Vol. 20: 71–97, 2013 ENDANGERED SPECIES RESEARCH Published online March 21 doi: 10.3354/esr00481 Endang Species Res Contribution to the Theme Section ‘Techniques for reducing bycatch of mammals in gillnets’ OPENPEN ACCESSCCESS REVIEW Marine mammal bycatch in gillnet and other entangling net fisheries, 1990 to 2011 Randall R. Reeves1, Kate McClellan2,3,*, Timothy B. Werner2,4 1Okapi W ildlife Associates, 27 Chandler Lane, Hudson, Quebec J0P 1H0, Canada 2Consortium for Wildlife Bycatch Reduction, John H. Prescott Marine Laboratory, New England Aquarium, Central Wharf, Boston, Massachusetts 02118, USA 3Department of Environmental Conservation, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA 4Department of Biology, Boston University, 5 Cummington Mall, Boston, Massachusetts 02115, USA ABSTRACT: Since the 1970s the role of fishery bycatch as a factor reducing, or limiting the recov- ery of, marine mammal populations has been increasingly recognized. The proceedings of a 1990 International Whaling Commission symposium and workshop summarized fishery and bycatch data by region, fishery, and species, and estimated the significance of the ‘impacts’ of bycatch in passive gear on all cetacean species and subspecies or geographically defined populations. A global review of pinniped bycatch in 1991 concluded that incidental mortality in passive gear had contributed to declines of several species and populations. Here we update the information on cetacean gillnet bycatch, assess bycatch data on marine mammals other than cetaceans (i.e. pin- nipeds, sirenians, and 2 otter species), determine where important data gaps exist, and identify species and populations known or likely to be at high risk from bycatch in gillnets. We found that at least 75% of odontocete species, 64% of mysticetes, 66% of pinnipeds, and all sirenians and marine mustelids have been recorded as gillnet bycatch over the past 20-plus years. Cetacean bycatch information in some areas has improved, facilitating our ability to identify species and populations at high risk, although major gaps remain. Understanding of the scale of pinniped and sirenian bycatch has also improved, but this bycatch remains poorly documented, especially at the population level. This study reveals how little is known about marine mammal bycatch in gillnets in much of the world. Even as other significant threats to marine mammals have become better documented and understood, bycatch remains a critical issue demanding urgent attention if there is to be any hope of preventing further losses of marine mammal diversity and abundance, and of protecting, or restoring, ecological health. KEY WORDS: Bycatch · Entanglement · Gillnets · Marine mammals Resale or republication not permitted without written consent of the publisher INTRODUCTION organized and convened by the Scientific Committee of the International Whaling Commission (IWC) (Per- Bycatch (incidental mortality and injury in fishing rin et al. 1994). The published proceedings of that gear; see ‘Materials and methods’ for a more exact event included a global summary of fishery and definition) has been increasingly recognized since bycatch data by region, fishery, and species, as well the 1970s as a factor limiting or reducing marine as an experts’ evaluation of the ‘impacts’ of bycatch mammal populations (Mitchell 1975, Hofman 1995, on many cetacean species and geographically Read 2005, 2008). A benchmark for this recognition defined populations (IWC 1994). The workshop was the 1990 Symposium and Workshop on the Mor- report contained a series of recommendations related tality of Cetaceans in Passive Fishing Nets and Traps to bycatch documentation, mitigation, and monitor- *Corresponding author. Email: [email protected] © Inter-Research 2013 · www.int-res.com 72 Endang Species Res 20: 71–97, 2013 ing. Six species or populations were highlighted as of the potential population-level impacts (Ross et al. urgently needing action to reduce unsustainable 1989, Cockcroft 1990, Cockcroft et al. 1991, 1992). bycatch: the Yangtze River dolphin or baiji Lipotes Finally, sperm whales Physeter macrocephalus in the vexillifer, the Gulf of California porpoise or vaquita Mediterranean Sea are thought to number only in the Phocoena sinus, coastal populations of humpback 100s, and they are still dying in drift nets (largely ille- dolphins Sousa sp. and bottlenose dolphins Tursiops gal since 2002 when the European Union imposed a sp. in KwaZulu-Natal (South Africa), striped dolphins total ban on driftnetting by member states); a major Stenella coeruleoalba in the Mediterranean Sea, and difference now is that the evidence for demographic harbor porpoises Phocoena phocoena in the western isolation of Mediterranean sperm whales is much North