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 (Jefferson et al. 2002b). Finless porpoises are caught in nets in Iranian, Indian, Pakistani and Malaysian coastal waters, although there are no good estimates of the magnitude of such catches (e.g., see Collins et al. 2005, Jaaman et al. 2009, Braulik et al. 2010). In East Malaysia, finless porpoises are caught as bycatch in ‘fish stakes’ in Sabah (Jaaman et al. 2009) and one individual was found entangled in the line of a crab trap in Sarawak (G. Minton pers. comm. 2011). 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 (where the two species of finless porpoises are sympatric), a considerable number of both species are taken in trammel nets, trawl nets, stow nets, and other gear (Wang unpublished data). In Hong Kong waters, porpoises are caught regularly by trawl nets and gillnets, but there are no estimates of bycatch levels (Parsons and Jefferson 2000, Jefferson et al. 2002b). Given the numbers and types of net fisheries in Chinese coastal waters, the apparently high bycatch of finless porpoises (e.g. Zhou and Wang 1994, Yang et al. 1999) is a serious concern. No large-scale direct hunting of this species is known to occur. In some parts of their range, people apparently are averse to eating finless porpoises (Kasuya 1999, Wang unpublished data) but in other areas they are consumed.
As a coastal species, the Indo-Pacific Finless Porpoise is also affected by habitat loss and degradation, boat traffic, and pollution. The extensive modification of coastlines for shrimp farming, causeways and harbor (and other) development throughout Asia (including the Arabian Gulf) means that there is less habitat for finless porpoises (Reeves et al. 2003, Braulik et al. 2010). Although pathology related to contaminant exposure has not been reported in finless porpoises, pollution is considered a potentially serious threat (Kasuya 1999, Iwata et al. 1994, Parsons and Chan 1998, Minh et al. 1999, Parsons 1999, Jefferson et al. 2002b, Ramu et al. 2005). The number of reported small cetacean strandings in Hong Kong has increased in recent years, partly due to increasing public awareness of local cetaceans, but also possibly due to escalating levels of human disturbance and pollution (T.A. Jefferson pers. comm. 2007). In the Arabian Gulf and Gulf of Oman/Sea of Oman, chemicals from recent wars exacerbate total pollution loads in coastal waters from the usual sources such as industry, agriculture and households (Braulik et al. 2010). Vessel collisions, especially involving high-speed ferries, may be a particular problem for porpoises in Hong Kong (Parsons and Jefferson 2000).
The following summary of examples illustrates the problems faced by this species:
In Chinese waters (excluding the considerable fisheries operating in Taiwanese waters), more than 3.5 million gillnets were estimated to be in use in the early 1990s (Zhou and Wang 1994). In India, the use of gillnets reportedly had increased from about 18,000 in 1950 to more than 216,000 by 1980 and gillnets were also being used extensively in Sri Lanka, Bangladesh and Myanmar (Mohan 1994). Since the time of those reports, gillnetting effort certainly would have increased considerably in these and other regions because there are few (if any) restrictions on this kind of fishing gear anywhere in the range of the species. Porpoise habitat in Myanmar is heavily fished with gillnets and other types of gear capable of taking small cetaceans incidentally (Smith and Than Tun 2008). A drift gillnet fishery for elasmobranchs is of concern in the Bangladesh Sundarbans (Smith et al. 2008). In East Malaysia, Finless Porpoise numbers are thought to have ‘greatly declined’ due to bycatch in fisheries (Jaaman et al. 2009), and commercial fishing (gillnetting, trawling, purse-seining) is intensive off Langkawi, Peninsular Malaysia, where bycatch of Finless Porpoises is known to occur even though it is not monitored regularly (L. Ponnampalam pers. comm. 2011). Surveys in the coastal waters of Vietnam resulted in very few sightings of cetaceans; none of the sightings were of Finless Porpoises yet local whale temples contained numerous cetacean skulls, including many from finless porpoises (Smith et al. 1997). The impacts of war several decades ago, compounded by intense fishing (especially the use of gillnets) in recent years and ongoing, are likely at least partly responsible for the near absence of cetaceans from these waters. In Iran (Arabian Gulf and Gulf of Oman/Sea of Oman), bycatch in fisheries appears to be the greatest threat to marine mammals and takes from apparently small, localized populations of Finless Porpoises may be unsustainable (Braulik et al. 2010). Two major marine mammal die-offs occurred in the Arabian Gulf in the 1980s and 1990s, possibly related to major oil spills (Collins et al. 2005). Even in the 1970s, Finless Porpoise sightings in Pakistani coastal waters around the Indus Delta region were apparently decreasing (Pilleri and Gihr 1972, Pilleri and Pilleri 1979) and by the 1990s the species may have virtually disappeared from this badly degraded area (see Reeves et al. 1997). The Finless Porpoise was one of several cetacean species thought to have disappeared from large parts of their previous ranges in Thailand, largely as a result of coastal habitat degradation and fisheries (IWC 1994).
Direct killing or capture of Finless Porpoises appears to be relatively rare (Reeves et al. 1997) but development of intentional fisheries for marine mammals from incidental captures (and economic benefits from subsequent consumption and marketing) may have increased in parts of Southeast Asia as preferred marine resources have been over-fished (see Perrin 2002).