Accelerating population decline of Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis)
Highlights
► Comparative life tables were constructed before and after 1993. ► The instantaneous rate of increase () revealed an accelerating population decline. ► Status of N. p. asiaeorientalis meets Critically Endangered on the IUCN Red List. ► The probability of extinction (PE) is 86.06% within the next 100 years. ► Effective conservation measures must be enacted immediately.
Introduction
The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) is a subspecies of the narrow-ridged finless porpoise (N. asiaeorientalis), it is endemic to the middle and lower reaches of the Yangtze River, and is the only freshwater porpoise (Gao and Zhou, 1993, Zhao et al., 2008, Committee on Taxonomy, 2009). Following the probable extinction of the Yangtze River dolphin or baiji (Lipotes vexillifer) in the first decade of the twenty-first century (Turvey et al., 2007), the Yangtze finless porpoise has become the only cetacean species to be found in the Yangtze River. However, progressive declines in the number of porpoises observed during boat surveys (Zhang et al., 1993, Wei et al., 2002, Zhao et al., 2008), drastic loss of suitable habitats resulting in apparent distribution gaps in the formerly contiguous population (Wang et al., 2000, Zhao et al., 2008, Wang, 2009), and low levels of genetic diversity (Yang et al., 2002, Yang et al., 2008b, Xia et al., 2005, Zheng et al., 2005) all suggest that the Yangtze finless porpoise may follow the baiji and also become extinct in the wild in the near future. The apparent decline of the porpoise population in the Yangtze mainstem, from more than 2550 animals in 1991 (Zhang et al., 1993) to fewer than 1225 animals in 2006 (Zhao et al., 2008), raises an important question: how many years do we have left to reverse the decline of this cetacean?
The answer to this question depends on the actual rate of porpoise decline. This decline is being driven by high levels of mortality resulting from anthropogenic impacts such as incidental by-catch in legal and illegal fishing gear, ship collisions, widespread sand dredging, pollution, and water development projects along the middle and lower reaches of the Yangtze River (Wang et al., 2000, Wang et al., 2005, Yang et al., 2002, Yang et al., 2008a, Xia et al., 2005, Zheng et al., 2005, Zhao et al., 2008, Wang, 2009). The Yangtze is known as the “golden channel” of central China, and supports the livelihoods of hundreds of millions of people through agriculture, aquaculture and industrial activities. The status of cetaceans and other megafaunal taxa (e.g. acipenseriform fishes) inhabiting the Yangtze River has deteriorated continuously since the early 1990s, when economic growth began to escalate in China and led to increasing environmental degradation. Understanding the severity of anthropogenic threats to the surviving finless porpoise population in the Yangtze system is a necessary step before appropriate conservation interventions can be developed and implemented. This key question can be informed by understanding the rate of porpoise population decline.
Cetacean population decline rates can be estimated directly by using historical abundance estimates (Stevick et al., 2003, George, 2004, Zerbini et al., 2006, Zhao et al., 2008), or by demographic approaches (Fujiwara and Caswell, 2001, Stolen and Barlow, 2003, Moore and Read, 2008, Currey et al., 2009a, Currey et al., 2009b, Huang et al., 2012b) such as life table analysis (Caswell et al., 1998, Dans et al., 2003, Stolen and Barlow, 2003, Moore and Read, 2008, Huang et al., 2012b). However, direct detection of population trends, either increases or declines, usually requires decades of field investigations, and the target population may have declined substantially before a change in abundance is detectable (Gerrodette, 1987, Taylor and Gerrodette, 1993, Thompson et al., 2000, Taylor et al., 2007, Wilson et al., 2011, Huang et al., 2012a). Conversely, demographic approaches also provide quantitative predictions of future population trends and extinction risk that can inform precautionary management (Harwood, 2000, Lacy, 1993, Thompson et al., 2000, Fujiwara and Caswell, 2001). Comparative study of rate of decline or risk of extinction between different scenarios of anthropogenic impact using a demographic framework (Moore and Read, 2008, Currey et al., 2009b) can also quantify how uncontrolled environment deterioration may impact long-term survival of threatened populations.
In this study, we constructed life tables for the Yangtze finless porpoise before and after 1993 using data from stranded animals. An individual-based Leslie matrix model was used to predict the porpoise population trend and its risk of extinction over the next 100 years. Our research shows how rapidly the current porpoise population is likely to decline to extinction, and highlights the urgency of implementing robust conservation measures.
Section snippets
Specimen collection and age estimation
All age-at-death data (n = 279 animals) used in this analysis came from porpoises that were opportunistically collected along the middle and lower reaches of the Yangtze River from 1978 onwards (Fig. 1). In 66 of these cases (reported in Yang et al., 1998) no body length was recorded, and age was estimated by counting dentinal growth layer groups (GLGs) in tooth sections (Myrck et al., 1983, Hohn et al., 1989, Stolen and Barlow, 2003). In the other 213 cases, no teeth were collected or preserved,
Life table and demography rate estimates
Age-at-death data included 183 animals from the Pre93 population (including data up to and including 1993) and 96 animals from the Post93 population. The ages of all individuals from the Post93 population were estimated from body length using Eq. (1), while age data for the Pre93 population were based on both GLG counts (66 animals) and estimation from body length (117 animals). Life table parameters, including number of animals (nx) and survivorship (lx) for each age class, are listed in Table
Uncertainties of predicted risk
The accurate prediction of rate of decline and future extinction date depends upon the accuracy of demographic parameter values and the number and magnitude of different impacts that are included in a population model. In this analysis, most porpoise individuals (213/279) were aged using length–age relationships (Zhang, 1992) rather than standard GLG counts from teeth sections. However, the comparison of Pre93 lx based on different aging methods does not show any statistical difference.
Acknowledgments
The authors thank our colleagues for their wonderful teamwork. This project was supported by Grants from the National Natural Science Foundation of China (30730018) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KSCX2-EW-Z-4 and KSCX2-YW-Z1023). Our sincere thanks go to three anonymous reviewers for their valuable comments, which substantially improved our manuscript.
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