Biology Reference
In-Depth Information
Underwater acoustic observations were performed by stereo acoustic data loggers (A-
tags; Little Leonardo Ltd., Tokyo, Japan, in 2006; Marine Micro Technology, MMT, Saitama,
Japan, in 2007). Each A-tag consisted of two hydrophones, approximately 170 mm apart,
which were used to identify the sound source direction, a preamplifier with band-pass filter
between 70-300 kHz (in 2006) or 55-235 kHz (in 2007) to eliminate noise outside the
frequency bands, a PIC18F6620 CPU, a 128 MB flash memory, and a waterproof tube to
encase a CR2 lithium battery cell for the boat-based observation at Stations 0, 1, and 2 or two
alkaline UM-1 battery cells for the buoy-based long-term observation at Station C. The
lifetime of A-tag with CR2 lithium battery cell is over 30 hours, and the lifetime with two
alkaline UM-1 battery cells is about one month. To ensure getting unabridged data in the
long-term observation, each deployment period would be approximately month. The
hydrophone sensitivity is -201 dB re 1 V/µPa at 120 kHz (100-160 kHz within 5 dB), which
is close to the dominant frequency of sonar signal of finless porpoises (Li et al., 2005). Each
A-tag is an event data logger that records sound pressure and the travel time difference (Td)
between two hydrophones every 0.5 ms (2 kHz event sampling frequency). It does not record
the waveforms of received sound. The active range of the A-tags for porpoise observation is
approximately 300 m, according to the source levels of porpoise signals (Li et al., 2006).
For the boat-based acoustic observation at Stations 0, 1, and 2, a bamboo rod was used to
fix the A-tag to be an underwater depth of 1-m at the side of each double-anchored boat. The
two hydrophones of each A-tag were roughly set parallel to the current direction to monitor
the moving direction of porpoises. The primary hydrophone of the A-tag was directed
upstream towards Poyang Lake, and the secondary hydrophone was directed downstream
towards the Yangtze River. This would mean that if the travel time difference of porpoise
signal between the two hydrophones of each A-tag was changing from positive to negative,
the phonating porpoise was moving from the Poyang Lake direction to the Yangtze River
direction. For the buoy-based long-term acoustic observation at Station C, an iron bar was
used to tightly fix the A-tag approximately at a depth of 1 m. As the direction of buoy is not
fixed, the relative direction of the two hydrophones in each A-tag to the current direction is
uncertain, and the moving direction of phonating porpoise upstream to the Poyang Lake or
downstream to the Yangtze River would not be identified in this case.
Data Analysis
The acoustic data were analyzed by using a custom-made program developed on Igor Pro
5.03 (WaveMetrics, Lake Oswego, OR, USA). The high-frequency click trains produced by
porpoises present regular or gradual changes in sound pressure and interclick interval (ICI).
The interclick intervals are typically between 10-80 ms (Akamatsu et al., 2005; 2007). These
characteristics can distinguish porpoises click trains from the noise of background or cargo
ships passing nearby, which have randomly changing sound pressures and interclick intervals.
Figures 2 and 3 illustrate sound pressure (SP), travel time difference (Td) between two
hydrophones of each A-tag, and interclick interval (ICI) of typical porpoise click trains and
cargo ship noise, respectively. Since the interclick intervals of porpoise click trains are
usually shorter than 130 ms (Li et al, 2007; Akamatsu et al., 2007), a click train was defined
as a series of over 5 clicks with ICIs shorter than 130 ms.
Search WWH ::
Custom Search