Atlantic. Three other populations were ‘of par- stronger than it was in 1990 (Notarbartolo di Sciara et ticular concern’ because of large known bycatch lev- al. 2006, Engelhaupt et al. 2009). els thought to be unsustainable: dusky dolphins At least 2 of the cetacean populations highlighted Lagenorhynchus obscurus in the eastern South in 1990 would probably not be ranked as being of Pacific (specifically Peru), northern right whale dol- such high concern today, at a global scale, as they phins Lissodelphis borealis in the central North were then. The United Nations ban on the use of Pacific, and sperm whales Physeter macrocephalus large-scale, high-seas driftnets, which took effect at in the Mediterranean Sea. the end of 1992 (Northridge & Hofman 1999), greatly In a separate effort, Woodley & Lavigne (1991) re - reduced the driftnet mortality of northern right whale viewed the literature for information on bycatch of dolphins. Although gillnetting of billfish, sharks, pinnipeds and concluded that incidental mortality in squid, and tuna inside the exclusive economic zones passive gear had contributed to declines in popula- (EEZs) of some North Pacific countries probably con- tions of northern fur seals Callorhinus ursinus and tinue to kill 100s of these dolphins each year, the total harbor seals Phoca vitulina in the North Pacific and number of living right whale dolphins remains fairly harp seals Pagophilus groenlandica in the Barents high: there were estimated to be 10 000s to 100 000s Sea. They also believed that mortality in commercial in the central North Pacific in the early 1990s (Buck- trawl fisheries was at least partly responsible for a land et al. 1993) and about 8000 in the United States decline in Steller sea lions Eumetopias jubatus and EEZ in 2005 to 2008 (Carretta et al. 2011). Given the that bycatch had had ‘detrimental impacts’ on New ongoing driftnet ban on the high seas and these rela- Zealand sea lions Phocarctos hookeri, harbor seals tively high estimates of abundance, the need for con- Phoca vitulina off Newfoundland and Alaska, gray servation measures directed at northern right whale seals Halichoerus grypus in the eastern Baltic Sea, dolphins seems less urgent now than it did 2 decades and endangered Mediterranean and Hawaiian monk ago. seals Monachus schauinslandi and M. monachus, Further, harbor porpoises have been found to be respectively. much more abundant in the western North Atlantic Over the 20-plus years since 1990 much has than was assumed in 1990, when the ‘best available changed. One of the cetacean species singled out in estimates’ ranged between 8000 and 15 300 (north- 1990 as being in great peril, the baiji, is now probably eastern USA, Bay of Fundy, and southwestern Nova extinct (Turvey et al. 2007). The vaquita has contin- Scotia region; IWC 1994, p. 31) compared with a 2006 ued to decline as a result of unsustainable bycatch in estimate of 89 054 (coefficient of variation, CV = 0.47) fishing gear (Rojas-Bracho et al. 2006, Jaramillo- (Gulf of Maine/Bay of Fundy stock) (Waring et al. Legorreta et al. 2007); it is now widely regarded as 2011). Annual porpoise bycatch in gillnets and other the world’s most endangered cetacean species. passive gear in this region were estimated at 300 to Dusky dolphins have continued to be killed in Peru, 800 in 1990 (IWC 1994, p. 25) compared with an esti- and the subspecies there (Lagenorhynchus obscurus mate of total annual human-caused mortality in 2004 posidonia) may still be declining as a result, despite a to 2008 of 928+ (CV = 0.16) in all United States and series of legislative measures intended to reduce Canadian fisheries in the Gulf of Maine and Bay of mortality from the deliberate targeting of cetaceans Fundy (Waring et al. 2011). Although this high (Van Waerebeek et al. 2002, Mangel et al. 2010). number of annual porpoise deaths is cause for Similarly, humpback dolphins and bottlenose dol- ongoing concern, the situation appears less grave phins have continued to be subjected to incidental than it did in 1990 in terms of sustainability (Or- mortality in anti-shark nets off KwaZulu-Natal (Ped- phanides & Palka 2013, this Theme Section, Read demors et al. 1997, Peddemors 1998, Best 2007), with 2013, this Theme Section). The situation for striped no clear assessment since the late 1980s/early 1990s dolphins in the Mediterranean Sea is broadly similar Reeves et al.: Marine mammal bycatch 73 to that of harbor porpoises in the western North At- cially in developing countries (Moore et al. 2010); lantic, with the caveat that besides continuing to sus- and (3) what Read (2008) described as a ‘transition tain considerable bycatch, they have been strongly af- from bycatch to market value,’ that is, animals that fected since 1990