BioacousticsThe International Journal of Animal Sound and its Recording, 2009, Vol. 19, pp. 49–65© 2009 AB Academic Publishers

CHANGES IN CLICK SOURCE LEVELS WITH DISTANCE TO TARGETS: STUDIES OF FREE-RANGING WHITE-BEAKED DOLPHINS LAGENORHYNCHUS ALBIROSTRIS AND CAPTIVE HARBOUR PORPOISES PHOCOENA PHOCOENA

ANA CAROLINA G. ATEM1, MARIANNE H. RASMUSSEN1, MAGNUS WAHLBERG1,2, HANS C. PETERSEN3 AND LEE A. MILLER1*

1 Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark 2 Fjord & Bælt, Margrethes Plads 1, DK 5300 Kerteminde, Denmark 3 Department of Statistics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark

ABSTRACT

Probably all odontocetes use echolocation for spatial orientation and detection of prey. We used a four hydrophone “Y” array to record the high frequency clicks from free-ranging White-beaked Dolphins Lagenorhynchus albirostris and captive Harbour Porpoises Phocoena phocoena. From the recordings we calculated distances to the animals and source levels of the clicks. The recordings from White-beaked Dolphins were made in Iceland and those from Harbour Porpoises at Fjord & Bælt, Kerteminde, Denmark during prey capture. We used stringent criteria to determine which clicks could be defined as being on the acoustic axis. Two dolphin and nine porpoise click series could be used to track individual animals, which presumably focused on the array hydrophones or a fish right in front of the array. The apparent source levels of clicks in the individual tracks increased with range. One individual White-beaked Dolphin and three Harbour Porpoises regulate their output signal level to nearly compensate for one-way transmission loss while approaching a target. The other dolphin regulated the output differently. For most of the recordings the sound level at the target remains nearly constant and the echo level at the animal increases as it closes on the target.

Keywords: Echolocation, biosonar, source level, apparent source level, prey capture, hydrophone array, White-beaked Dolphin, Lagenorhynchus albirostris, Harbour Porpoise, Phocoena phocoena

*Corresponding author. E-mail: lee@biology.sdu.dk

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INTRODUCTION

All odontocetes studied thus far emit brief clicks at varying repetition rates presumably as biosonar or echolocation signals to probe their environment. Some important characteristics of their click signals are the transmission beam pattern, click intervals, and source levels. The click is emitted in a directional beam, which has been measured at vertical and horizontal angles for some species of dolphins. For the White-beaked Dolphin the estimated 3 and 10 dB beam widths are 8˚ and 10˚ respectively (Rasmussen et al. 2004). This is narrower than the 10.2˚ and 22.5˚ beam measured for the Bottlenose Dolphin (Au 1993). Echolocation clicks are sent in pulsed modes. When a click is emitted and the echo is received the next click is transmitted after a certain lag-time (Au 2000). During aggressive communication (burst pulses), pulse intervals can be 1 ms or less (Caldwell & Caldwell 1967; Blomqvist & Amundin 2004). Captive dolphins use decreasing click intervals during prey capture (Morozov et al. 1972) while free-ranging dolphins use a wide range of intervals from over 100 ms to very short intervals depending on behaviour (Lammers et al. 2004; Rasmussen & Miller 2004).

The sound pressure level of a click at 1 m from the source is defined as the source level (Urick 1983). Source levels of clicks are often expressed in dB re 1µPa peak-to-peak (p-p) values. These values, all in dB re 1µPa (p-p), vary among species and have been described for many odontocetes such as: the Pygmy Killer Whale Feresa attenuata ranging from 197-223 dB (Madsen et al. 2004b); False Killer Whale Pseudorca crassidens between 201-225 dB, Risso’s Dolphin Grampus griseus 202-222 dB (Madsen et al. 2004a); Killer Whales Orcinus orca 195-224 dB (Au et al. 2004, Simon et al. 2007) and White-beaked Dolphins Lagenorhynchus albirostris (Rasmussen et al. 2002). Source levels around 210 dB were found for Dusky Dolphins Lagenorhynchus obscurus (Au & Würsig 2004) and for the Atlantic Spotted Dolphin Stenella frontalis (Au & Herzing 2003). Zimmer et al. (2005) reported source levels of up to 214 dB for Cuvier’s Beaked Whales Ziphius cavirostris. The highest of all source levels are those reported for Sperm Whales Physeter macrocephalus, 240 dB re 1 µPa (p-p) (Møhl et al. 2003). There is also great variation in source levels with distance to the target (Au & Benoit Bird 2003) as well as amplitude variation from a single individual depending on the ambient noise level (Au et al. 1985).

