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. Changes in fishing methods may have reduced the incidental catch in some areas such as western Kyushu (Kasuya 1999), but substantial numbers are still being taken in gill nets 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 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 gill nets (58), surface gill nets (17), trap nets (7), trawl nets (1) and drifting (ghost) nets (1). Such fishing gears aree common in Japan and probably kill finless porpoises off other coasts, although usually such catches go unreported. 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a considerable number of finless porpoises are taken in trammel nets, trawl nets, stow nets, and other gear (J.Y. Wang unpublished). 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. Although illegal, electric fishing became widespread in the Yangtze River during the 1990s, and it probably kills porpoises outright and 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In some parts of their range, there is an aversion to eating finless porpoises (Kasuya 1999, J.Y. Wang unpublished). However, there is a long history of porpoises taken incidentally being sold for human consumption in at least parts of Japan (e.g., Mizue et al. 1965) and on the Korean peninsula (IWC 2000). From an analysis of the species composition of odontocete products in Korean markets during 2003–2004, it was 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 harbor (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 1300 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). Although pathology related to contaminant exposure has not been reported in Narrow-ridged Finless Porpoises, pollution is considered a potentially serious threat (Kasuya 1999; see Kannan et al. 1989; Iwata et al. 1994, 1995; Le et al. 1999; Zhao et al. 2008).
A recent 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 (S.T. Turvey pers. comm. 2011). 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 change was 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 has quadrupled over the course of two decades. Turvey concluded that entanglement in rolling hook long-lines is probably responsible for the greatest number of porpoise deaths in fishing gear and that vessel strikes and other anthropogenic factors, such as electrofishing, are likely now the dominant and increasing causes of mortality in the Yangtze.
The causes of an apparent decline in porpoise numbers in the Inland Sea of Japan are not fully understood but include incidental mortality in fisheries as well as various forms of habitat degradation (IWC 2000, 2006). Parts of the coastal regions of the Inland Sea are highly developed and industrialized so the associated threats of habitat loss and pollution are suspected to have had some (undetermined) level of impact
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