Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
Tursiops are opportunistic feeders, taking a wide variety of fishes, cephalopods, and shrimp (Wells
and Scott 1999) using a wide variety of feeding strategies (Shane 1990). In addition to use of active
echolocation to find food, bottlenose dolphins likely detect and orient to fish prey by listening for the
sounds they produce – so-called passive listening (Barros and Myrberg 1987; Gannon et al. 2005).
Nearshore bottlenose dolphins prey predominately on coastal fish and cephalopods, while offshore
individuals prey on pelagic cephalopods and a large variety of epi- and mesopelagic fish species
(Walker 1981; Van Waerebeek et al. 1990; Mead and Potter 1995).
Navy bottlenose dolphins have been trained to reach maximum diving depths of about 300 m
(Ridgway et al. 1969). Reeves et al. (2002) noted that the presence of deep-sea fish in the stomachs
of some offshore individual bottlenose dolphins suggests that they dive to depths of more than 500 m.
Dive durations up to 15 min have been recorded for trained individuals (Ridgway et al. 1969). Typical
dives, however, are more shallow and of a much shorter duration.
Acoustics and Hearing—Sounds emitted by bottlenose dolphins have been classified into two broad
categories: pulsed sounds (including clicks and burst-pulses) and narrow-band continuous sounds
(whistles), which usually are frequency-modulated. Clicks and whistles have a dominant frequency
range of 110 to 130 kHz and a source level of 218 to 228 dB re 1 µPa-m (Au 1993) and 3.5 to 14.5
kHz and 125 to 173 dB re 1 µPa-m, respectively (Ketten 1998). Generally, whistles range in
frequency from 0.8 to 24 kHz (Thomson and Richardson 1995).
The bottlenose dolphin has a functional high-frequency hearing limit of 160 kHz (Au 1993) and can
hear sounds at frequencies as low as 40 to 125 Hz (Turl 1993). Inner ear anatomy of this species has
been described (Ketten 1992). Electrophysiological experiments suggest that the bottlenose dolphin
brain has a dual analysis system: one specialized for ultrasonic clicks and the other for lowerfrequency
sounds, such as whistles (Ridgway 2000). The audiogram of the bottlenose dolphin shows
that the lowest thresholds occurred near 50 kHz at a level around 45 dB re 1 µPa-m (Nachtigall et al.
2000). Below the maximum sensitivity, thresholds increased continuously up to a level of 137 dB at
75 Hz. Above 50 kHz, thresholds increased slowly up to a level of 55 dB at 100 kHz, then increased
rapidly above this to about 135 dB at 150 kHz. Scientists have reported a range of best sensitivity
between 25 and 70 kHz, with peaks in sensitivity occurring at 25 and 50 kHz at levels of 47 and 46 dB
re 1 µPa-m (Nachtigall et al. 2000). Richardson (1995) noted that the differences between the
reported audiograms for these two studies might be attributable in part due to conducting the
experiments in tanks. A neurophysiological method was used to determine the high-frequency
audiograms (5 to 200 kHz) of five bottlenose dolphins (Richardson 1995). Temporary threshold shifts
(TTS) in hearing have been experimentally induced in captive bottlenose dolphins (Ridgway et al.
1997; Schlundt et al. 2000; Nachtigall et al. 2003). Ridgway et al. (1997) observed changes in
behavior at the following minimum levels for 1 sec tones: 186 dB at 3 kHz, 181 dB at 20 kHz, and 178
dB at 75 kHz (all re 1 µPa-m). TTS levels were 194 to 201 dB at 3 kHz, 193 to 196 dB at 20 kHz, and
192 to 194 dB at 75 kHz (all re 1 µPa-m). Schlundt et al. (2000) exposed bottlenose dolphins to
intense tones (0.4, 3, 10, 20, and 75 kHz); the animals demonstrated altered behavior at source
levels of 178 to 193 dB re 1 µPa-m, with TTS after exposures generally between 192 and 201 dB re 1
µPa-m (though one dolphin exhibited TTS after exposure at 182 dB re 1 µPa-m). Nachtigall et al.
(2003) determined threshold for a 7.5 kHz pure tone stimulus. No shifts were observed at 165 or 171
dB re 1 µPa-m, but when the noise level reached 179 dB re 1 µPa-m, the animal showed the first sign
of TTS. Recovery apparently occurred rapidly, with full recovery apparently within 45 min following
noise exposure. TTS measured between 8 and 16 kHz (negligible or absent at higher frequencies)
after 30 minutes of noise exposure (4 to 11 kHz) at 160 dB re 1 µPa-m (Nachtigall et al. 2004).
♦ Pantropical Spotted Dolphin (Stenella attenuata)
Description—The pantropical spotted dolphin is a generally slender dolphin. This species has a dark
dorsal cape, while the lower sides and belly of adults are gray. The beak is long and thin; the lips and
beak tend to be bright white. A dark gray band encircles each eye and continues forward to the apex
of the melon; there also is a dark gape-to-flipper stripe (Jefferson et al. 1993). Pantropical spotted
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