim 1994). Given that the click and call 

plus high frequency tone were likely outside the bandwidth of the vessel noise, it is not 

surprising that these results were observed. 

24
Hearing Loss Due to Sound Exposure 

Another auditory effect of sound exposure is hearing loss. There are temporary and 

permanent forms of hearing loss. Temporary hearing loss or temporary threshold shift (TTS) 

involves recovery of baseline hearing over a period of time and occurs from physiological 

fatigue of the hair cells of the inner ear. The magnitude of the threshold shift depends on the 

amplitude, duration, temporal pattern, frequency, and energy content of fatiguing sound. For 

example, exposure to a more intense but shorter sound may result in similar TTS compared to 

that measured after exposure to a less intense but longer sound. With some types of sound 

exposure, subjects may experience large but fully recoverable shifts in threshold (≈40 dB) that 

become asymptotic with longer exposure durations (asymptotic TTS). The largest threshold 

shifts often occur at frequencies one-half to one octave above the frequency of exposure (Yost 

2000). 

Permanent hearing loss or permanent threshold shift (PTS) does not show recovery over 

time and is the manifestation of auditory injury. PTS results from damage or death of the hair 

cells in the inner ear. PTS can occur from repeated exposures that induce TTS or from a single 

intense exposure. The relationship between TTS and PTS is often quite complex despite intense 

research in humans and terrestrial models. For example, even very large threshold shifts (up to 

40 dB) resulting from sound exposure can be fully recovered. For ethical reasons, PTS is not 

studied in marine mammals. TTS has been investigated to describe how temporary hearing loss 

is dependent on the type, frequency, amplitude, and duration of the exposure, and to model PTS 

from TTS data despite the complicated relationship between the two. This work is typically 

done in a laboratory setting in which trained animals are tested for baseline hearing and exposed 

to noise, then hearing is tested again to measure a threshold shift. 

Both behavioral and electrophysiological methods have been used to measure TTS. 

While there are no TTS studies on killer whales, there are a handful of TTS investigations on 

bottlenose dolphins and beluga whales. These include measuring TTS after exposure to 

broadband noise (Nachtigall et al. 2003, 2004), tones (Schlundt et al. 2000, 2006, Finneran et al. 

2005, 2007), and impulsive sounds (Finneran et al. 2000, 2002b). Since TTS is dependent on 

both amplitude and duration of exposure, sound exposure level (SEL in dB re 1 μPa2

s) is a 

common way to quantify exposure, because it describes both duration and amplitude in one 

metric. However, SEL inherently assumes that different combinations of amplitude and duration 

that yield equivalent SELs (i.e., equal energy) result in the same amount of TTS. Some data 

measured in delphinids suggest that this may be an appropriate assumption under some 

conditions (Finneran et al. 2005). 

Using a behavioral procedure to measure threshold shifts of a 7.5 kHz tone in a 

bottlenose dolphin, an average TTS of 11 dB resulted after exposure to broadband noise having a 

flat spectrum between 4 and 11 kHz at 179 dB re 1 μPa for 41 to 54 minutes (Nachtigall et al. 

2003). This corresponded to an SEL of 213.8 dB re 1 μPa2

s when the average exposure duration 

of 50 minutes was used. Using an electrophysiological approach to measure threshold shifts at 8, 

11.2, 16, 22.5, and 32 kHz in the same subject with the same fatiguing stimulus at 160 dB re 1 

μPa for 30 minutes (SEL of 192.6 dB re 1 μPa2

s), the largest threshold shifts occurred after 5 

minutes of exposure (TTS5) at frequencies between 8 and 16 kHz. The maximum threshold shift 

of 8 dB occurred at 16 kHz with shifts equal to approximately 5 and 6 dB at 8 and 11.2 kHz, 

25
respectively. Recovery to baseline hearing levels was approximately 1.5 dB per doubling of time 

(Nachtigall et al. 2004). 

TTS was also measured in bottlenose dolphins and belugas whales exposed to 1-second 

tones ranging between 3 and 75 kHz and at levels between 182 and 201 dB re 1 μPa. Masked 

temporary threshold shifts (MTTS) at frequencies equal to or higher than (by one-half or one 

octave) the exposure frequency ranged between 6 and 17 dB (Schlundt et al. 2000). Only 

threshold shifts greater than or equal to 6 dB were reported as noise induced TTS because this 

value seemed sufficiently above the 3–4 dB variability of baseline hearing thresholds. The 

beluga whale exhibited equivalent threshold shifts (between 6 and 8 dB) at higher exposure 

levels to that of the dolphin when the same exposure and hearing test frequencies was used 

between the subjects. Only masked thresholds could be measured since behavioral testing was 

conducted in San Diego Bay, where ambient noise levels were high and variable, so masking 

noise (between 3 and 100 kHz and varied in spectral levels between 63 and 95 dB re 1μPa2

/Hz) 

was used to create a consistent noise floor during preexposure, exposure, and postexposure 

conditions (Schlundt et al. 2000). 

Because the presence of masking noise often results in smaller threshold shifts in 

terrestrial mammals, these results reported by Schlundt et al. (2000) were questioned and testing 

was replicated in a quiet pool again using a behavioral response procedure. In this study, two 

bottlenose dolphins were exposed to a 3 kHz tone at levels ranging between 100 and 200 dB re 1 

μPa for 1–8 seconds, corresponding to SELs of 100 to 203 dB re 1 μPa2

s. In most cases, 

significant threshold shifts (different from control exposures) measured 4 minutes after exposure 

(TTS4) were observed at 4.5 kHz for SELs above 190 dB re 1 μPa2s. No significant TTS was 

observed at the same frequency as the exposure up to levels of 200 dB re 1 μPa2

s (Finneran et al. 

2005). For SELs above 200 dB re 1 μPa2s, recovery was not complete by 10 minutes, but 

hearing sensitivity typically returned to baseline values within a day. All data summarized for 

exposures to continuous noise or tones (nonimpulsive sounds) in bottlenose dolphins and beluga 

whales showed that, in most cases, significant threshold shifts occurred at SELs greater than or 

equal to 195 dB re 1 μPa2s despite differences in exposure duration, sound pressure level, 

experimental approaches, and subjects (Finneran et al. 2005). 

More recent work has extended exposure durations at the same frequency up to 128 

seconds corresponding to SELs of up to 217 dB re 1 μPa2

s, and preliminary results indicated that 

TTS4 of up to 23 dB occurred with full recovery within 30 minutes (Schlundt et al. 2006). 

Hearing sensitivity was measured both behaviorally and electrophysiologically in a bottlenose 

dolphin after exposure to a 20 kHz tone that ranged in SELs of 203–206 dB re 1 μPa2

s in a more 

recent study by Finneran et al. (2007). Threshold shifts up to 40–45 dB using the physiological 

approach were observed with frequency-dependent results. The largest shifts were observed at 

30 kHz, followed by 40 and 20 kHz with no measurable TTS occurring at 10, 50, 60, and 70 

kHz. Threshold shifts were smaller (19–33 dB) and recovery time was faster when the 

behavioral method was used even when the exposure and the time taken to measure hearing 

sensitivity after exposure was comparable between the two approaches (Finneran et al. 2007). 

Such differences are likely attributable to inherent differences between the two approaches. 

Further investigations are required to work out these differences and to interpret TTS results 

using evoked potential audiometry (Finneran et al. 2007). 

26
A few studies have also investigated the effects of impulsive sounds (transient sounds 

having rapid rise times and high peak levels) on MTTS in bottlenose dolphins and beluga 

whales. In these studies, behavioral methods were used to measure masked hearing thresholds 

before and after exposure in San Diego Bay. No MTTS (defined as ≥ 6 dB) was observed in 

either species after exposure to sounds simulating underwater explosions with SELs up to 179 

dB re 1 μPa2s, but behavioral responses above 156 dB re 1 μPa2s were sometimes noted 

(Finneran et al. 2000). In another study, MTTS was measured after exposure to single 

underwater impulses from a seismic water gun. MTTSs of 7 and 6 dB were measured at 0.4 and 

30 kHz, respectively, in a beluga whale after exposures of 186 dB re 1 μPa2

s. Recovery 

occurred within 4 minutes after exposure. No MTTS greater than or equal to 6 dB was observed 

in the bottlenose dolphin up to SELs of 188 dB re 1 μPa2

s (Finneran et al. 2002b). 

It is unclear what effect the masking noise had on the threshold shifts from impulsive 

sound exposure because these experiments were not repeated in quieter conditions. In some 

cases, physiological responses to sound exposure were also investigated (Romano et al. 2004) 

and these are reviewed in the Nonauditory Effects section below. 

The TTS investigations on delphinids reviewed here involved sound exposures with 

constant exposure levels. However, free-ranging animals are often exposed to sounds that are 

intermittent in time and space. While very little work has been done on the issue of sound 

exposure intermittence on TTS in marine mammals, such exposures in people yield somewhat 

complicated results, partly because of partial recovery during the quieter periods. For very short 

interruptions or small fluctuations, TTS is proportional to the average exposure level. With more 

variability in sound exposure levels, TTS is not a consistent function of the total energy of the 

exposure (Ward 1997). 