Echolocation clicks have other properties such as peak frequencies (frequency of maximum amplitude in the spectrum), centroid frequency (the spectrum divided in two parts of equal energy), -3 dB bandwidth and rms bandwidth (Au 1993; Madsen et al. 2004a). White-beaked Dolphin clicks have average peak frequencies of 115 kHz with a secondary peak of approximately 250 kHz, centre frequency at

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about 82 ± 4 kHz, and -3 dB and rms bandwidths of 70±12 kHz and 36 ± 2 kHz respectively (Rasmussen & Miller 2002; 2004). The same values for wild Harbour Porpoises are: peak frequency 129-145 kHz, centre frequency 130-142 kHz, -3 dB bandwidth 6-26 kHz and rms bandwidth 5-12 kHz (Villadsgaard et al. 2007).

Localization of a phonating dolphin can be achieved using hydrophone arrays like a symmetrical “Y” hydrophone array. The “Y” hydrophone array has one hydrophone in the centre and the other three hydrophones at the ends of plastic pipes spaced at angles of 120˚ (Schotten et al. 2004). The distance to a sound source can be determined by measuring the time of arrival differences of the signal at the centre hydrophone with respect to the arrival times of the signal at the other three hydrophones (Au & Herzing 2003). This can be done by using the hydrophone array in a Cartesian coordinate system where the distance between the dolphin and the centre hydrophone is R and the horizontal and vertical angles between the dolphin and the centre hydrophone are φ and θ (Schotten et al. 2004). Since the distances to the animals are fairly short (a few tens of meters) the absorption coefficient of sound in water can be neglected. Assuming the animal directs its sonar beam to the centre hydrophone, the source level (at 1 m) can be calculated from the received level and the calculated distance to the animal for each emitted click. If the received level is constant over short distances then the animal is compensating for the one-way transmission loss by halving the emitted sound level for each halving of distance to the target (-20 Log R). Near compensation for one-way transmission loss has been shown for several species of free-ranging dolphins (Lagenorhynchus albirostris Rasmussen et al. 2002; Stenella frontalis Au & Herzing 2003; Orcinus orca Au et al. 2004; Lagenorhynchus obscurus Au & Würsig 2004; Tursiops sp Jensen et al. 2009), and for a Harbour Porpoise (Beedholm & Miller 2007).

The first purpose of our study was to determine the change in click source levels as a function of distance to a target (our hydro-phone array) from individual, free-ranging White-beaked Dolphins Lagenorhynchus albirostris. We used several criteria to judge if a signal was on axis. All previous studies, save one, of source levels of clicks emitted by free-ranging dolphins deal with populations of animals. Our second purpose was to determine the change in click source levels from three captive Harbour Porpoises Phocoena phocoena as a function of distance to fish prey. The data from one Harbour Porpoise blindfolded did not differ from those of the same animal without being blindfolded, underscoring the sole use of biosonar during prey capture. We found that one individual White-beaked Dolphin, presumably focused on our array, and the Harbour Porpoises reduced the source level to nearly adjust for the one-way transmission loss. Determining on-axis signals from free-ranging dolphins presents formidable problems even when applying strict criteria. Also the

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source levels from all dolphins at distances between 5 and 15 m nearly compensates for one-way transmission loss, similar to what is reported earlier.

