Although it is believed that the harbor porpoise population is not seriously in danger at present, incidental catch of the species in gillnets continues to be the primary threat throughout its range (Hammond et al 2008). Off the eastern coast of the US, an estimated 1200 to 2900 harbor porpoises were killed annually in gillnets between 1990 and 1996 (Waring et al 1998). In the North Sea, Danish annual estimates of bycatch ranged from 2867 to 7566 between 1990 and 2002 (Vinther & Larsen 2002). In the Celtic Sea, bycatch estimates continue to be in the hundreds (ICES 2008; Northridge & Kingston 2009). In Newfoundland and Labrador, the annual level of bycatch was estimated at 2228 animals in 2003 (Benjamins et al 2007). Gillnet bycatch of the Endangered Black Sea harbor porpoise likely continues at high levels, as illegal, unreported, and unregulated fishing has increased in this area (Reeves & Notarbartolo Di Sciara 2006), and bycatch of the Critically Endangered Baltic Sea harbor porpoise is also thought to be occurring at unsustainable levels (Berggren 1994; Koschinski 2002).
Kraus et al (1997) tested the effectiveness of acoustic pingers in the Gulf of Maine to warn harbor porpoises of the presence of nets, and found that they significantly reduce bycatch. Pingers, in combination with time-area closures in this region, reduced gillnet bycatch by about half (Waring et al 2011). Pingers have also been shown to reduce harbor porpoise bycatch in the North Sea (Larsen 1999; Larsen et al 2002), the northeast Pacific (Gearin et al 2000), and the Black Sea (Goener & Bilgrin 2009). Harbor porpoises also avoided nets with active pingers in the Baltic Sea (Carlstrom et al 2009). Overall decreases in fishing effort have also contributed to fewer incidental porpoise catches in several other areas, such as Canada's Maritime Provinces (Read et al 2006; Stevenson 2003).
References
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