Another consideration is that even between similarly related species, TTS at a given SEL 

can vary dramatically. This might be due to differential sensitivity at the frequencies of the 

sound exposure. Bottlenose dolphins and beluga whales have similar audiograms (Schlundt et 

al. 2000) which might explain why similar sound exposure levels produced comparable amounts 

of TTS. Hearing sensitivity at both 4 and 20 kHz is roughly 10–20 dB better in the killer whale 

than in the bottlenose dolphin, as shown in Figure 8. However, these results are based on the 

performance of only a few individuals (n = 1 or 2). Some investigators argue that it might be 

more appropriate to normalize SEL relative to baseline hearing sensitivity to account for these 

differences (Kastak et al. 2005). Thus while TTS results of the bottlenose dolphin and beluga 

whale provide the best available data to assess potential hearing loss due to sound exposure in 

killer whales, differences in hearing sensitivity at a given frequency might produce more or less 

TTS as a result of sound exposure at that frequency. 

27
20

30

40

50

60

70

80

90

100

110

120

130

140

150

Killer whale

Bottlenose dolphin

Beluga whale

10 100 1000 10000 100000 000000 1

equency

Threshold (dB re: 1 μPa)

Fr

Threshold (dB re 1μPa) 

10 100 1,000 1 1 1 0,000 00,000 ,000,000

Frequ (Hz) ency 

Figure 8. Behavioral audiograms plus electrophysiological thresholds below 2 kHz for the killer whale 

(based on Szymanski et al. 1999), bottlenose dolphin (based on Johnson 1967), and beluga whale 

(based on White et al. 1977, Awbry et al. 1988, Johnson et al. 1989). 

28
Behavioral Changes in the Presence of 

Unwanted Sound 

Other effects of sound exposure include behavioral changes that might have 

repercussions on life functions of affected animals. Behavioral responses to sound exposure can 

be highly variable and depend not only on factors related to the characteristics of the sound 

source, its transmission, and background ambient noise levels, but also on individual, group, 

population, and species-level differences arising from factors such as age, sex, dependent 

offspring presence, hearing sensitivity/loudness perception, activity patterns, motivational states, 

previous experience that may have resulted in habituation or sensitization, and noise tolerance 

(NRC 2003, 2005). 

There have been many studies investigating the effects of sound exposure on the behavior 

of marine mammals, including those on killer whales. These responses may be short-term 

changes such as changing swimming direction, dive duration, or vocal behavior, or long-term 

changes such as avoiding a once popular area for foraging, breeding, or socializing (Morton and 

Symonds 2002, Foote et al. 2004, Bain et al. 2006, Williams and Ashe 2006, Williams et al. 

2006). Most of these studies involve investigating behavioral changes in the presence of 

anthropogenic sources relative to some baseline behavioral measurement. However, in some 

cases, it is unclear whether the physical presence of the sound-producing device (such as a 

moving vessel) or the sound itself caused behavioral changes. 

Behavioral Patterns 

Behavioral activities of SRKWs were observed in the presence and absence of vessels 

within 1,000 m over the summers of 2003–2005 (Bain et al. 2006). Activity budgets (foraging, 

rest, traveling, and socializing) were reported at two sites along the west side of San Juan Island, 

a north site (lat 48°30.561′N, long 123°8.494′W) and a south site (lat 48°27.421′N, long 

122°59.401′W). Whales spent more time traveling and less time foraging in the presence of 

boats within 100 and 400 m of the focal whale group than in their absence. No difference 

between foraging, resting, and traveling between study sites was found, but whales spent more 

time socializing in the north than in the south site. They were more likely to continue foraging 

when boats were absent compared to when they were present within 100 and 400 m, but it was 

possible the responses at 400 m were a result of the effect at 100 m. Whales also traveled in less 

direct paths and had longer average durations between breaths when vessels were present 

compared to when they were absent within 1,000 m. Surface active behavior was not notably 

more frequent in the presence of vessels than in their absence within 1,000 m, and there was no 

significant difference in swim course or speed due to vessel traffic (Bain et al. 2006). 

Williams et al. (2006) also found that vessel presence changed the behavioral patterns of 

Northern Resident killer whales in Johnstone Strait, British Columbia. Whales spent 

significantly less time feeding and more time traveling, socializing, and resting in the presence of 

29
vessels than in their absence within eight zones, four within a reserve where no boats were 

allowed and four adjacent zones. While the overall energetic demand was estimated to be 3% 

higher in the presence of boats, the lost opportunity to feed had a larger energetic effect 

estimated to be about a 28% decrease in energetic gain (Williams et al. 2006). 

The significance of these results as well as those of Bain et al. (2006) is contingent on the 

ability of researchers to accurately discriminate different behavioral states of free-ranging killer 

whales. Williams and Ashe (2006) also conducted controlled experiments in which adult focal 

male Northern Residents in Johnstone Strait were tracked when no boats were within 1,000 m, 

then in the presence of a few (1–3) or many (>3) boats within 1,000 m. Males swam in less 

direct paths in the presence of a few boats within 1,000 m, but swam in more direct paths when 

there were many boats present within 1,000 m. If swimming directedness had been analyzed 

simply in the presence or absence of boats, then this difference in behavior would not have been 

apparent. An investigation of Southern Resident behavior with varying number of boats would 

also be useful to determine boat number effects. However, unlike in the Northern Residents, 

there are fewer opportunities to observe Southern Resident behavior when no boats or a few 

boats are near them. 

Active Sonar and other Anthropogenic Sounds 

There was also a documented case of exposure by J pod to military active sonar by the 

USS Shoup on 5 May 2003 in the Haro Strait (U.S. Navy 2004). The USS Shoup, a guided 

missile destroyer, was equipped with a AN/SQS-53C(V)4 hull-mounted sonar with kingfisher 

mine avoidance system that produces frequency-modulated signals between 2.6 and 3.3 kHz at a 

nominal source level of 235 dB re 1 μPa at 1 m. This system’s midfrequency tactical sonar is 

designed to detect, localize, and avoid mines and other objects. The goal of the Navy on that day 

was to train ship personnel to use the sonar system in a “sweep channel” exercise in which 1–2 

second signals were emitted once every 28 seconds from 1123 to 1438 hours (U.S. Navy 2004). 

During part of the exercise, J pod was in the area and observed by several researchers familiar 

with SRKW behavior who described the whales as exhibiting “abnormal behavior” including 

bunching as a group close to shore (NMFS 2005a). 

The sonar signals were recorded by Val Veirs on bottom mounted hydrophones and 

recordings were provided to NMFS, but the dynamic range of the system prevented accurate 

assessments of the highest received levels. Thus the Navy Research Laboratory estimated 

received levels relative to the whales. Mean received levels relative to J pod were estimated to 

range between 121 and 175 dB re 1 μPa and likely received SELs were 169.1–187.4 dB re 1 

μPa2

s with a worse case range estimated to be 177.7–195.8 dB re 1 μPa2s. NMFS (2005a) 

reported that it was likely that these received levels were audible to the whales and that 

reverberation from the complex underwater environment may have reduced the ability of the 

whales to localize the sound source. 

TTS onset was estimated based on TTS studies conducted on dolphins (reviewed in the 

Hearing Loss Due to Sound Exposure subsection above) and except in the worse case scenario, 

NMFS (2005a) stated that it was not likely that the whales experienced temporary or permanent 

hearing loss (based on a TTS onset level of 195 dB re 1 μPa2

s), but that the received levels were 

high enough to induce a behavioral reaction that was consistent with observer reports. 

30
Longer-term effects of sound exposure were observed in both resident and transient killer 

whales in the Broughton Archipelago, British Columbia, when acoustic harassment devices 

(AHDs) were in operation. The AHD signals were designed to repel harbor seals from salmon 

farms by producing a 10 kHz signal at a source level of 194 dB (re 1 μPa at 1m). It was 

estimated that the received levels of the AHD signals would reach ambient noise levels at about 

50 km from the source. The number of days killer whales were sighted in the area was 

significantly reduced after the AHD broadcasts, compared to before (Morton and Symonds 

2002). 

Other anthropogenic sounds described in SRKW critical habitat are those associated with 

construction activity, particularly pile driving given its relatively loud and impulsive acoustic 

signature at close ranges. Although behavioral effects of pile driving or other marine 

construction noise on killer whales has not been reported, Wursig et al. (2000) found that IndoPacific humpback dolphins (Sousa chinensis) significantly increased their group swim speed 

during pile driving even with the deployment of a bubble curtain, but such human activity did 

not have a long-term overall effect on dolphin abundance in the area. 