MATERIAL AND METHODS

Data recording

The recordings of White-beaked Dolphin clicks took place in Iceland during the summer of 2003. The recording equipment consisted of a hydrophone array (mounted on a three meter long pole) with four matched hydrophones (TC4034, frequency range 1 Hz-350 kHz ± 3 dB, Reson, Slangerup, Denmark) at the ends of 0.5 m or 1 m plastic pipes arranged as a symmetrical “Y” connected to a multi channel amplifier (Etec, Copenhagen, Denmark) and from there to a “lunch box” computer. The sounds were recorded at 800 ksamples/s on each of the four channels simultaneously and stored in the hard drive synchronously with video recordings from an underwater camera mounted about 10 cm above the centre hydrophone (Rasmussen et al. 2004). The output from a Brüel & Kjær (Nærum, Denmark) piston phone (B&K 4223) via a special adaptor was used to check the voltage sensitivity of the hydrophones, which, in each case, did not deviate from the specified sensitivity. Calibration files each representing 163 dB re 1 µPa rms with noted amplification were stored for later converting to peak-to-peak (p-p) values by adding 9 dB. The recording equipment was calibrated out to 13 m at 0˚ (in line with the centre hydrophone). A calibration curve of the hydrophone array can be found in Rasmussen et al. (2004). We limited the calculated distances from 1 m to 15 m in this study.

Data analysis

The sound files were separated into four channels, one for each hydrophone, using the software SigPro (Simon Boel Pedersen, Copenhagen, Denmark) to confirm that clicks were recorded on all hydrophones. The program was mainly used to identify click series from individual animals. An animal was considered clicking towards the array when the highest amplitude was recorded on the centre hydrophone or equally high amplitudes on one or all of the other three hydrophones (criterion 1 in Table 1). To identify if a dolphin was clicking alone towards the array, a sequence should contain only one obvious click train.

After selecting individual click trains and making sure none of the clicks had overloaded our equipment, clicks were edited

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using CoolEdit Pro (version 2, Syntrillium Software, Phoenix, AZ, USA). The edited click trains were then loaded into MatLab (The MathWorks, Inc. Cambridge, MA, USA) to calculate the position of the dolphins, apparent source levels (ASL), angles to the dolphins, and centre frequencies (using a specially written script). The script uses the time of click arrivals at the four hydrophones and the speed of sound in the water to calculate the position of the vocalizing dolphin according to Schotten et al. (2004). In this paper we use the term apparent source level (ASL) back calculated to 1 m instead of source level, which implies that the signal level is reported at 1 m distance, irrespectively of angle to the source. The clicks were selected as being on-axis by sorting them with a set of criteria (see below). All apparent source levels (ASL) given here are expressed in dB re. 1 µPa peak-to-peak (p-p).

After analyzing each sequence carefully, we realized that clicks in some sequences recorded from free-ranging White-beaked Dolphins could be off-axis. Therefore, we used two more criteria to select presumed on axis clicks for source level analysis (criteria 2 and 3 in Table 1), a criterion to make sure the dolphins were in the far field (criterion 4 in Table 1) and a criterion to assure a single animal was on axis and moving toward our array (criterion 5 in Table 1).

The echolocation clicks from three captive Harbour Porpoises (one male, Eigil, and two females, Freja and Sif) were recorded during prey capture trials performed periodically between 2004 and 2008 at Fjord & Bælt, Kerteminde, Denmark. The recording equipment was the same as that used for the White-beaked Dolphin studies. During prey capture trials, the animals were sent from one end to another end of the pool, where a dead fish was thrown into the water 20 to 30 cm directly in front of the array. Fish captures were documented in video recordings. (an example can be seen here-http://www.acoustics.org/press/156th/miller.html). The female porpoise, Freja, performed the task both with and without being “blindfolded” with suction cups over her eyes.

TABLE 1

Five criteria used to select on-axis clicks. Criteria 1 to 4 are fromRasmussen et al. 2004.

