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it was possible to count the number and determine the swimming direction of the phonating
animals by analysis of their click trains. During off-line analysis, click trains of 10 s or less
(apart from each other) and having smoothly changed traces of travel time difference were
considered produced by one individual; these trains were defined as a single track. The
number of independent tracks in each 1-min time bin was defined as the observed number of
animals (or group size) in a unit of time (1 minute). The swimming direction of an animal was
determined by the gradual change of travel time difference traces (see above).
For the buoy-based long-term acoustic observation at Station C, on account of the
uncertainty of the buoy direction, the relative direction between the two hydrophones of each
A-tag and uncertain current direction, and it was difficult to determine the number and
swimming directions of phonating animals. Instead, only detection (or group) of porpoise was
documented. A detection (or group) was determined when porpoise click trains were
identified and the click trains were within 5 minutes of each other. Porpoise presence time
was defined as the start time of the first click train in a detection; and detection duration was
the time period between the start time and the end time of a detection. Once a detection was
identified, the presence time and detection duration were documented. To describe the
temporal presence pattern of porpoises in Station C, the presence ratio of animals was
analyzed. The presence ratio of animals was the ratio of the accumulated detection duration
over the unit duration such as one day or one hour.
Number of passing cargo ships and hydrology data were also collected to compare with
the presence of porpoises in the long-term acoustic observation station, Station C. These
cargo ships could be identified and counted acoustically by their changing travel time
difference (Td) (Figure 3). The hydrology data of the study area during deployment of the
long-term acoustic observation, including flux and direction of water current, were acquired
from the Hydrological Bureau of the Yangtze River Water Resources Commission.
R
ESULTS
In the boat-based acoustic observation, we obtained 1,216 and 504 minutes of effective
observation time at Stations 1 and 2, respectively, during April 27-29 of 2006, and 464
minutes at Station 0 during May 9-10 of 2007. At Stations 0, 1, and 2, animals were detected
acoustically in 92.9, 76.2, and 76.0% of the effective observation time, respectively. In unit
time (1 minute), Station 0 counted the most porpoises, which was on average 1.85
individuals/min; Station 1 counted an average of 1.41 individuals/min; and Station 2 counted
the least porpoises, at only 0.83 individuals/min (Figure 4b). Swimming direction could only
be determined for a few porpoises. At Stations 0, 1, and 2, averages of only 0.35, 0.30, and
0.01 individuals/min were determined with swimming directions, respectively (Figure 4a).
Porpoises were observed swimming both upstream to the Poyang Lake and downstream to the
Yangtze River at all three stations (Figure 4a).
The buoy-based long-term acoustic observation has been deployed since June 27, 2007,
at Station C (Figure 1). The data shown in this chapter were obtained between June 27 and
September 28, 2007, with a total observation time of 80 days (Table 1). In total, 578 porpoise
detections were identified, and the total detection duration was 15,411 minutes, which
occupied 13.9% of the effective observation time (Table 1). The presence pattern of porpoises
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