Vocal Response to Background Noise 

Another response to sound exposure involves changes in vocal behavior. In humans, an 

unconscious response to speak louder with higher background noise levels is called the Lombard 

effect/response (Lombard 1911) and is often observed when people wear headphones and talk 

while listening to loud music. Many animals also call louder, longer, more often, or at different 

frequencies in the presence of masking noise (Brumm and Slabbekoorn 2005). For example, 

beluga whales in the St. Lawrence River responded to increases in ambient noise from boat 

traffic by increasing call amplitude (Schiefele et al. 2005), call repetition rate, and frequency of 

the call (Lesage et al. 1999). SRKWs significantly increased the duration of their primary 

stereotyped call in the presence of boats compared to in their absence in the more recent time 

periods, which correlated with a dramatic increase in the number of whale watching boats in 

their habitat (Foote et al. 2004). In laboratory settings, bottlenose dolphins increased the average 

number of clicks emitted per trial as white noise levels were increased during an echolocation 

target detection task (Au et al. 1982). 

Biogenic noise from other animals also results in vocal compensation effects. For 

example, Antarctic killer whales shifted the frequency modulated points of their calls below or 

above the frequency range of predominant leopard seal (Hydruga leptonyx) calls during the 

season when leopard seals were calling (Mossbridge and Thomas 1999). A captive beluga whale 

produced louder and higher frequency echolocation clicks when it was moved from San Diego 

Bay, California, to Kaneohe Bay, Hawaii, which had ambient noise levels that were typically 12–

17 dB higher than in San Diego due to snapping shrimp presence (Au et al. 1985). Thus when it 

comes to sound production, there are strategies that animals use to attempt to compensate for 

increased ambient sound levels. When animals exhibit such vocal compensation behaviors, it 

can be interpreted that auditory masking is a potential challenge, particularly when ambient 

sound levels reach a point where animals can no longer compensate by increasing their vocal 

output or other sound emission parameter. 

31
Strandings and other Nonauditory Effects 

of Sound Exposure 

An extreme behavioral outcome to sound exposure is stranding. In some cases, cetacean 

strandings have coincided in space and time with military sonar exercises. There are three welldocumented cases, one in Greece in 1996, one in the Bahamas in 2000, and one in the Canary 

Islands in 2002, and all involved the mass stranding of 11–14 individual beaked whales (and 

sometimes other cetacean species). While there is a general consensus among the scientific 

community that military sonar was a likely causal link to the strandings, the mechanisms which 

caused the observed pathologies are still a mystery (NRC 2005). For example, results from 

necropsies performed on 10 of the 14 individuals involved in the Canary Island stranding event 

revealed extensive bubble formation and tissue damage in various organs. A number of 

hypotheses including decompression sickness and acoustically mediated bubble growth due to 

tissue supersaturation of nitrogen (via rectified diffusion) were proposed, but are highly debated 

explanations (Crum and Mao 1996, Fernandez et al. 2004, 2005, Jepson et al. 2003, Piantodosi 

and Thalmann 2004). 

There are no documented cases of killer whale strandings that coincide with military 

sonar exercises; however, several harbor porpoises (Phocoena phocoena) stranded before, on, 

and after the USS Shoup exercise on 5 May 2003. Forensic examination revealed no evidence of 

acoustic trauma but inadequate preservation of the samples produced equivocal results (NMFS 

2005a). 

Other less extreme, but potentially serious, nonauditory effects of noise include 

physiological effects related to stress. Most studies investigating these effects have been 

conducted on terrestrial mammals and include changes in heart, respiration, and metabolic rate, 

and immune and reproductive function. 

A few studies have investigated the production of stress hormones including 

catecholamines (such as norepinephrine, epinephrine, and dopamine) and aldosterone 

(implicated in a longer-term stress response in marine mammals) as well as decreased immune 

function resulting from controlled sound exposures in delphinid subjects. For instance, Thomas 

et al. (1990) found no difference in the levels of catecholamines in beluga whales after exposure 

to playbacks from an oil drilling platform. Romano et al. (2004) also measured the effects of 

sound exposure on nervous system and immune function in laboratory subjects of ongoing TTS 

studies (Finneran et al. 2002b). 

Norepinephrine, epinephrine, and dopamine levels were significantly higher in the beluga 

whale after high-level (< 183 dB re 1 μPa2

s) compared to low-level (>183 dB re 1 μPa2s) or 

control exposures of impulses from a seismic water gun. The bottlenose dolphin did not show 

differences in catecholamine levels but did have higher aldosterone levels and lower absolute 

monocyte (cells of the immune response) counts after experimental exposures compared to those 

32
measured after control exposures (Romano et al. 2004). It appears that sound exposure in some 

instances results in measurable differences in the production of stress hormones and cells related 

to immune function that could have undesirable physiological effects in free-ranging delphinids. 

33
Zones of Influence 

The previous sections have reviewed studies on the effects of sound exposure in killer 

whales and other odontocetes. These studies have shown that 1) marine mammals, including 

killer whales and closely related species, are vulnerable to auditory effects of noise exposure 

including masking and hearing loss effects; 2) individuals and groups of killer whales, including 

Southern Residents, exhibit behavioral changes that are consistent with avoidance responses in 

the presence of vessels and AHD signals; 3) physiological changes with undesirable effects on 

endocrine and immune function in smaller dolphins have resulted from sound exposure; and 4) in 

extreme cases, beaked whale strandings have been linked spatiotemporally with midfrequency 

military sonar exercises. 

One objective of this review is to use results from previous investigations to assess 

potential impacts of sound exposure from various sources on SRKWs. An approach to this 

assessment is to estimate the zone where an acoustic effect is expected based on available data 

(Richardson et al. 1995). Four zones are defined based on different response effects. In general, 

these are listed in order of smaller radii of influence relative to the acoustic source as follows: 1) 

zone of audibility is the area where the received levels of sound exposure are high enough that a 

killer whale can hear it; 2) zone of responsiveness is the area where the received level is such 

that the whale responds behaviorally or physiologically; 3) zone of masking is the area where 

received levels can mask the detection of sound emissions for biosonar and vocal 

communication, prey sounds, or other biologically important sounds; and 4) zone of hearing loss 

or injury is the area where received levels (or sometimes more appropriately SELs) are high 

enough to cause temporary or permanent hearing loss. 

Richardson et al. (1995) took the approach of describing these zones by distance, but 

accurate assessments of range require adequate knowledge of sound propagation characteristics. 

Here, zones of influence are by and large described by received levels given the complex shallow 

water environments inhabited by Southern Residents and the uncertainty involved in predicting 

propagation loss in space and time in these areas. This focus on received levels ignores the 

influence of temporal factors such as duration, pulse repetition rate, or duty cycle of the sound 

exposure that also likely contribute to potential response effects. Where information is available 

and appropriate, such as in the discussion of zones of hearing loss, SELs that take the duration of 

exposure into account will also be discussed. 

Zone of Audibility 

Whether or not a sound produced by human activity will be audible to a Southern 

Resident killer whale depends on the interplay between source, path, and receiver variables (for 

review, see Richardson et al. 1995). Source variables include source level, duration, and 

frequency of the sound; path variables include properties of transmission loss that affect received 

levels; and receiver variables include hearing sensitivity and responsiveness of the whales with 

respect to the received sound. If the received sound level falls below the detection threshold of 

34
the animal, then the signal is said to be “threshold limited,” while if it is above the detection 

threshold but below the ambient noise level at all frequencies that killer whales are sensitive to, it 

is said to be “noise limited” (Richardson et al. 1995). The absolute hearing sensitivity of killer 

whales is generally below the typical one-third octave ambient noise level for frequencies less 

than 3 kHz and greater than 70 kHz (arrows in Figure 9) in low ambient noise situations such as 

in calm conditions (e.g., a sea state of 0) in about 200 m of water. Thus killer whale hearing will 

be threshold limited in these conditions. 

Whales will likely be threshold rather than noise limited at lower frequencies. This 

prediction is based on the assumption that the critical bandwidths of the killer whale are 

approximated by one-third of an octave, which is a typical assumption with lack of direct 

measurements of critical bandwidths. More direct predictions can be made using the critical 

ratios of 20 dB re 1μPa at 10 kHz and 40 dB re 1μPa at 80 kHz (Bain et al. unpublished data, 

cited in Bain and Dahlheim 1994) and spectrum levels measured in the areas that SRKWs 

frequent such as those reported by Veirs and Veirs (2005). For example, the received level of a 

10 kHz tonal signal would have to be approximately 78 dB in order to be detected at night in 

Haro Strait because ambient spectrum levels at this frequency are 58 dB re 1μPa2

/Hz and the 

critical ratio is 20 dB. 