1) A maximum apparent source level (p-p) on the centre hydrophone or equal to one of the outer hydrophones2) Centre frequency on the centre hydrophone should be above 85 kHz;3) The vertical and horizontal angles between the dolphin and the centre hydrophones should be within 35˚;4) Dolphin should be at least 1 m from the array;5) Distance swum with continuous on-axis clicks should be more than 1 m.

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The statistical analyses were performed using the procedure for mixed linear models (“proc mixed”) in the SAS 9.1.3 system (SAS Institute Inc. 2004). Individuals were considered as random effects, with trials nested within individuals, and meter (distance to target) was considered as a fixed continuous independent variable related to the time of the experiment. The modelling followed standard procedures with F and t-tests, as illustrated in West et al. (2007), chapter 7.

RESULTS

White-beaked dolphins

In total, we analyzed 804 White-beaked Dolphin clicks from 144 click sequences, in which only 10 sequences (134 clicks) fulfilled the first criterion in Table 1; long click sequences from dolphins ensonifying the centre hydrophone with the highest amplitude clicks (or equally high). Clicks from these 10 sequences are plotted in Figure 1 and show that apparent source levels increase with Log range (R). Figure 1 (a) shows all the 134 clicks while Figure 1 (b) shows only those clicks from dolphins at ranges between 5 and 12 m. There is considerable scatter in source levels with the maximum value at about 195 dB re. 1 µPa (p-p) and a regression line with a slope of 21 Log R, or nearly the one-way transmission loss.

Two click sequences fulfilled all five criteria in Table 1, thus signifying click sequences from two individual dolphins. Table 2 shows the mean values for apparent source levels, maximum distance, distance swum, centre frequencies and the vertical and horizontal angles between the dolphin and the hydrophone. The two sequences are depicted in Figure 2 showing that apparent source levels decrease while the dolphins approach the array. Apparently, dolphin 1 (black triangles in Figure 2) follows rather closely the one-way transmission loss and dolphin 2 does not. The two click sequences were recorded on different days, so they were emitted most likely from two different dolphins.

Harbour Porpoises

The source levels and distance swam for three Harbour Porpoises performing prey detection and capture tasks are show in Figures 3 to 5, where one Harbour Porpoise, Freja, performed the task blindfolded as well (Figure 5b). Similar to the White-beaked Dolphin graphs, source levels decrease with Log range in all sequences and follow closely the one-way transmission loss. The slopes given in Figure 3 to

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Figure 1. Apparent Source Levels (ASL) for on axis clicks from 10 free-ranging White-beaked Dolphins as a function of distance. (a) shows all dolphin clicks at ranges from 1 to 15 m. The regression line is given in the figure, n= 134 (b) shows only those clicks from dolphins at distances greater than 5 m. The regression line is given in the figure, n=120. Note the 25 dB variation in source levels.

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5 are not significantly different (p > 0.2) with the common slope being 20.4 Log(R), 95% confidence interval 17.0 to 23.9. The common slope was not significantly different from the 20 Log(R) slope (p > 0.5), but was significantly different from the zero and 40 Log(R) slopes (p < 0.0001 for each). However, the dB values at the y-intercepts shown in Figures 3 to 5 are significantly different (p = 0.0008) with an average y-intercept of 147 dB. Freja emitted the most intense clicks with an average y-intercept of 149 dB. The average y-intercept for Sif was 147 dB and that for Eigil was 146 dB.

TABLE 2

Apparent Source Levels (ASL), maximum distance (dmax) , distance swum (Δd), centre frequency (Cf), vertical angle (θ) and horizontal angle (φ), for sequences 1

and 2. SD is the standard deviation.

