The recognition of two externally distinct morphological forms of Finless Porpoises as separate biological species (Neophocaena phocaenoides and N. asiaeorientalis) was accepted when it was demonstrated that the two forms are reproductively isolated (and likely have been separated since the last glacial maximum) even though they are sympatric in a fairly large area of East Asia (Wang et al. 2008, Jefferson and Wang 2011). Differences in the external morphology of the dorsal aspect of the two species are distinguishable even amongst free-ranging animals (as opposed to animals in-hand for close inspection) (Wang et al. 2010) and intermediate individuals have never been reported even though several hundred to thousands of carcasses have been examined. The two species also clearly differ in craniometry (Amano et al. 1992, Jefferson 2002). Within the Narrow-ridged species, two subspecies are retained: the Yangtze Finless Porpoise, N. a. asiaeorientalis, and the East Asian Finless Porpoise or Sunameri, N. a. sunameri (Gao 1991, Wang 1992a,b; Gao and Zhou 1995). Where Narrow-ridged Finless Porpoises have been studied fairly well (e.g., Yoshida et al. 1995, 2001; Yoshida 2002, Jefferson 2002, Zheng et al. 2005, Yang et al. 2008, Chen et al. 2010, Xu et al. 2010, Li et al. 2011, Ju et al. 2012, L. Li et al. 2013, S. Li et al. 2013, Chen et al. 2014, Jia et al. 2014, Lin et al. 2014), there is evidence to suggest subpopulation structure.

• 2012 – Vulnerable (VU)
  Download assessment
• 2011 – Vulnerable (VU)
  Download assessment

The type specimen was from the Yangtze River, Kiangsu [=Jiangsu] Province, China, ~129 km (80 miles) northwest of Shanghai (Pilleri and Gihr 1972).

In general, this species occurs in a narrow strip of shallow (usually <50 m deep) coastal water around the western Pacific Ocean from the Taiwan Strait to the waters of northern China, Korea and northern Honshu, Japan. (Note that a Narrow-ridged Finless Porpoise was discovered recently in the waters of Okinawa Island, Japan (Yoshida et al. 2010), but it probably represents an extralimital stray rather than an extension of the species’ range.) The distribution also includes a few estuaries and rivers (e.g., the N. a. asiaeorientalis subspecies appears to be found exclusively in the Yangtze River system; Gao 1991, Gao and Zhou 1995). Human coastal development and other activities already may have substantially reduced and fragmented the distribution of Finless Porpoises (Reeves et al. 1997).

• N. a. sunameri: Coastal waters from the Taiwan Strait through the East China Sea north to the Bohai/Yellow Sea in China and the waters of Korea and Japan (Gao 1991, Gao and Zhou 1995). Korean and Japanese populations are geographically separated (Shirakihara et al. 1992, Yoshida et al. 2001, Yoshida 2002).

• N. a. asiaeorientalis: Middle and lower reaches of the Yangtze River (= Chang Jiang), where it ranges (or did until recently) to 1,600 km upstream, i.e., to the gorges above Yichang (200 m above sea level). The range includes Poyang and Dongting lakes and their tributaries, the Gan Jiang and the Xiang Jiang (Gao 1991, Gao and Zhou 1995). The subspecies is thought to be restricted to fresh water.

The distirbution map shows where the species may occur based on oceanography and known habitat preferences. Given the paucity of dedicated cetacean survey effort in much of the species’ suspected range, the distribution map includes not only the known range of the species but also the possible or projected range where oceanic features are consistent with the its known preferred habitat. As such, the species has not been recorded for all the states within the hypothetical range as shown. States for which confirmed records of the species exist are indicated in the occurrence list "Extant." States within the hypothetical range, but for which no confirmed records exist, are indicated as "Presence Uncertain."

Narrow-ridged Finless Porpoises are small and cryptic and therefore difficult to survey. Estimates of abundance have been made for only a few areas (IWC 2006). Estimates from the early 2000s are available for five subpopulations in Japanese waters, distinguished on the basis of skull morphology and mtDNA variability (Yoshida et al. 1995, Yoshida 2002), as follows: 3,807 (coefficient of variation (CV) 16%) in Ariake Sound/Tachibana Bay (Shirakihara and Shirakihara 2002); 289 (CV 19%) in Omura Bay (Shirakihara and Shirakihara 2002); 3,743 (CV 24%) in Ise/Mikawa Bay (Yoshida 2002); 3,387 (CV 33%) in Chiba/Sendai Bay (Amano et al. 2003); and 7,572 (CV 17%) in the Inland Sea (Shirakihara et al. 2007). Most of the estimates were summarized in IWC (2006). For the Inland Sea subpopulation, Kasuya et al. (2002) recorded a decline in sighting rate of 18-100% for 18 tracklines surveyed in 1976–78 and again in 1999–2000. The declines for 11 of the tracklines were statistically significant. Using abundance indices calculated by multiplying the density indices (no. individuals/cruise distance) and area size of each stratum, an overall decline of 69% is inferred over the 22 years between the two sets of surveys (T. Kasuya pers. comm. 2007, data from Kasuya et al. 2002).