20

30

40

50

60

70

80

90

100

110

120

Killer whale thresholds

SS 0 1/3 Octave levels

100 1000 10000 100000 1000000

Frequency

Threshold (dB re 1μPa) Threshold (dB re: 1 μPa)

100 1,000 1 1 1, 0,000 00,000 000,000 

Frequency (Hz) 

Figure 9. Killer whale behavioral audiogram based on Szymanski et al. 1999 (reprinted from Figure 5c in 

Szymanski et al., copyright 1999, with permission from Acoustic Society of America), and onethird octave ambient levels in a sea state of 0 from Richardson et al. 1995 (reprinted from Figure 

5.1b in Richardson et al., copyright 1995, with permission from Academic Press). Dashed lines 

are extrapolated values and the two × marks indicate physiological thresholds to supplement 

information on sensitivity at the lower frequencies. Arrows indicate the frequency boundaries at 

which hearing would be noise versus threshold limited. 

35
The degree to which hearing will be noise limited will depend on the ambient conditions 

which are quite variable. Based on the Szymanski et al. (1999) audiograms, Erbe (2002) 

estimated that vessel noise levels based on one-twelfth octave band analysis was sufficiently 

loud enough to exceed the hearing thresholds of killer whales at frequencies above 2 kHz. It was 

estimated that a fast inflatable operating at speed of 51 km/hour and a source level of 162 dB re 

1μPa would be audible to killer whales over a range of about 16 km in a sea state of one half. A 

slow inflatable operating at a speed of 10 km/hour would be audible over a shorter range of 1 km 

in the same sea state (Erbe 2002). The model used assumed that the limit of audibility occurs 

when the critical band level becomes equal to or less than the ambient noise. However, onetwelfth octave critical bandwidths were assumed across all frequencies based on Fletcher’s 

(1940) equal power assumption. The zone of audibility for this boat noise might be different if 

direct measurements of killer whale critical bandwidths were available. 

Zone of Responsiveness 

In this section, the zone of responsiveness is defined as the area where the received level 

is such that killer whales would respond behaviorally. Most studies that illustrate statistically 

significant effects on the behavior of killer whales in the presence of anthropogenic sound 

provide insufficient information related to the source or received sound levels, frequencies, and 

duration of exposure, making it extremely challenging to predict the zone of responsiveness. 

The level of ambient noise, which can fluctuate in space and time and the resulting effective 

signal-to-noise ratio, also adds complexity to this issue. Furthermore, reactions based on other 

sensory information such as visual cues cannot be ruled out in the absence of controlled studies 

(such as those involving playbacks). 

Based on the little information available, it appears that killer whales will avoid an area 

within about 4 km of an AHD producing a 10 kHz signal at a source level of 194 dB re 1 μPa at 

1 m (Morton and Symonds 2002, NMFS 2005a). The transmission loss characteristics of the 

waters surrounding the AHD broadcasts are unknown; but, if we assume that at the most it would 

take the form of spherical spreading loss and at the least cylindrical spreading loss, the likely 

range for received levels at 4 km can be determined. The validity of these spreading loss 

assumptions is tentative. Using the following equation to calculate received levels, 

RL = SL – TL (6) 

where RL is the received level, SL is the source level of the signal, and TL is the transmission 

loss, where TL = 20 log R + αR for spherical spreading or TL = 10 log R + αR for cylindrical 

spreading, and where α is the absorption loss at 10 kHz and is approximately 0.00118 dB/m 

(Urick 1983), this would correspond to a AHD received level ranging from 117 to 153 dB re 1 

μPa. 

Based on the response of J pod to the sonar exercise by USS Shoup in May 2003, 

Southern Residents are predicted to behaviorally respond to frequency modulated signals 

between 2.6 and 3.3 kHz at received levels estimated between 121 and 175 dB re 1 μPa and 

received SELs of 169.1–187.4 dB re 1 μPa2

s. 

36
In the absence of data on the effects of vessel noise levels on killer whale behavior, Erbe 

(2002) assumed that a received broadband sound pressure level of 120 dB re 1μPa based on 

Richardson et al. (1995) would produce a behavioral reaction by killer whales. A behavioral 

response was predicted to occur when the vessel was within a 200 m and a 50 m range for an 

inflatable operating at 51 km/hour and 10 km/hour, respectively. Effects on the activity budgets 

of Southern Residents in the presence of vessels within 400 m as reported by Bain et al. (2006) 

agree with these predictions by Erbe (2002). In addition, the lower limits of the estimated 

received levels of both AHDs and midfrequency sonar signals that produced behavioral 

responses in Northern and Southern Resident killer whales fall within this range. 

Furthermore, if it is assumed that received levels above 120 dB would produce a 

significant behavioral response, then this would occur 50% of the time during summer days at 

the position of the recording hydrophones based on 2-second sound pressure level averages 

reported in the Veirs and Veirs (2005) study. This type of criterion based on one broadband 

sound pressure level measurement does not take into account frequencies of best hearing 

sensitivity of killer whales. For example, most energy of the received levels measured by Veirs 

and Veirs (2005) falls below 1 kHz, which is outside the range of best hearing sensitivity of 

killer whales (Szymanski et al. 1999). Weighting functions that emphasize frequencies of best 

hearing sensitivity while deemphasizing those of least hearing sensitivity based on the killer 

whale audiogram (or equal loudness level contours if available) would be useful to measure 

exposure levels in order to address these frequency effects. 

Zone of Masking and Effects on the Active Space 

of Sound Emissions 

The active space of a signal is the range over which it can be detected. Any sound that 

masks the reception of a communicative signal, such as conspecific call or echolocation click, 

will reduce the active space of that signal. Detection of natural signals a few dB below ambient 

noise levels has been demonstrated in humans and some marine mammals (Richardson et al. 

1995). Captive killer whales have demonstrated that detection thresholds of clicks and calls are 

not impeded by boat noise, although in some cases, frequency overlap was not a likely 

contributing factor (Bain and Dahlheim 1994). The sonar equation provides a useful way to 

describe the active space of an acoustic signal. To determine the range at which a sound might 

be heard by a listener, the sonar equation may be defined as follows: 

RL = SL – TL – NL + DI (7) 

where NL is the noise level and DI is the directivity index of the auditory system (Urick 1983). 

In most cases where background noise exists, the received level would have to exceed the 

CR in order for detection to take place. In the case of auditory masking, NL increases, reducing 

the received level of the signal. If the source level of a signal is not below the hearing threshold, 

the excess level (in dB) can be used to calculate the distance over which it might be heard after 

transmission loss has been taken into account. Assumptions about transmission loss vary 

considerably as well as the bandwidth appropriate for analyzing background noise. Based on a 

one-twelfth octave bandwidth analysis, Erbe (2002) estimated an inflatable operating at a speed 

of 51 km/hour would mask the reception of a killer whale call to the point of being 

37
unrecognizable (when spectral components at 4.7 and 5.8 kHz were inaudible) within a range of 

14 km. An inflatable operating at a speed of 10 km/hour was predicted to mask killer whale calls 

within a 1 km range. However, the killer whale call level incorporated into the model was based 

on received levels (up to 124 dB re 1μPa) and not source levels which are known to be 

considerable higher (135–168 dB re 1 μPa at 1 m, Miller 2006). Thus the range of masking 

effects is likely overestimated in this instance. 

Miller (2006) reported that the mean active space of stereotyped calls with highfrequency components was 10–16 km in sea state 0 while the mean active space of stereotyped 

calls without high-frequency components, whistles, and variable calls was 5–9 km. In these 

cases, a killer whale call was assumed to be detectable by another whale when the received level 

in at least 1 one-third octave band exceeded the hearing threshold or was 6 dB below the 

background noise level, whichever was greater. Relative to the sea state of 0 in which noise 

spectral levels are about 26 dB re 1μPa2

/Hz lower, estimated active space of calls in a sea state of 

6 were 75%, 83%, and 91% lower for stereotyped calls, variable calls, and whistles, respectively, 

using Fletcher’s (1940) equal power assumption (Miller 2006). The frequency and time structure 

of signals relative to that of the noise as well as the directional hearing system might result in 

increased sensitivity to these signals, but these considerations were not incorporated in the 

models for simplicity (Erbe and Farmer 2000, Erbe, 2002, Miller 2006). 

For echolocation signals, the sonar equation is modified to include two-way transmission 

loss as well as the target strength (TS) of the ensonified object as follows: 

RL = SL – 2TL + TS – NL +DI (8) 

Au et al. (2004) used these equations to estimate the horizontal range at which a foraging 

killer whale would be able to perceive biosonar echoes off prey in quiet conditions. This was 

based on a model of Chinook salmon (Oncorhynchus tshawytscha) target strength and an 

estimated echo level (EL) received by a whale using the following equation: 

EL = SL – 2TL + TS (9) 

Based on these assumptions and using dB peak-to-peak levels, it was estimated that an 

echolocating killer whale would receive echoes of a Chinook salmon at 65 m depth between 29 

and 33 dB above threshold at a horizontal range of 100 m. In noisier conditions, the noise level 

along with the DI must also be considered. The DI of a killer whale was estimated to be 21 dB at 

50 kHz based on a scaling factor to convert the DI of a bottlenose dolphin, since no direct 

measurements of DI in killer whales were available (Au et al. 2004). For moderately heavy rain, 

the ambient noise spectral density level was estimated to be 52 dB re 1μPa2

/Hz and it was 

predicted that the echo level would not extend above the noise level until the whale was within a 

horizontal range of 40 m from the prey item (Au et al. 2004). Thus the active space of the 

echolocation signal was reduced considerably from quiet to noisier conditions created by heavy 

rain. It is important to note that the quiet condition assumed a complete lack of ambient noise, 

which is not realistic. Free ranging animals will never be in an environment that is completely 

lacking of ambient noise. 

