94
The Biology of Sea Turtles, Vol. II
and muscular activity. Thresholds were recorded for both tonal and click stimuli.
Best sensitivity was found in the low-frequency region of 250 –1000 Hz. The decline
in sensitivity was rapid after 1000 Hz, and the most sensitive threshold tested was
at 250 Hz (the lowest frequency tested), with a mean threshold of ~26.3 dB re 1 g
root mean square (rms) + 2.3 dB standard deviation (SD).
3.3.3 BEHAVIOR
Two research studies have examined the response of juvenile loggerheads to sound
in their natural environment (Moein et al., 1995; O’Hara and Wilcox, 1990). In both
cases, these studies were initiated to assist in the development of an acoustic repelling
device for sea turtles. O’Hara and Wilcox (1990) attempted to create a sound barrier
for loggerhead turtles at the end of a canal of Florida Power & Light using seismic
air guns. The test results indicated that at 140 kg/cm 2 the air guns were effective as
a deterrent for a distance of about 30 m. The sound output of this system was
characterized as approximately 220 dB re 1 mPa at 1 m in the 25–1000 Hz frequency
range. However, this study did not account for the reflection of sound by the canal
walls. Consequently, the stimulus frequency and intensity levels are ambiguous
(O’Hara and Wilcox, 1990).
Moein et al. (1995) investigated the use of pneumatic energy sources (air guns)
to repel juvenile loggerhead sea turtles from hopper dredges. A net enclosure
(approximately 18 m ¥ 61 m ¥ 3.6 m) was erected in the York River, VA, to contain
the turtles, and an air gun was stationed at each end of the net. A float attached to
the posterior of the carapace was used to note the position of the turtle as the air
guns fired. Sound frequencies of the air guns ranged from 100 to 1000 Hz (Zawila,
1995). Three decibel levels (175, 177, and 179 dB re 1 mPa at 1 m) were used every
5 sec for 5 min. Avoidance of the air guns was observed upon first exposure for the
juvenile loggerheads. However, these animals also appeared to habituate to the sound
stimuli. After three separate exposures to the air guns, the turtles no longer avoided
the stimuli (Moein et al., 1995).
3.3.4 CONCLUDING REMARKS
These studies highlight the need for more research on the auditory capabilities
of sea turtles. It is believed that physiological and behavioral adaptations may
have evolved for sea turtles based on their selection of aquatic niches with each
ontogenetic stage. For these three stages of life, the sensory environment also
changes. Shallow-water habitats of the juvenile and adult stages are a much
“noisier” world than the open ocean environment of the hatchling stage. Ambient
noise in the inshore environment is heavily weighted to low-frequency sound
(Hawkins and Myrberg, 1983). In highly developed areas (coastal waters) lowfrequency noises associated with shipping lanes, recreational boat traffic, and
biological organisms are prominent. Differences in functional morphology and
behavioral hearing capabilities among species and life history stages have not
been documented for sea turtles in the literature. In fact, only juvenile loggerhead
and green sea turtles have undergone any auditory investigations. Both the middle
1123 book.book Page 94 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
and muscular activity. Thresholds were recorded for both tonal and click stimuli.
Best sensitivity was found in the low-frequency region of 250 –1000 Hz. The decline
in sensitivity was rapid after 1000 Hz, and the most sensitive threshold tested was
at 250 Hz (the lowest frequency tested), with a mean threshold of ~26.3 dB re 1 g
root mean square (rms) + 2.3 dB standard deviation (SD).
3.3.3 BEHAVIOR
Two research studies have examined the response of juvenile loggerheads to sound
in their natural environment (Moein et al., 1995; O’Hara and Wilcox, 1990). In both
cases, these studies were initiated to assist in the development of an acoustic repelling
device for sea turtles. O’Hara and Wilcox (1990) attempted to create a sound barrier
for loggerhead turtles at the end of a canal of Florida Power & Light using seismic
air guns. The test results indicated that at 140 kg/cm 2 the air guns were effective as
a deterrent for a distance of about 30 m. The sound output of this system was
characterized as approximately 220 dB re 1 mPa at 1 m in the 25–1000 Hz frequency
range. However, this study did not account for the reflection of sound by the canal
walls. Consequently, the stimulus frequency and intensity levels are ambiguous
(O’Hara and Wilcox, 1990).
Moein et al. (1995) investigated the use of pneumatic energy sources (air guns)
to repel juvenile loggerhead sea turtles from hopper dredges. A net enclosure
(approximately 18 m ¥ 61 m ¥ 3.6 m) was erected in the York River, VA, to contain
the turtles, and an air gun was stationed at each end of the net. A float attached to
the posterior of the carapace was used to note the position of the turtle as the air
guns fired. Sound frequencies of the air guns ranged from 100 to 1000 Hz (Zawila,
1995). Three decibel levels (175, 177, and 179 dB re 1 mPa at 1 m) were used every
5 sec for 5 min. Avoidance of the air guns was observed upon first exposure for the
juvenile loggerheads. However, these animals also appeared to habituate to the sound
stimuli. After three separate exposures to the air guns, the turtles no longer avoided
the stimuli (Moein et al., 1995).
3.3.4 CONCLUDING REMARKS
These studies highlight the need for more research on the auditory capabilities
of sea turtles. It is believed that physiological and behavioral adaptations may
have evolved for sea turtles based on their selection of aquatic niches with each
ontogenetic stage. For these three stages of life, the sensory environment also
changes. Shallow-water habitats of the juvenile and adult stages are a much
“noisier” world than the open ocean environment of the hatchling stage. Ambient
noise in the inshore environment is heavily weighted to low-frequency sound
(Hawkins and Myrberg, 1983). In highly developed areas (coastal waters) lowfrequency noises associated with shipping lanes, recreational boat traffic, and
biological organisms are prominent. Differences in functional morphology and
behavioral hearing capabilities among species and life history stages have not
been documented for sea turtles in the literature. In fact, only juvenile loggerhead
and green sea turtles have undergone any auditory investigations. Both the middle
1123 book.book Page 94 Monday, November 11, 2002 11:11 AM