Chapter 3 Population trend of Chinese white dolphins (Sousa chinensis) in the
3.2 Material and methods
3.4.5 Conservation Implications
Considering the long life span of these dolphins (at least 38 years, Jefferson et al.
2012) , the low birth rate and high mortality rate estimates by this study caused serious ecological modeling issue. Since both the birth rate and mortality are density-dependent parameters in population trend analysis, which means as the population became smaller, the changes of these parameters will be dramatic, making the population drop exponentially. In Moray Firth, Wilson et al.(1999) used power analysis on a coastal bottlenose dolphin population indicated the detection of population decline in 10% only occur by at least 8 years of efforts, and if the population decline was detected in less
As Ross et al. (2010) suggested before, the Chinese white dolphins population in Taiwan faced a wide range of anthropogenic threats from fishery mortality (e.g.
bycatch), freshwater diversion, chemical and biological pollution, to noise and habitat degradation. Unfortunately, over the past years these impacts were rarely reduced, worsening the habitat status for the dolphins. As the plan for the government-proposed essential habitat progresses, it is strongly recommended to focus additional studies on not only the impacts but mitigation of anthropogenic threats to the dolphins, while their habitats, at least the high density area, need to be improved immediately. As the population is in a critical declining crisis, all related conservation actions must take place soon before it’s too late.
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Table 3.1 Results of the first six best models in robust design mark-recapture
Table 3.2 Annual estimates of abundance and other parameters based on the best model
Efforts
*P=Capture rate; C=Re-capture rate; N=abundance/population size θ=unmarked ratio; Nt=corrected total population size
Table 3.3 Annual estimates of abundance and other parameters based on the second best model with survivorship varying with time “S(t)”
S SE P=C SE ̂ 95%CI 𝜽̂ SE ̂𝒕 95%CI
2008 0.57 0.059 65 65-68 0.120 0.023 74 73-75
2009 0.99 0.015 0.61 0.054 66 65-70 0.104 0.023 74 72-76 2010 0.97 0.021 0.56 0.055 69 67-74 0.084 0.017 75 72-78 2011 0.94 0.029 0.51 0.052 65 63-71 0.129 0.030 74 71-77 2012 0.91 0.037 0.69 0.059 63 62-66 0.027 0.008 64 63-66
*S= survival rate; P=Capture rate; C=Re-capture rate; N=abundance/population size θ=unmarked ratio;
Nt=corrected total population size
Table 3.4 Annual crude birth rate (CBR) of the Chinese white dolphins in Taiwan
Year 2008 2009 2010 2011 2012
n 2 6 4 3 0
i 65 65 68 64 63
mc 17 19 15 13 6
CBR(%) 2.44 7.14 4.82 3.90 0
*n=number of neonates; i=number of individuals observed; mc=number of mother-calf pairs
Table 3.5 Population abundances of the Chinese white dolphins in South China Seas estimated by the mark-recapture method
Region Country N(95%CI) Reference
Khanom Thailand 49(N.A.) Jaroensutasinee et al. 2011
Hong Kong and east Pearl River Estuary
China 753(635-943) Jefferson and Hung 2004
Pearl River Estuary
China 2517-2555(N.A.) Chen et al. 2010
Leizhou Bay China 237(189-318) Zhou et al. 2007
Xiamen China 76(43-109) Chen et al. 2009
Western Taiwan coast
Taiwan 75(69-81) Yu et al. 2010
Western Taiwan coast
Taiwan 74(68-80) Wang et al. 2012
Western Taiwan coast
Taiwan 64(63-66) This study
Table 3.6 Yearly estimates of population abundance of the Chinese white dolphins in Taiwan
2007 2008 2009 2010 2011 2012 Survey Trips
Yearly Efforts(km)
Reference
Simple closed population
80 75 114-151 6001-7051.7 Yu et al.
2010
Robust design
65 68 54 74 17-28 1138.4-1292 Wang et al.
2012
Robust design
74 74 75 74 64 50-151 2189.8-7051.7 This study
Fig. 3.1 Monthly variation in survey trips between 2008 and 2012 (showing only monthly surveys >5 trips, n=439 surveys)
Fig. 3.2 Frequency distribution of various dolphin group sizes (n=323 sightings from 478-day surveys between 2008 and 2012).
0 10 20 30 40 50 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Number of sightings
Group Size
Fig. 3.3 Discovery curve of the Chinese white dolphins in Taiwan between 2007 and 2012 (* indicates that new individuals were recruited from grown-up juveniles)
1 51 101 151 201 251 301
<5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55
Number of individual
2007 2008 2009 2010 2011 2012
*
Fig. 3.5 Annual changes in dolphin population abundance with mean and 95%
confidence interval (N: abundance estimated; Nt: total population with unmarked ratio correction)
40 45 50 55 60 65 70 75 80
2008 2009 2010 2011 2012
Abundance
Year
N
Nt