38
The analysis of active space for echolocation signals can be extended for conditions when 

vessels are operating in the vicinity of killer whales. Based on data from Hildebrand et al. 

(2006), spectral levels of vessel and Haro Strait ambient noise were used to estimate the 

horizontal detection range of killer whales echolocating on Chinook salmon. For this analysis, 

source spectral density levels at 50 kHz for cruise and power up operating speeds were available 

to predict received levels of a whale at 100, 200, and 400 m from two whale watching vessels. 

Received levels for these conditions are based on the following equation: 

RL = SL − TL = SL − 20 log R + αR (10) 

where SL is the source level at 50 kHz for a given vessel at a given speed, R is the range in 

meters, and α is the absorption loss at 50 kHz and is approximately 0.016 dB/m (Urick 1983) as 

shown in Table 1. 

Table 1. Source levels for whale watching vessels at different speeds, calculated received levels 

assuming spherical spreading loss for three distances, and measured received level of a container 

ship as reported by Hildebrand et al. (2006). 

Vessel Speed 

SL at 50 

kHz dB 

re 1μPa^2/Hz Range (m) 

Calculated RL 

at 50 kHz 

in dB 

re 1μPa^2/Hz 

Measured RL 

at 50 kHz 

in dB 

re 1μPa^2/Hz 

Boat B Cruise 101 100 59.4 

101 200 51.8 

101 400 42.6 

Power 92 100 50.4 

92 200 42.8 

92 400 33.6 

Boat C Cruise 93 100 51.4 

93 200 43.8 

93 400 34.6 

Power 95 100 53.4 

95 200 45.8 

95 400 36.6 

Boat D Cruise 111 100 69.4 

111 200 61.8 

111 400 52.6 

Power 104 100 62.4 

104 200 54.8 

104 400 45.6 

Ship (Hanjin 

Marseilles) 

21 

knots 

107 100 65.4 

107 200 57.8 

442 48 

Ambient (measured on day after MV Hanjin Marseilles pass) 26 

39
Transmission loss was assumed to be spherical which is likely only accurate in deep 

water. The range of distances between whales and vessel noise is pertinent based on the 

voluntary “Be Whale Wise” guidelines, which included maintaining cruising speeds of less than 

7 knots (13 km/hour) within 400 m of the whales and a “no go” zone within 100 m of the whales 

(http://www.nwr.noaa.gov/Marine-Mammals/upload/BeWhaleWise.pdf). Additionally, the 

received spectral density level at 50 kHz of a container ship at 442 m was used. The NL that a 

killer whale auditory system would receive was calculated as 

NL = N0 + BW – DI (11) 

where N0 is the noise spectral density of the noise, BW is the received bandwidth, and DI is the 

directivity index used in the previous investigation (Au et al. 2004). 

This NL was based on RMS measurements, so 9 dB were added to convert to peak-topeak levels as in the previous analysis by Au et al. (2004). The maximum horizontal range (m) 

was taken as the horizontal distance between a whale at 1 m and a salmon at 65 m of depth in 

which the echo levels off the fish would extend into the noise floor. Echo levels were 

determined by the same methods as in Au et al. (2004). Ambient levels in the Haro Strait were 

those reported by Hildebrand et al. (2006) and are comparable to ambient levels between a sea 

state of 1 in deep water (Urick 1983). It was assumed that only the noise from one vessel was 

present and other sources, including natural ones, were not included in the calculation of NL. 

The results of this analysis are shown in Table 2 and Figure 10. 

Compared to ambient levels in the Haro Strait in which the maximum horizontal 

detection range was predicted to be 400 m, boat noise generated by cruise and power up speeds 

up to 400 m relative to the whales was predicted to significantly reduce the active space of an 

echolocation click at 50 kHz. At a distance of 400 m between vessel and whale, Boat B at power 

up speed and the Boat C at cruise speed were predicted to have the least impact (maximum 

horizontal detection range was 250 m, resulting in a 150 m reduction in active space compared to 

Haro Strait ambient levels), while Boat D at cruise was predicted to have the greatest impact that 

was similar to the masking effects of heavy rain (maximum detection range was 40 m, thus the 

active space was effectively reduced by 360 m). 

The predicted distances at which noise produced by cruising (> 20 knots) whale watching 

vessels would approach ambient levels in Haro Strait are 650, 950, and 1,400 m for Boat C, Boat 

B, and Boat D, respectively. The zones of masking for these vessel types are defined within 

these ranges. Even the large container ship (MV Hanjin Marseilles) passing at 442 m relative to 

a whale was predicted to reduce the active space of the biosonar signal by 340 m relative to Haro 

Strait ambient conditions. 

The above analysis is oversimplified because it only considers one frequency and 

echolocation clicks are broadband. Whales most likely use acoustic information across a range 

of frequencies to detect prey through biosonar. Additionally, as stated by Au et al. (2004), the 

DI assumes that the noise is isotropic (independent of direction) and vessel noise at this range is 

likely anisotropic. For example, the whales might be able to swim to some depth to avoid vessel 

noise at the surface, but such behavior might also have energetic costs. The target strength of a 

40
Table 2. Predicted maximum horizontal detection ranges at 50 kHz for a killer whale at the surface 

echolocating on a Chinook salmon at 65 m depth, reduction in range relative to ambient Haro 

Strait measurement, and percent of reduction in range relative to ambient Haro Strait 

measurement. 

Noise condition 

Horizontal echo 

detection range 

(m) 

Reduction in 

range re ambient 

Haro Strait (m) 

% reduction in 

range re ambient 

Haro Strait 

No noise 650 – – 

Sea state of 1 450 – – 

Ambient Haro Strait 400 – – 

Heavy rain 40 – – 

Boat B cruise at 100 m 20 380 95 

Boat B cruise at 200 m 40 360 90 

Boat B cruise at 400 m 100 300 75 

Boat B power at 100 m 50 350 88 

Boat B power at 200 m 100 300 75 

Boat B power at 400 m 250 150 38 

Boat C cruise at 100 m 40 360 90 

Boat C cruise at 200 m 100 300 75 

Boat C cruise at 400 m 250 150 38 

Boat C power at 100 m 40 360 90 

Boat C power at 200 m 80 320 80 

Boat C power at 400 m 200 200 50 

Boat D cruise at 100 m 0 400 100 

Boat D cruise at 200 m 20 380 95 

Boat D cruise at 400 m 40 360 90 

Boat D power at 100 m 10 390 98 

Boat D power at 200 m 30 370 93 

Boat D power at 400 m 80 320 80 

Hanjin (ship) at 100 m 0 400 100 

Hanjin (ship) at 200 m 20 380 95 

Hanjin (ship) at 442 m 60 340 85 

Chinook salmon was modeled using tonal sonar signals and a model of a fish (Au et al. 2004) 

and not with killer whale-like echolocation signals and real fish to determine target strength 

values (see Au et al. 2007). Furthermore, information on target detection thresholds in killer 

whales in quiet conditions is lacking, so interpreting appropriate detection ranges predicted from 

this analysis is unknown. 

The range at which killer whales would detect, discriminate, and consider pursuing prey 

is debatable as discussed in Au et al. (2004). For example, even if the detection of prey is 

predicted at 400 m in quiet ambient conditions (Figure 10), it does not necessarily mean that 

whales will choose to pursue the prey. For these reasons, the data from this analysis should be 

interpreted with caution, given the lack of information about predator-prey interactions in freeranging killer whales. 

41
0

50

100

150

200

250

300

350

400

450

500

Sea State of 1

Ambient Haro Strait

Heavy Rain

Boat B cruise at 100m

Boat B cruise at 200m

Boat B cruise at 400m

Boat B power at 100m

Boat B power at 200m

Boat B power at 400m

Boat C cruise at 100m

Boat C cruise at 200m

Boat C cruise at 400m

Boat C power at 100m

Boat C power at 200m

Boat C power at 400m

Boat D cruise at 100m

Boat D cruise at 200m

Boat D cruise at 400m

Boat D power at 100m

Boat D power at 200m

Boat D power at 400m

Hanjin (ship) at 100m

Hanjin (ship) at 200m

Hanjin (ship) at 442m

Max. Horizontal Detection Range (m) ion Range (m) ) Max. horizontal detection range (m

Noise Condition

Figure 10. Predicted maximum horizontal detection ranges at 50 kHz for a killer whale at the surface 

echolocating on a Chinook salmon at 65 m depth for various noise conditions. 