Click Number ASL dmax Δd Cf θ φseq. of (dB) (m) (m) (kHz) (degrees) (degrees) on-axis (average± (average± average± average± clicks SD) SD) (SD) SD)

1 16 177±0.7 8.50 1.34 90.6±3.3 -27±2 5.9±0.82 18 186±3.1 12.56 2.72 90.9±3.6 -14.3±2.3 1.1±2.7

Figure 2. Apparent source levels (ASL) from two presumably different individual, free-ranging White-beaked Dolphins as a function of range. The regression line for dolphin 1 (▼) is given by y=26 Log R +153 dB, n= 16. The regression line for dolphin 2 (•) is given by y=73 Log (R) +111 dB, n= 18. Apparently dolphin 2 is not focusing on the array even though all of the signals were classified by us as being on axis. See text for further details.

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Figure 3. Apparent source level (ASL) as a function of range during prey capture by a male Harbour Porpoise, Eigil. The data cover three different tracks recorded during three different years (symbols). The regression line is given in the figure, n= 45.

Figure 4. Apparent source level (ASL) as a function of range during prey capture by a young female Harbour Porpoise, Sif. The data cover two tracks made during different years (symbols). The regression line is given in the figure, n= 15.

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Figure 5. Apparent source level (ASL) as a function of range during prey capture by an older female Harbour Porpoise, Freja. The data in (a) cover three tracks made during different years (symbols) and those in (b), with Freja blindfolded, cover two tracks made during different years (symbols). The regression line in (a) is given in the figure, n= 35. The regression line in (b) is given in the figure, n= 39. (See text for more information.)

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DISCUSSION

Previous studies on four species of delphinids, including a study of White-beaked Dolphins using different equipment, showed that source levels are higher at greater distances and that they decrease with decreasing range (Rasmussen et al. 2002; Au & Herzing 2003; Au & Benoit-Bird 2003; Au et al. 2004; Au & Würsig 2004). The same is reported in this study (Figure 1). In addition, presumed individual White-beaked Dolphins show the same behaviour (Figure 2).

If the emitted signal is constant in amplitude, the echo level at the dolphin will increase with decreasing range since the target strength is constant. Therefore, it is possible that a dolphin approaching a target will compensate for this increase in echo level by decreasing the level of its outgoing echolocation signal as suggested in earlier studies. Similar to the studies cited above, the majority of click amplitudes in our study decrease in accordance with a one-way transmission loss or close to a 20 Log R function. With a decrease in distance, the amplitude of the echoes will double at the dolphin’s ear if it attenuates the signal by 6 dB per distance halved (-20 Log R), which maintains a constant ensonification level on the target. Au and Benoit-Bird (2003) suggested that this reduction in source level with decrease in range represents a time-varying automatic gain control (AGC). Another possibility would be for the dolphin to compensate for the two-way transmission loss (40 Log R) thus keeping the echo level constant at the ear (Rasmussen et al. 2002; Au & Benoit-Bird 2003). The source levels of White-beaked Dolphin clicks in our study had a maximum of 26 Log R (95% confidence interval) (except for one animal described below) and thus were far from compensating for the two-way transmission loss.

Dolphin echolocation signals are emitted in a directional beam, and signals recorded distant from the axis of the beam (off-axis clicks) are distorted, with distorted signal waveforms, attenuation of higher frequencies components, and decreasing peak frequencies (Au 1993; Au & Nachtigall 1997). Simon et al. (2007) suggested that when using only the criteria of highest (or equally high) amplitude at the centre hydrophone, some of the clicks may be off-axis and were recorded from a direction where the apparent source level was not changing with off axis angle. And, as pointed out earlier (Madsen et al. 2004b, Madsen & Wahlberg 2007), there is still no analytical method that can discriminate exactly on and off axis clicks. The highest amplitude at the centre hydrophone is definitely the major indication of on-axis clicks. However, when more conservative criteria were considered, such as centre frequency, angles between dolphin and the equipment and others (Table 1), many clicks that were previously considered on–axis were discarded. On the other hand, these conservative criteria will exclude weak on-axis clicks (Madsen & Wahlberg 2007). Spectral

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parameters are influenced by signal level (Au 1993) and choosing a centre frequency above 85 kHz could have excluded on-axis clicks with lower intensities from our data set.

