Biology Reference
In-Depth Information
As described above, both Stations 2 and C are situated in the section between the two
bridges (highway and railway bridges).
The distance between Station 2 and C is only
approximately 2,400 m, without any other visible physical barriers between them. Porpoises
could be acoustically detected at stations 2 (Figure 4) and C (Figures 5a and 6b), and detected
swimming both upstream to Poyang Lake and downstream to the Yangtze River at stations 0,
1, and 2 (Figure 4a), despite being difficult to visually observe the animals in the section
between the two bridges (unpublished data). It is supposed that the ―back and forth‖
movement behaviors, which were often observed visually before (Zhang et al., 1993), might
still occur in the mouth area of Poyang Lake. There could still be a chance of genetic
communication among the groups in the Yangtze River main stem and Poyang Lake.
However, the proportion or degree of the ―back and forth‖ movement and genetic
communication might be limited, since the animal density at Stations 2 was relatively low
(0.83 individuals/min on average), which is about half of the presence at Stations 0 and 1
(1.85 and 1.41 individuals/min, respectively). The presence ratio of porpoises in Station C is
only 13.9% of the effective observation time. These data suggest that the lowest density area
exists between the two bridges, a busy construction area. We need to carefully monitor finless
porpoises in these areas to hopefully prevent population fragmentation. The data presented in
this chapter verify the usefulness of stereo A-tags for stationary acoustic monitoring and the
detection of porpoise presence, especially in the cases, where the animal density is relatively
low and finding animals by visual observations is hard.
The low animal density detected in Stations 2 and C, situated in the section between the
two bridges (Figure 1), might be a bridge effect. The bridges might have blocked the
movement of porpoises through them by both changing local bottom topography and
environments of hydrology and underwater noise. All vibration transferred by piers, engine
noise produced by vehicular traffic and the construction activity at the upstream railway
bridge (the railway bridge was still under construction during the observation period) could
have produced extra underwater noise.
Effects of Shipping Traffic and Water Current
At Station C, porpoise presence was observed in both day and night over the entire
observation period between June 27 and September 28, 2007. There were two distinct time-
periods when fewer porpoises were detected; between 05:00 and 10:00, and between 15:00
and 18:00 o'clock, respectively (Figure 5a). It seems that the shipping traffic did have a
negative effect on the porpoise presence (Figure 5). When the shipping traffic was high, the
presence ratio of porpoises was low, and vice versa (Figure 5). The shipping traffic might
affect the porpoise presence by strong engine noise and generated water waves.
Porpoises were almost observed everyday during the acoustic observation period at
Station C, except for the period when the system did not work, and between July 12 and July
28, 2007, and between August 5 and August 22, 2007, when the presence ratio data was
reduced (Figure 6b). Figure 6 shows that there were some matches between hollows of day-
by-day presence ratio of porpoises in each 1-day time unit and turbulences or reversing of
current direction. When the turbulence or reversing of current direction appeared, the
presence ratio of porpoises dropped dramatically (see the grey transparent panes in Figure 6).
The turbulence or reversing of current direction might have indirectly affected the presence or
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