Noise condition 

Zone of Hearing Loss or Injury 

The zone of hearing loss or injury is the area where received levels or sound exposure 

levels are high enough to cause temporary or permanent hearing loss. Studies on TTS in various 

marine mammal groups are needed (NRC 2003) because of their utility to predict auditory injury 

or PTS. Hearing loss effectively reduces the active space of important signals such as 

conspecific calls or biosonar signals. That is, whales that experience temporary or permanently 

elevated hearing thresholds must be closer to a source to detect it. TTS data from studies 

conducted on bottlenose dolphins and beluga whales have been used to predict the zones of 

hearing loss (TTS) and injury (PTS) in killer whales. In particular, the TTS studies that have 

used white noise as fatiguing stimuli (Nachtigall et al. 2003, 2004) are most appropriate for 

extrapolating potential hearing loss in killer whales due to vessel noise exposure, those using 

pure tone fatiguing stimuli (Finneran et al. 2005, 2007) are most appropriate for sonar exposure, 

and those using impulsive sounds (Finneran et al. 2000, 2002b) are most appropriate for pile 

driving activity. 

The zone of hearing loss or injury is the area where received levels or sound exposure 

levels are high enough to cause temporary or permanent hearing loss. Studies on TTS in various 

marine mammal groups are needed (NRC 2003) because of their utility to predict auditory injury 

or PTS. Hearing loss effectively reduces the active space of important signals such as 

conspecific calls or biosonar signals. That is, whales that experience temporary or permanently 

elevated hearing thresholds must be closer to a source to detect it. TTS data from studies 

conducted on bottlenose dolphins and beluga whales have been used to predict the zones of 

hearing loss (TTS) and injury (PTS) in killer whales. In particular, the TTS studies that have 

used white noise as fatiguing stimuli (Nachtigall et al. 2003, 2004) are most appropriate for 

extrapolating potential hearing loss in killer whales due to vessel noise exposure, those using 

pure tone fatiguing stimuli (Finneran et al. 2005, 2007) are most appropriate for sonar exposure, 

and those using impulsive sounds (Finneran et al. 2000, 2002b) are most appropriate for pile 

driving activity. 

Erbe (2002) used preliminary data from the Nachtigall et al. (2003) study as cited in Au 

et al. (1999) in which a TTS of 12–18 dB at 7.5 kHz occurred in a bottlenose dolphin after 

exposure to an octave band of noise at 179 dB re 1μPa for approximately 50 minutes (resulting 

in an SEL of 213 dB re 1μPa2

s). This corresponded to a sensation level of 96 dB (amount above 

the pure tone threshold of the subject at 7.5 kHz). Within a few meters, octave band noise levels 

of whale watching boats were usually less than 96 dB above the threshold of killer whales in this 

bandwidth. Therefore, Erbe (2002) used data from humans and other terrestrial animals to scale 

down the amount of TTS and assumed that exposure levels of 68–74 dB above threshold for the 

Erbe (2002) used preliminary data from the Nachtigall et al. (2003) study as cited in Au 

et al. (1999) in which a TTS of 12–18 dB at 7.5 kHz occurred in a bottlenose dolphin after 

exposure to an octave band of noise at 179 dB re 1μPa for approximately 50 minutes (resulting 

in an SEL of 213 dB re 1μPa2s). This corresponded to a sensation level of 96 dB (amount above 

the pure tone threshold of the subject at 7.5 kHz). Within a few meters, octave band noise levels 

of whale watching boats were usually less than 96 dB above the threshold of killer whales in this 

bandwidth. Therefore, Erbe (2002) used data from humans and other terrestrial animals to scale 

down the amount of TTS and assumed that exposure levels of 68–74 dB above threshold for the 

42
same exposure period would result in a 5 dB threshold shift. At that time, the accuracy of this 

assumption was questionable because no TTS was observed in the bottlenose dolphin for 

exposure levels of 87 dB above threshold (Au et al. 1999). It was predicted that a TTS of 12–18 

dB would result from exposure of a fast inflatable (traveling at 51 km/hour) within a 10 m range 

after 30–50 minutes. A TTS of 5 dB was predicted from exposure of the same inflatable over the 

same duration within 450 m range. Exposure to a slower moving inflatable (10 km/hour) would 

result in TTS of 5 dB after 30–50 minutes within a 20 m range (Erbe 2002). 

Based on more current work by Nachtigall et al. (2003), a TTS of 11 dB at 7.5 kHz 

would result from exposure to a fast inflatable within a 10 m range after 50 minutes of 

continuous exposure. In another study using the same octave band of noise for exposure, an 

electrophysiological approach was used to measure postexposure hearing thresholds more 

rapidly and over a wider range of frequency. Lower amplitude and shorter duration exposures 

produced measurable TTS (Nachtigall et al. 2004) compared to a lack of TTS measured through 

behavioral techniques (Nachtigall et al. 2003), but differences in results from these two 

approaches might be related to methodological issues that are still being resolved (Finneran et al. 

2007). Based on the current data, a killer whale would experience a TTS of 5 dB at 

approximately the same frequency (8 kHz) and a TTS of 8 dB at 16 kHz after 30 minutes of 

continuous exposure within a range from the vessel corresponding to 70 dB above hearing 

sensitivity. 

Based on TTS results in which a 3 kHz pure tone was used for exposure, onset TTS onehalf octave higher than the frequency of exposure (4.5 kHz) was predicted at sound exposure 

levels of 195 dB re 1μPa2

s (Finneran et al. 2005). Killer whales, however, are about 10–15 dB 

more sensitive than bottlenose dolphins within this frequency range, but these behavioral results 

are only based on a one or two individuals (Johnson 1967, Szymanski et al. 1999). When 

differences in hearing sensitivity are taken into account, sound exposure levels that are predicted 

to result in killer whale TTS onset should be considered to be lower by approximately 10–15 dB 

within this frequency range. This is a conservative estimate given the lack of direct 

measurements of TTS in killer whales. 

Pile driving activity in Friday Harbor had an average SEL that ranged between 171 and 

187 dB re 1 μPa2

s (Lauglin 2005). Exposure to a seismic water gun resulted in masked TTS of 7 

and 6 dB at 0.4 and 30 kHz, respectively but no TTS greater than or equal to 6 dB at 4 kHz 

occurred in a beluga whale after a comparable SEL of 186 dB re 1 μPa2

s (peak-to-peak pressures 

of 226 dB re 1 μPa). No MTTS greater than or equal to 6 dB was observed in the bottlenose 

dolphin at 0.4, 4, or 30 kHz up to SELs of 188 dB re 1 μPa2

s (peak-to-peak pressures of 228 dB 

re 1 μPa, Finneran et al. 2002b). While the absolute thresholds of the bottlenose dolphin and 

beluga whale are very similar at 0.4 and 4 kHz, the beluga whales hearing is better by about 10 

dB at 30 kHz (Johnson 1967, White et al. 1977, Awbry et al. 1988, Johnson et al. 1989). The 

killer whale’s threshold at 4 kHz is about 18 dB better while at 30 kHz is 5 dB worse than the 

beluga whale’s thresholds (Syzmanski et al. 1999). No absolute threshold measurements at 0.4 

kHz are available in the killer whale. With these sensation level differences in mind, killer 

whales exposed to the same stimulus might not exhibit a threshold shift greater than or equal to 

6 dB at 30 kHz but might experience TTS at 4 kHz. The magnitude of TTS at this frequency 

cannot be predicted with accuracy without further data. 

43
The range of auditory injury or PTS in killer whales exposed to boat noise has been 

predicted by Erbe (2002). An approach based on those used for humans was taken in which a 

weighting function used to predict PTS from broadband exposures was modeled. The killer 

whale weighting function was taken as the killer whale audiogram subtracted from the critical 

band levels of noise and the energy was integrated across all frequencies. A PTS of 2–5 dB was 

predicted for whales exposed within 1 km of a fast inflatable or within 50 m of a slow inflatable, 

continuously for an 8 hour day, 5 days a week, over 50 years (Erbe 2002). Another approach to 

predict PTS onset from TTS data is to extrapolate curves of TTS as a function of weighted and 

unweighted levels (depending on the exposure type) up to a value that would likely be associated 

with PTS in terrestrial mammals (i.e., TTS ≥ 40 dB). This approach is being considered for a 

number of groups based on hearing function in order to define noise exposure criteria for freeranging marine mammals (Southall et al. 2007). 

44
Likelihood of Acoustic Impacts on the 

SRKW Population 

Determining whether sound exposure results in measurable differences in hearing or 

behavior at the individual or group level has been explored in a few species (Richardson et al. 

