Biology Reference
In-Depth Information
Figure 2. Historical distribution of the baiji until the mid-twentieth century. The baiji formerly occurred
in the main Yangtze channel as far upstream as Huanglingmiao and Liantuo (downstream section of the
Three Gorges), approximately 1900 kilometers from the estuary (see Zhou et al., 1977). It also occurred
in Dongting and Poyang Lakes, two large lake systems appended to the main Yangtze channel, and the
neighboring Qiantang River.
Continued wide-scale anthropogenic impacts caused by increasing human population
density, aggressive environmental exploitation and industrialization over the past 50 or so
years (Shapiro, 2001) were responsible for a further precipitous decline in the remaining baiji
population. However, the range of different potential extinction drivers operating in the
Yangtze region, and the limited available data on baiji mortality, complicates our
understanding of the relative importance of different threat processes in this population
collapse. The primary factor was probably unsustainable by-catch in local fisheries; at least
half of all known baiji deaths observed by Chinese researchers from the 1950s to the 1980s
were caused by rolling hook long-lines and other fishing gear (Figure 3), and electro-fishing
accounted for 40% of the limited number of known baiji deaths recorded during the 1990s.
Further mortality is also known to have been caused by collisions with boat hulls and
propellers, explosives used in channel clearance, and chemical spills (Lin et al., 1985; Chen &
Hua, 1989; Zhou & Li, 1989; Zhou & Zhang, 1991; Zhou & Wang, 1994; Zhou et al., 1994,
1998; Sheng, 1998b; Zhang et al., 2003). Agricultural and industrial intensification and water
development projects have led to escalating habitat degradation in the main Yangtze and
Qiantang river channels and their tributaries and appended lakes, including increased
siltation, elimination of optimal baiji counter-current habitat, and decreases or extirpation of
many fish species, all of which are likely to have had substantial further impacts on baiji
populations (Liu et al., 2000; Smith et al., 2000; Xie, 2003; Fang et al., 2006). In particular,
industrial and agricultural pollutants may have severely impacted baiji health and fertility, but
data to assess the significance of this likely extinction driver remain very limited (Yang
&
Liu, 2005; Shao et al., 2006). Although dolphins stranded on sandbars were sometimes beaten
to death by local residents (Perrin & Brownell, 1989), direct exploitation of baiji largely
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