After using all five criteria in Table 1, only two click sequences were found from presumed single individuals. The first sequence of clicks was short and emitted from a dolphin that swam only 1.3 m. But even so, a slight decrease in source level with decrease in range could be calculated, confirming the tendency observed in our results using a liberal criterion. The clicks from this individual fell close to the one-way transmission loss curve in a similar manner to what was already described for other dolphin species, including White-beaked Dolphin (Rasmussen et al. 2002; Au & Herzing 2003; Au & Benoit-Bird 2003; Au et al. 2004; Au & Würsig 2004).

Just because a dolphin’s clicks pass all our criteria for being on-axis does not mean the animal is interested in our recording array. The second sequence from an individual dolphin seen in Figure 2 shows that apparent source levels decreased drastically (72 Log R) with decreasing change in range, even though the range was not long (≈ 3 m). In this case the clicks did not fall anywhere close to the one-way or two-way transmission loss curves, and this pattern does not seem to be from a dolphin interested in our array. This dolphin did not regulate its output level either by 6 or 12 dB per distance halved, but by 22 dB per distance halved while approaching the array. Nevertheless, the dolphin still reduces its output signal while approaching a “target”, just like in the first sequence. Either the dolphin was not interested in the array, or there is more plasticity in the source level-to-range regulation than that indicated from analyses of pooled results and from prey capture sequences by individual Harbour Porpoises.

The source levels given in Rasmussen et al. (2002) are about 20 dB greater than those given here, which is significant. An increase in environmental noise caused an 18 dB increase in source levels of a Beluga Delphinapterus leucas (Au et al. 1985). We feel that differences in environmental and recording conditions may have influenced source levels. Data for the Rasmussen et al. (2002) paper were collected in 1998 using a different vessel and different recording system from that used by us in 2003 for this study. However, background noise measurements were not made during recordings made in 1998 or 2003.

In addition, our criteria are more rigorous for selecting click sequences that could be attributed to a single individual. Rasmussen et al. (2002) used the first criterion from Table 1 to select on-axis clicks and found one sequence of one dolphin looking straight to the underwater video camera. Here we used five criteria to select on-axis clicks and found two sequences from individual dolphins emitting clicks towards the hydrophone array. Therefore, it is more likely that

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the clicks presented here are on-axis than those shown in Rasmussen et al. (2002).

The Harbour Porpoises at Fjord & Bælt were trained to capture dead fish in front of the hydrophone array and, therefore, swam directly towards the array. Even so signals must qualify criterion 1 in Table 1 to be considered on axis. The Harbour Porpoises in our study reduced the level of their outgoing signals while approaching the target (Figures 3-5). The levels of their signals decreased by about 6 dB per distance halved, closely following the one-way transmission loss curve (20 Log R), in a similar fashion as described for the White-beaked Dolphins mentioned above. The regression lines of the three porpoises were not significantly different and had a common slope of 20.4 Log R.

It was clear in our study and documented in another study (Verfuss et al. 2009) that the inter click interval (ICI) decreases (click rate increases) as the amplitude decreases when the Harbour Porpoises approach the fish prey. However, we could not quantify this relationship since not all emitted clicks during a prey capture sequences were on the acoustic axis according to our criteria. However, when one of the porpoises, Eigil, spontaneously changed the rate of his echolocation clicks while stationary at a small plastic square; the amplitude of his signals followed rather closely a 20 Log (ICI) function (Beedholm & Miller 2007). Thus the decrease in amplitude with increasing click rate as the animal closes on the target may reflect limitations imposed by the sound production mechanism at high clicks rates (Beedholm et al. 2006).

Some odontocete species do not regulate the source level in the manner observed in the species studied here. Beaked whales Mesoplodon densirostris and Ziphius cavirostris and the Sperm Whale Physeter macrocephalus maintain high output levels until abruptly changing to low levels and high click rates during the “buzz” indicating prey capture (Madsen et al. 2002, 2005).