1995, Nowacek et al. 2007, Finneran et al. 2007). Determining whether the effect is biologically 

significant, that is, having overall deleterious effects at the level of the population, is difficult 

(NRC 2005). Currently available information is sparse enough to preclude an accurate 

assessment of the deleterious effects of sound exposure and related cumulative effects at the 

individual as well as the population level for Southern Resident killer whales. However, the 

following discussion, based on the framework provided in NRC (2005), may serve as an outline 

for recommendations for future work (discussed in the next section) while suggesting some 

provisional approaches for acoustic impact assessment in the interim period. 

Risk Assessment 

One approach is that based on risk assessment. Risk assessment requires that the 

problems are clearly identified and the probability of exposure and the types of biological effects 

are defined, leading to an estimate of risk. There are many types of potential biological effects, 

but the focus should be on those that have repercussions on life functions that ultimately affect 

population status as outlined in the Population Consequences of Acoustic Disturbance model 

(NRC 2005). 

Both the risk of behavioral disturbance and auditory injury at the population level require 

specific information regarding the type, amplitude, and duration of the exposure; the range over 

which the exposure might affect free-ranging animals including sound propagation properties; 

the fraction of the population affected (e.g., through dose-response curves); the intensity of the 

reaction; and the ease with which animals of the population might recovery from the exposure 

(such as finding alternative habitats or recovering hearing function). 

In many cases, it is very difficult to interpret the biological significance of behavioral 

disturbances. For example, does the disturbance result from the acoustic signal resembling the 

sounds of a predator, prey, or conspecific? Or is the response just indicative of annoyance that 

scales to exposure level? It is often impossible to determine differences in the reasons why 

animals respond in a particular way because of a lack of power to determine causal relationships 

between exposure and behavioral changes (NRC 2005). For healthy populations, recovery from 

sound exposure disturbance might be relatively easy, having no long-term effects on that 

population. However, for small populations that are constrained by other factors that affect 

survival, such as food availability, recovery from sound exposure disturbance that affects 

foraging success, for example, might not be as easy. 

45
Related to these considerations is the area affected versus the habitat available 

(Richardson et al. 1995). The core habitat of Southern Residents is particularly concentrated on 

the west side of San Juan Island during the summer (Hauser 2006), and it is considered a central 

area for feeding (Baird and Hanson 2004, Hanson et al. in prep.). This is also an area where few 

or no parts of the range are absent of human-produced noise. Broadband received levels (0.1–15 

kHz) were more than 120 dB re 1 μPa approximately 50% of the time during summer days in 

2005 in this area due to vessel traffic (Veirs and Veirs 2005). This broadband level has been 

used to estimate zones of behavioral responses in resident killer whales without direct empirical 

evidence (Erbe 2002). 

Assuming these measurements are representative of summer exposure levels on a regular 

basis and that Southern Residents would show a behavioral response to these exposure levels, 

then it is likely that behavioral effects of noise exposure would be manifested on a regular basis. 

However, these broadband SPLs reported by Veirs and Veirs (2005) do not include the 

frequency range of best hearing sensitivity of killer whales (18–42 kHz, Syzmanski et al. 1999). 

Extent of Masking Effects 

The masking effects of sound exposure on communication signals (pulsed calls) are 

likely to extend up to several kilometers away in some conditions. While whale watching 

vessels may shut down when within a few hundred meters of the whales, they commonly motor 

around the area adjacent to the whales and within the zone of masking when arriving, 

repositioning, or exiting the area. Other vessels, such as larger ships and fishing vessels, as well 

as sound from natural sources contribute to particularly high levels of ambient noise in SRKW 

summer habitat. Thus masking of communication signals is probably a consistent challenge for 

Southern Residents. Pulsed calls are most often heard while killer whales are foraging and 

traveling (Ford 1989, Miller 2002). Some individuals, particularly females, share prey items 

(Ford and Ellis 2006) and calls used for communication during cooperative foraging could be 

masked by a variety of sound sources. 

The levels of many anthropogenic sounds, such as vessel traffic, are strongest at 

frequencies below 10 kHz (Figure 5). A common assumption is that masking of echolocation 

signals is not much of a concern compared to communication signals because echolocation 

signals are strongest above 20 kHz (Au et al. 2004). However, killer whale hearing is tuned to 

higher frequencies and whales probably need to hear faint echoes from ensonified objects. Thus 

any sounds that increase background levels at higher frequencies above ambient levels have the 

potential to decrease the range at which echolocation activities are effective, including those 

associated with foraging. 

The analyses of active space of echolocation signals in the presence of both natural and 

anthropogenic sources clearly illustrate potential impacts in terms of the reductions in the range 

at which killer whales can detect prey items. The fact that whale watching vessels idling at 200 

m produced source spectral levels that were comparable to ambient levels (Hildebrand et al. 

2006) suggests that an idling vessel at this distance alone would not reduce the active space of 

echolocation signals. However, this assumption needs to be investigated further. According to 

the “Be Whale Wise” guidelines, 400 m is the shortest distance where vessels can power up to 

normal cruising speeds relative to whales. At this range, vessel noise reduced the horizontal 

46
detection range of a 50 kHz echolocation signal by as much as 360 m (from 400 to 40 m) relative 

to ambient conditions. The distance at which noise from vessel operations would be reduced to 

ambient is considerably larger than 400 m for the vessel measured by Hildebrand et al. (2006). 

Additional noise level measurements of vessels of different propulsion types operating at slower 

speeds (less than 7 knots) in SRKW habitat are needed to evaluate potential masking effects 

further, particularly those that include high frequency measurements. 

Most studies that aim to identify biological effects of noise exposure do not consider 

cumulative acoustic effects. Even if only a very small number of individuals are exposed to 

sounds that have the potential for auditory injury, other individuals in the area might still be 

affected by masking or behavioral disturbance, given the larger range at which a sound source 

would have an impact. Thus the assessment of sound exposure impacts should not ignore less 

severe events. For example, if a severity score is calculated based on models used for 

determining the Potential for Biological Removal, the scores must reflect the cumulative effects 

to determine total number of individuals affects and the effects on the population should be 

determined from this total number (NRC 2005). 

Furthermore, there are other auditory processes besides detection that are affected by 

noise exposure such as sound localization, discrimination, and recognition. For example, if a 

killer whale detects the call of a conspecific but cannot localize or recognize the call, then the 

function of the call for social cohesion may be impaired. There are very few studies on the 

effects of noise on these types of auditory processes in marine mammals. Nonetheless, these 

types of impacts should also be considered. 

Interaction with Nonacoustic Variables 

Sound exposure effects might also interact with nonacoustic variables that have been 

identified as possible factors related to the population decline of SRKWs. For example, when 

they are within 100–400 m of vessels, there is evidence that they switch from foraging behaviors 

to other behaviors such as traveling (Bain et al. 2006). It is unknown whether this behavioral 

change is in response to the presence of the vessels, to the noise produced by vessels, to other 

variables, or some combination of these factors. In addition, most vessels, such as those used for 

whale watching, operate during the day and there is evidence that foraging behavior is also 

diurnally dependent (Baird et al. 2005). Thus Southern Residents might be temporally and 

spatially restricted for activities related to foraging. There are cases in which individuals 

including those with dependent offspring have physical signs of poor body condition before they 

disappear and are assumed to have died. These observations indicate that starvation is a threat. 

Many marine mammals go through periods when food is less abundant. Entering such 

periods with insufficient body reserves can have significant biological effects at the level of the 

individual as well as for an endangered population. Acoustic variables that affect caloric intake 

in SRKWs would affect population recovery. Other anthropogenic effects might also interact 

with the metabolic effects of periods of low caloric intake such as the mobilization of lipophilic 

contaminants. For example, fasting northern elephant seals (Mirounga angustirostris) showed 

higher levels of mobilized polychlorinated biphenyls (PCBs) into the bloodstream during the end 

of their postweaning fast compared to before the fast. Additionally, PCB concentrations in the 

blood were higher in leaner animals, suggesting that they might be more at risk to potential toxic 

47
effects (Debier et al. 2006). The interaction of acoustic effects with other threats on the SRKW 

population is of significant concern. However, it is extremely difficult to link potentially small 

but consistent changes in response to noise exposure to population level effects. 

48
Recommendations for Future Work 

Future studies that would increase confidence in predicting potential acoustic impacts on 

SRKWs include those that accurately measure source characteristics and sound propagation 

effects, particularly of anthropogenic sounds in core habitat. For example, knowing the source 

level, frequency content, and typical duration of exposure can enable better predictions of what 

the received levels might be at the whales’ location. Because the propagation of vessel sounds is 

not equal in all directions, a study that measured propagation from the bow, sides, and stern of 

the vessel would be able to determine if received levels are higher for a particular orientation and 

how this might interact with directional hearing and the ability to detect signals in noise. Such 

information could be used to mitigate killer whale and whale watching vessel interactions, 

particularly with respect to maneuvering around the whales. 

