Harbor porpoises, which inhabit nearshore waters of the temperate to sub-Arctic Northern Hemisphere, are among the world’s smallest odontocetes. Unlike many cetaceans, they are short-lived, early to mature, and capable of reproducing annually. (1) Although harbor porpoises are abundant as a species, stocks or subspecies in some regions are considered threatened, particularly in the Black and Baltic seas. (2,3,4)
Entanglement in bottom-set or “sink” gillnets targeting groundfish is the leading cause of anthropogenic mortality of harbor porpoises in the North Atlantic (2,5) High rates of bycatch have occurred in the Bay of Fundy, Gulf of Maine, Gulf of St. Lawrence, and the waters off Newfoundland and Labrador; as well as in the North, Baltic, and Celtic Seas (5,6,7). In the eastern North Pacific, harbor porpoise bycatch has been documented in halibut and salmon set nets and in Alaskan gillnet and driftnet fisheries (5). Incidental capture in Japanese set net fisheries has been observed as well (8).
Some bycatch mitigation strategies have attempted to make use of harbor porpoises’ echolocation abilities, though it is uncertain whether harbor porpoises suffer entanglement because they fail to detect fishing gear, or if entanglement occurs when they are not echolocating (9,10). Researchers have attempted to increase the acoustic reflectivity or “target strength” of gillnets through chemical enhancement or physical modifications, but passive acoustic strategies have proven to be an effective bycatch mitigation measure. (9,10,11). High frequency acoustic alarms or “pingers” have been shown to reduce rates of entanglement, though potential concerns include habitat exclusion or long-term habituation (7,11,12). Additional bycatch mitigation strategies include seasonal and spatial closures, gear modifications, and the replacement of high-risk gillnets with traps, pots and longlines (13,14)
At a global level, the International Whaling Commission’s (IWC) maintains a scientific sub-committee on small cetaceans that conducts reviews and offers scientific advice. (15) Other major conservation initiatives operate at the regional or national level. Harbor porpoises are protected in the United States under the Marine Mammal Protection Act of 1972 (MMPA), which requires NMFS to manage interactions between marine mammals and commercial fishing gear. In the North and Baltic Seas, harbor porpoises are managed under the Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS), developed under the UNEP Convention on Migratory Species. The ASCOBANS recovery plan for Baltic Sea harbor porpoises, known as the Jastarnia Plan, recommends drastic measures to prevent bycatch interactions, including effort reductions in the driftnet and bottom-set gillnet fisheries. (14)
References and additional resources:
1. Read, A.J. 1999. Harbour porpoise - Phocoena phocoena (Linnaeus,
> 1758). Pp. 323-355 in: S.H. Ridgway and R.J. Harrison (editors), Handbook of Marine Mammals, Volume 6. Academic Press, New York.
2. Berggren, P., P.R. Wade, J. Carlström, and A.J. Read. 2002. Potential limits to anthropogenic mortality for harbour porpoises in the Baltic region. Biological Conservation 103: 313-322
3. Reeves, R.R., Smith, B.D., Crespo, E.A. and di Sciara, G.N. (compilers) 2003. Dolphins, Whales and Porpoises: 2002-2010 Conservation Action Plan for the World's Cetaceans. IUCN/SSC Cetacean Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.
4. Cetacean Specialist Group 1996. Phocoena phocoena. In: IUCN 2007. 2007 IUCN Red List of Threatened Species. < >. Downloaded on 17 October 2007.
5. Jefferson, T.A., and B.E. Curry. 1994. A global review of porpoise (Cetacea: Phocoenidae) mortality in gillnets. Biological Conservation 67: 167-183
6. Tregenza, N.J.C., S.D. Berrow, P.S. Hammond, and R. Leaper. 1997. Harbour porpoise (Phocoena phocoena) by-catch in set gillnets in the Celtic Sea. ICES Journal of Marine Science 54: 896-904
7. Culik, B.M., S. Koschinski, N. Tregenza, and G.M. Ellis. 2001. Reactions of harbor porpoises Phocoena phocoena and herring Clupea harengus to acoustic alarms. Marine Ecology Progress Series 211: 255-260
8. Iwata, T., S. Shimizu, Y. Fujimori and T. Miura. 2003. Incidental catch of harbor porpoises in set nets in the coastal waters of southern Hokkaido, Japan. Fisheries Science 69: 657-659
9. Kastelein, R.A., W.W.L. Au, and D. de Haan. 2000. Detection distances of bottom-set gillnets by harbour porpoises (Phocoena phocoena) and bottlenose dolphins (Tursiops truncatus). Marine Environmental Research 49: 359-375
10. Cox, T.M. and A.J. Read. 2004. Echolocation behavior of harbor porpoises Phoecoena phocoena around chemically enhanced gill nets. Marine Ecology Progress Series 279: 275-282
11. Koschinski, S., B.M. Culik, E.A. Trippel and L. Ginzkey. 2006. Behavioral reactions of free-ranging harbor porpoises Phocoena phocoena encountering standard nylon and BaSO4 mesh gillnets and warning sound. Marine Ecology Progress Series 313: 285-294
12. Cox, T.M., A.J. Read, A. Solow and N. Tregenza. 2001. Will harbor porpoises (Phocoena phocoena) habituate to pingers? Journal of Cetacean Research and Management 3(1): 81-86
13. Bisack, K.D. and J.G. Sutinen. 2006. Harbor porpoise bycatch: ITQs or time/area closures in the New England gillnet fishery. Land Economics 82 (1): 85-102
14. Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS). 2003. ASCOBANS Fact Sheet #1: What is the ASCOBANS Recovery Plan for Baltic Harbour Porpoises (Jastarnia Plan)? [http://www.service-board.de/ascobans_neu/files/factsheet1.pdf]
15. International Whaling Commission. “Small Cetaceans.” [http://www.iwcoffice.org/conservation/smallcetacean.htm]