Besides regulating the output from the sound generator, animals may also control their hearing abilities. Such an “auditory controlled” AGC system has been described for the False Killer Whale Pseudorca crassidens by recording auditory evoked brain potentials (Supin et al. 2004; 2005; 2007). These authors show that Pseudorca crassidens was capable of regulating its hearing sensitivity, presumably by a forward masking mechanism, thus compensating for the changes in echo level. Pseudorca crassidens did not change the level of transmitted sonar signals with changing target distances (Supin et al. 2007).

Two questions arise at this point. Why do some dolphins and the Harbour Porpoise show an apparent “motor controlled” AGC while approaching a target? Why does the false killer whale apparently have an auditory controlled AGC and keep the transmitted signal level constant?

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As the White-beaked Dolphin and the Harbour Porpoise inhabit highly reverberant environments such as near shore and shallow waters, they are forced to find their prey in a cluttered environment. As described by Au and Turl (1983) and Turl et al. (1991), reverberation is the sum of echoes scattered from objects and in-homogeneities in the medium. Reducing the level of the outgoing signal while approaching a target would reduce echoes from “uninteresting” objects (clutter) smaller than the target and improve the signal-to-clutter ratio. The level of some clutter echoes may actually fall below the auditory threshold and “disappear” from the animal’s auditory image. Why does the level of reduction follow more closely the one-way transmission loss? In this case the ensonification of the target (prey) will be nearly constant and independent of distance. Provided that the echolocation clicks of odontocete predators are sufficiently intense (source level about 200 dB re. 1 µPa (p-p) for the Harbour Porpoise, see Villadsgaard et al. 2007), prey that are able to hear these signals, like several fish species in the herring family Alosinae (Wilson et al. 2008), will lack information on proximity of the predator, giving it an advantage over the prey (Verfuss et al. 2009).

If clutter is not a problem then keeping the outgoing signal level independent of distance for long ranges, as reported for some beaked whales and the sperm whale (Madsen et al. 2005), will maintain a high signal-to-noise ratio and a better image of the target. Should these species have a central auditory AGC like that reported for the false killer whale (Supin et al. 2007), this, in addition, might improve target identification by providing finer sensory control and reducing specialization for sound production.

In conclusion, the present study supports the general tendency shown by some dolphins and the Harbour Porpoise to reduce the apparent source level while closing in on a target. This source level regulation may improve signal-to-clutter ratios, since it would be advantageous to reduce clutter echoes while approaching a target. Since the signal attenuation closely follows the one-way transmission loss, a constant ensonification level is maintained on a prey target and a prey that can hear odontocete signal frequencies cannot sense the change in distance to an approaching porpoise or dolphin.

We are only beginning to uncover the biosonar world of free-ranging odontocetes. Especially important are comparative studies of species in different habitats to determine the flexibility of their biosonar. For example, are there really differences in biosonar systems of coastal species contra pelagic species and if so how are the biosonar systems adapted to specific environments? Are there odontocete/prey interactions or do odontocetes enjoy unhindered access to prey species? The increasing use of archival tags (Jones et al. 2008) attached to individual odontocete species will profoundly increase our understanding of how these echolocators

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use their biosonar during orientation and prey capture in different environments.

ACKNOWLEDGMENTS

We thank the field assistants, and especially Helga Ingimundardóttir for her help in Iceland. Thanks to Dr. Peter T. Madsen, Aarhus University, and anonymous referees for valuable suggestions to improve the manuscript. Data for Harbour Porpoise source levels were obtained during courses in Marine Mammal Biology held at the Marine Biology Research Centre and Fjord & Bælt, Kerteminde, Denmark. The first author would like to thank the Biological Institute for financial support and her family and friends. The Harbour Porpoises are maintained by Fjord & Bælt, Kerteminde, Denmark, under Permit No. J.nr. SN 343/FY-0014 and 1996-3446-0021 from the Danish Forest and Nature Agency, Danish Ministry of Environment. We acknowledge the staff at the Fjord & Bælt for their cooperation.

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Received 23 December 2008, revised 18 February 2009 and accepted 27 February 2009