Killer whale hearing sensitivity has been described for frequencies between 1 and 100 

kHz. The whales will likely hear a sound if received levels are above the hearing threshold of 

the animal and if background levels are sufficiently low enough to allow it. The assessment of 

the zones of audibility and masking are contingent on knowledge of the auditory bandwidths of 

killer whales. As a first approach, one-third octave levels could be used but direct measurements 

of critical bandwidths across a wide range of frequencies within the hearing range is necessary to 

assess the validity of using one-third octave band analysis to describe sound sources and assess 

their impacts. In the interim, it may be useful to bracket the potential effects by using an upper 

and lower range of probable critical bandwidths of killer whales, such as one-third and onetwelfth octave analyses (e.g., see Southall 2003b). For these reasons, studies that include noise 

level measurements in SRKW habitat should report sound pressure levels in broadband, onethird octave band, one-twelfth octave band, and spectral density levels whenever possible. 

Masking Effect Assessment 

Further assessment of masking effects on echolocation signals would be useful, 

particularly for more types of whale watching vessels as well as other vessels in core habitat. 

Because the energy of most noise produced by vessels is concentrated at the lower frequencies, it 

is tempting to assume that the amount of noise generated at the higher frequencies will not affect 

killer whales. This assumption might be based in part on an anthropomorphic bias, since humans 

do not hear functionally above 20 kHz yet laboratory evidence shows that killer whales are most 

sensitive at and above this frequency. It is necessary to consider the whales’ subjective 

perceptual world (the “Umwelt”) that they experience. 

Currently, there are very few available measurements of ambient noise levels above 20 

kHz (see Hildebrand et al. 2006). The existing data suggests that maintaining slow cruise speeds 

within 400 m of the whales and shutting vessel engines down completely when within 100 m of 

a whale is sufficient to reduce the effects of masking of echolocation signals used for foraging. 

Predicting zones of masking will depend on the vessel type, operating speed, and propulsion type 

as well as the number of vessels actively motoring. Whale watching vessels operating at normal 

49
cruise speeds even beyond 400 m are predicted to reduce the active space of the 50 kHz 

echolocation signal, but there is difficulty in interpreting appropriate detection ranges. Further 

information about noise levels above 20 kHz for vessels of different sizes, propulsion types, and 

operating speeds, particularly for speeds less than 7 knots, would be useful to further assess the 

impacts of anthropogenic sounds on killer whales and how regulatory guidelines such as those 

based on “Be Whale Wise” may or may not be effective in reducing such potential impacts on 

animals using higher frequency biosonar for prey detection. 

Information about the target strength of key prey items such as various Pacific salmon 

species using more realistic signals as well as investigations on predator-prey interactions in 

free-ranging killer whales are needed to predict the potential masking impacts of anthropogenic 

sources on echolocation and foraging behavior. Studies that focus on foraging efficiency and 

sound production associated with foraging in the presence and absence of anthropogenic noise 

would provide additional information that could be used for this assessment. 

Behavioral Response 

Predicting the zone of responsiveness is dependent on knowing the quantitative aspects of 

the exposure relative to the whales. As is the case for most cetacean species (see Nowacek et al. 

2007), there is either a lack of information altogether or too broad a range of received levels that 

are potentially correlated with behavioral responses to provide confidence regarding what future 

exposure might result in behavioral avoidance or displacement in SRKWs. A broadband 

received level of 120 dB re 1 μPa without respect to frequency content and the sensitivity of the 

animals has sometimes been used as a generalized rule of thumb to define the onset of a 

behavioral response to noise (e.g., Erbe 2002). Two-second SPL averages were greater than or 

equal to 120 dB (re 1 μPa measured from 0.1 to 15 kHz) 50% of the time during summer days in 

Haro Strait (Veirs and Veirs 2005). 

One interpretation is that killer whales would potentially behaviorally or physiologically 

respond to sounds in the areas associated with summer foraging at least 50% of the time. This 

assumption requires further empirical evaluation, particularly since killer whales are most 

sensitive at frequencies between 18 and 42 kHz. Studies that investigate behavioral responses to 

vessel traffic should at the least report received sound levels, if not source levels, and preferably 

sound exposure levels to address the temporal aspects of the exposure. Moreover, it is unknown 

whether behavioral responses to vessel effects result from the presence of the vessel, the sounds 

it produces, or some combination of these factors. Future studies that tease apart these effects, 

such as controlled exposure experiments in which recordings of the vessel sounds are played 

back in the absence of the vessel to determine behavioral changes, are also needed to make more 

accurate predictions of the zone of responsiveness. 

Likelihood of Auditory Injury 

Determining the likelihood of auditory injury is probably one of the more imperative 

assessments of sound exposure in SRKWs. Even if the potential for such exposure is low 

relative to other possible acoustic risk factors, the uncertainty in predicting this type of auditory 

effect in an ESA-listed species warrants further research efforts. Auditory injury is usually 

estimated from studies on TTS because inflicting permanent hearing loss in marine mammals is a 

50
significant violation of the Marine Mammal Protection Act of 1972. No studies have measured 

TTS in killer whales but a few well documented studies have measured TTS in bottlenose 

dolphins and beluga whales. From these studies, TTS onset has been estimated at a sound 

exposure level of 195 dB re 1 μPa2

s for midrange exposure and test frequencies. Given the 

sensation level differences between killer whales, bottlenose dolphins, and beluga whales at 

these frequencies, this onset SEL criterion might not be conservative enough to prevent auditory 

injury in killer whales. TTS studies in killer whales would provide direct evidence to support or 

refute these assumptions. 

Research focused on assessing potential population effects of sound exposure is probably 

the most important (albeit most challenging) research priority. Dose-response curves, in which 

the percentage of individuals that show responses to sound exposure such as behavioral 

disturbance or onset-TTS as a function of sound exposure level, are needed to make accurate 

assessments at the level of the population (NRC 2005). Additionally, studies that measure equal 

loudness contours in killer whales would be extremely useful to determine weighting functions 

that could be used when measuring ambient noise in order to better predict behavioral and 

auditory disturbance. An approximation to this would be to use a weighting function based on 

the killer whale audiogram. 

Finally, there is a need for more information in published, peer-reviewed formats. For 

example, a working group is in the process of publishing noise exposure criteria for marine 

mammals that includes the development of interim weighting functions for different groups. 

These groups are categorized based on hearing abilities given the variety of auditory sensitivities 

at different frequencies across the different marine mammal taxa (Southall et al. 2007). Such 

documents would provide tools for policy makers and natural resource managers to make more 

confident decisions about potential acoustic impacts from human activity on SRKWs and other 

marine mammal populations. 

51
Conclusions 

This report offers the following conclusions on sound exposure and SRKWs: 

• Given the chronic nature of vessel traffic in areas designated as critical habitat of SRKWs 

and the potential for vessel noise to mask communication and echolocation signals of 

SRKWs, more measurements of these sounds within the relevant frequency range of 

killer whale hearing (i.e., 1–100 kHz) are needed. Such studies should further investigate 

the effects of vessel size, propulsion type, operating speed, and vessel orientation on the 

sound levels emitted. 

• Some vessel noise, depending on the vessel size, propulsion type, and operating speed, 

was predicted to significantly reduce the range at which echolocating killer whales could 

detect salmon in the water column. Interpretations of these effects are limited by a dearth 

of information on predator-prey interactions, such as the typical ranges at which killer 

whales detect and pursue salmon in quiet conditions. Studies that elucidate this 

information would enable a more accurate assessment of the impacts of vessel noise as 

well as other anthropogenic sources on the foraging success of echolocating killer 

whales. 

• The ability to assess zones of audibility and masking are dependent on knowing the 

appropriate bandwidth to quantify anthropogenic sounds. The effective filter bandwidth 

of the killer whale auditory system is unknown and has been estimated through indirect 

methods. However, these methods are often not accurate compared to direct 

measurements. Direct measurements of killer whale auditory filter bandwidths are 

needed to provide more confidence in assessing impacts of anthropogenic sounds. 

• Assessment of auditory injury in killer whales is estimated through surrogate species such 

as the bottlenose dolphin. However, differences in hearing sensitivity between the two 

species may result in inaccurate assessments of TTS in killer whales. Investigations that 

measure TTS directly in killer whales would provide very valuable data to assess the 

potential of auditory injury from sound exposure. 

• Predicting the zone of responsiveness is dependent on knowing the quantitative aspects of 

the exposure relative to the whales. There is either a lack of or too broad a range of 

received levels that are potentially correlated with behavioral responses to provide 

confidence regarding what future exposure might result in behavioral effects in SRKWs. 

Investigations that quantify both the received levels of the sound exposure (using 

appropriate metrics and within the appropriate frequency range) and the behavioral 

response of free-ranging killer whales are needed to assess zones of responsiveness. 

These studies should ideally measure the proportion of animals affected from controlled 

exposures. 

52
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84 Brand, E.J., I.C. Kaplan. C.J. Harvey, P.S. Levin, E.A. Fulton, A.J. Hermann, and J.C. Field. 2007. 

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Recent NOAA Technical Memorandums

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