Surveys in western Korean waters (Yellow Sea) in the early 2000s estimated 21,532 (CV 39%) Finless Porpoises in offshore waters and 5,464 (CV 20%) in inshore waters (Zhang et al. 2005). The Scientific Committee of the International Whaling Commission (IWC) considered these estimates to be negatively biased (IWC 2006). More recent surveys resulted in density estimates of 0.122 individuals/km² (offshore) and 0.151 ind./km² (inshore), both much lower than the corresponding estimates from surveys in 2004 (offshore, 0.565 ind./km²) and 2005 (inshore, 0.638 ind./km²) (Park et al. 2015). Park et al. (2015) concluded that these results indicated a decline in abundance of Narrow-ridged Finless Porpoises in this region of approximately 70% between 2004/2005 and 2011.

Based on surveys from 1984 to 1991, Zhang et al. (1993) estimated that there were about 2,700 porpoises in the Yangtze River, while Zhou et al. (2000) estimated that only 700 remained in the lower reaches between Nanjing and Hukou between 1989 and 1992. Wang et al. (2000) concluded that Finless Porpoise abundance in the Yangtze River had declined considerably and that there could be fewer than 2,000 animals (although this was not based on a rigorous assessment). A two-vessel survey of the Yangtze mainstem in 2006 yielded an estimate of 1,225 (CV 0.13) which, when combined with earlier estimates for Poyang and Dongting Lakes, was interpreted as implying an overall number of about 1,800 in the Yangtze system (Zhao et al. 2008). In some portions of the river where porpoises had been reported to be present in the past, none were seen during the 2006 survey (Zhao et al. 2012). Previous estimates of the annual rate of decline of Yangtze Finless Porpoises varied from 5 to 7.3% (see Wang 2009 for a review). However, a survey of the mainstem Yangtze in 2012 determined that only approximately 500 porpoises remained (Mei et al. 2014), indicating a roughly 60% decline in the main river in six years. However, Yangtze River cetacean surveys have been plagued by variable methods and coverage, and therefore decline rates must be viewed with some level of caution.

There are no abundance estimates for Finless Porpoises in Chinese marine waters other than Hong Kong (IWC 2006), which in any event refer to for N. phocaenoides and not N. asiaeorientalis.

Narrow-ridged Finless Porpoises are found mainly in coastal waters, including shallow bays, possibly mangrove swamps, estuaries, and some large rivers. However, they can also occur in shallow waters (<200 m deep) quite far from shore (up to 240 km). They appear to have a strong preference for waters with a sandy or soft bottom (Jefferson and Hung 2004).

In Japanese waters, Finless Porpoises prefer shallow depths (<50 m) and close proximity to the shore (<5 km). In the shallow East China Sea, proximity to the shore does not appear to be as important (Amano 2009).

Finless Porpoises, like other phocoenids (Jefferson and Curry 1994), are extremely susceptible to entanglement in gillnets, and large numbers have been, and continue to be, killed in many parts of their range.

In Japan, Narrow-ridged Finless Porpoises become entangled in a variety of types of fishing gear (Amano 2009). Changes in fishing methods and the use of acoustic deterrents may have reduced the incidental catch in some areas such as western Kyushu (Kasuya 1999, Amano et al. 2017), but substantial numbers are still being taken in gillnets and other fishing gear. A total of 114 specimens were collected in Japan during 1985–1992 from western and north-eastern Kyushu including parts of the western Inland Sea (Shirakihara et al. 1993): 84 (73%) of them had been killed incidentally in fisheries, 25 had been found dead on the beach or in the sea, and there was no information on the other five. Fishing gears that killed the 84 porpoises were bottom-set gillnets (58), surface gillnets (17), trap nets (7), trawl nets (1) and drifting (ghost) nets (1). Such fishing gears are common in Japan and probably kill Finless Porpoises off other coasts, although usually such catches go unreported. Shirakihara and Shirakihara (2013) estimated 238-270 Finless Porpoises are bycaught annually in Ariake Sound and Tachibana Bay.

In Korea (information is available only from the Republic of Korea), an official reporting system implemented in 2011 resulted in a count of 2,107 Finless Porpoises bycaught inshore, mainly along the west coast, in 2012 (Kim et al. 2013). Although reporting apparently has improved, there is still assumed to be substantial under-reporting for the following reasons according to Park et al. (2015): “First, each Finless Porpoise caught in a fish­ing net costs approximately $100 (100,000 Wons) [sic]. Therefore, the dead porpoise can be discarded because it has no signifi­cant value. Second, the bycatch of Finless Porpoises is usually reported only in large harbours, with deaths in smaller ports seldom reported (Kim et al., 2013). Therefore, it is likely that the impact of the bycatch of Finless Porpoises on its population is underestimated.”

Yang et al. (1999) reported that Finless Porpoises were the most frequently captured cetaceans in fishing gear along the Chinese coast and estimated that more than 2,000 were taken in 1994, mainly in trawl, gill, and stow nets. In the waters of western Taiwan, including the Matsu and Chinmen Islands (western Taiwan Strait), a considerable number of Finless Porpoises are taken in trammel nets, trawl nets, stow nets, and other gear (J. Y. Wang unpublished data). Given the numbers and types of net fisheries in Chinese coastal waters (e.g., Zhou and Wang 1994, Yang et al. 1999), there are serious concerns about the level of bycatch of both Finless Porpoise species. Illegal electric fishing became widespread in the Yangtze River during the 1990s, and it may kill porpoises outright and certainly contributes to the depletion of their prey (Reeves et al. 2000).

In some parts of their range, there is an aversion to eating Finless Porpoises (Kasuya 1999, J. Y. Wang unpublished data). However, there is a long history of porpoises taken incidentally being sold for human consumption in at least some parts of Japan (e.g., Mizue et al. 1965) and on the Korean peninsula (IWC 2000). According to M. Amano (pers. comm.), consumption of Finless Porpoises no longer occurs in Japan. An analysis of the species composition of odontocete products in Korean markets during 2003–2004 estimated that the true catch of Finless Porpoises in Korean waters during this period was probably about four times the officially reported catch of 142 animals (Baker et al. 2006).

As coastal and riverine animals, Narrow-ridged Finless Porpoises are also affected by habitat loss and degradation, boat traffic, and pollution. The extensive modification of coastlines for shrimp farming and rampant harbour (and other) development throughout East Asia means that there is less habitat for Finless Porpoises (Reeves et al. 2003). Porpoise habitat in the Yangtze River system has been degraded by water development, including the Gezhouba and Three Gorges dams and about 1,300 smaller dams in tributaries (Liu et al. 2000, Smith et al. 2000). Sand mining is intensive in Poyang Lake, until recently a stronghold for Yangtze Finless Porpoises (Zhao et al. 2008). Sand mining is also thought to have played a significant role in the severe decline of Finless Porpoises in the central and eastern Seto Inland Sea, Japan (Shirakihara et al. 2007). Although pathology related to contaminant exposure has not been reported in Narrow-ridged Finless Porpoises, pollution is considered a potentially serious threat (Kasuya 1999, Kannan et al. 1989, Iwata et al. 1994, 1995; Le et al. 1999, Zhao et al. 2008). A mass die-off of at least 249 Narrow-ridged Finless Porpoises along the Saemangeum dike area of the western Korean Peninsula was reported by Park et al. (2012) who speculated that the cause was unusually low temperatures. However, there have been news reports of large (>100) kills of Narrow-ridged Finless Porpoises in local coastal fisheries, so it may be difficult to rule out human activities as a causal factor in such large-scale die-offs.

A survey of levels and drivers of human-caused mortality carried out in 27 fishing settlements along the middle-lower Yangtze channel identified three main categories of observed porpoise deaths: (1) interaction with fishing gear, (2) vessel strikes, and (3) unknown cause (Turvey et al. 2013). A total of 344 dated porpoise mortality events directly observed by informants between 1950 and 2008 were reported, including 75 porpoise deaths from 1989–1998 and 147 from 1999–2008. This apparent increase was said to be driven by the greater number of deaths attributed to vessel strikes (19 to 35) and unknown causes (31 to 94) in the more recent decade. Comparison of the reported mortality with the estimated abundance suggests that the fraction of individuals removed from the declining porpoise population in the Yangtze had quadrupled over the course of two decades. Turvey et al. concluded that entanglement in rolling hook long-lines was probably responsible for the greatest number of porpoise deaths in fishing gear and that vessel strikes and other anthropogenic factors, such as electrofishing, had become the dominant and increasing causes of mortality in the Yangtze.