False Killer Whales are long-lived, slow to reproduce, upper-trophic level predators, and as such are subject to effects of a number of different anthropogenic stressors. The overlap of their diet with species targeted by fisheries, in particular high-value species such as tunas and billfishes, results in a number of types of interactions that can reduce local populations, although the sparseness of observer coverage in most hook-and-line and other fisheries within the range of False Killer Whales limits the information available. In addition, the False Killer Whale is one of several species known to mass strand (Caldwell et al. 1970, Alonso et al. 1999, Liebig et al. 2007), with the largest known mass stranding of 835 individuals in Argentina in 1946 (Caillet-Bois 1948). Such mass strandings have the potential to affect local populations.
False Killer Whales are hooked in longline fisheries as they engage in depredation on both the catch and bait (Hernandez-Milian et al. 2008, Thode et al. 2016). The U.S. Hawaii-based deep-set longline fishery has had approximately 20% observer coverage since 2001, and has provided detailed information on bycatch rates. In this fishery, False Killer Whales are the most frequently recorded species of cetacean hooked as bycatch (Forney and Kobayashi 2005, Bradford and Forney 2014). The first abundance estimates became available for the Hawaii pelagic stock in 2003 (Barlow 2006, Bradford et al. 2014b), and documented mortality and serious injury inside the U.S. EEZ surrounding the Hawaiian Archipelago exceeded the potential biological removal (PBR) level for 10 of the 12 years from 2003-2015 (Carretta et al. 2013, 2017). Rates of hooking or entanglement are approximately 1.5 individuals for every 1 million hooks set. This is a transboundary population and non-U.S. fisheries outside the U.S. EEZ surrounding the Hawaiian Archipelago represent the majority of longline effort, thus bycatch rates for the entire Hawaii pelagic stock are likely much higher than estimated. Non-lethal but potentially significant injuries (such as those that compromise health and ability to reproduce) may also occur if animals are hooked (Baird and Gorgone 2005, Baird et al. 2014, Alves et al. 2018). The rate of non-lethal line-related injuries is significantly higher in the main Hawaiian Islands insular stock than in the Hawaii pelagic stock (Baird et al. 2014). The main Hawaiian Islands insular stock has only limited overlap with the longline fishery, but there are no observers in the nearshore fisheries, and thus no bycatch estimates are available. Longline fisheries occur throughout the central and western tropical Pacific, and similar interactions with False Killer Whales occur in other regions (e.g., Indian Ocean, Kiszka et al. 2010, Anderson 2014; American Samoa, Bradford and Forney 2014; Mediterranean, Bearzi 2002; western North Atlantic, Waring et al. 2015). Observers on Spanish longline vessels documented two bycaught False Killer Whales off the Azores in 2006 in approximately 810,000 observed hooks (Hernandez-Milian et al. 2008), a rate similar to the Hawaii-based longline fishery. The incidental catch rate (including animals hooked but released alive) was estimated from 1992-2005 from observers in the Spanish longline fleet at 1.464, 1.685, and 0.797 individuals per million hooks for the Atlantic, Indian, and Pacific Oceans, respectively (Ramos-Cartelle and Mejuto 2008.) Observer programs to monitor bycatch in fisheries in most parts of the world are limited (if they exist at all), but given what is known regarding both the abundance of False Killer Whales and their predilection to take fish off lines, there is certainly potential for bycatch to result in population declines.
Based on observer reports, False Killer Whales were the most frequently reported species of cetacean killed in the tropical western and central Pacific purse seine fishery in 2009, with an estimated mortality of 281 (SPC-OFP 2010). Interactions with this fishery were observed within the EEZs of Papua New Guinea, Kiribati, the Federated States of Micronesia, and Nauru, and in international waters. Mortality of False Killer Whales has been documented in Chinese purse seine fisheries in the western Pacific (Dai et al. 2017). By contrast, the only mortality documented in the eastern tropical Pacific purse seine fishery since 2001 was in 2010 (IATTC 2011).
Incidental takes of small numbers of False Killer Whales in gill nets have been documented off northern Australia, Sri Lanka, India, Ghana, Brazil, Venezuela, Peru, Japan, Republic of Korea, and China (Perrin et al. 2005, Reeves et al. 2014, Song 2018). Wang et al. (2013) noted that entanglement in large-mesh drift gillnets is likely responsible for most False Killer Whale fishery-related mortality off eastern Taiwan. Yang et al. (1999) reported on cetacean bycatch rates in Chinese coastal fisheries (trawl, gill, and stow nets), which may number in the hundreds per year for False Killer Whales alone.
False Killer Whales have been taken in drive and harpoon fisheries in a number of areas, either for meat or oil, or to reduce populations due to real or perceived fishery conflicts. At least in some areas, the numbers taken have likely reduced local populations. False Killer Whales are occasionally taken at Saint Vincent in the Caribbean for meat and cooking oil (Caldwell and Caldwell 1975). Considerable numbers of False Killer Whales have also been killed in a past drive fishery in the Penghu Islands of Taiwan (Zhou et al. 1995). Official catch records were not recorded until 1972, but from 1958 to 1960, 900 individuals were killed in drive fisheries off northern Kyushu (Kasuya 2017). From 1972 to 2008, 2,643 were reported killed in all fishery types (including bycatch) in Japan, with the highest number in one year of 637 in 1978 (Kasuya 2017). Abundance for coastal Japan was estimated at 2,029 (Kasuya 2017), so this one-year take would have represented approximately 31% of the coastal population as estimated from 1983 to 1991. Over 1,500 were killed in drive fisheries off northern Kyushu between 1972 and 1993, with peak mortality occurring from 1978 to 1983 when 1,358 were reportedly taken (Kasuya 2017). Smaller numbers (171) were killed in a drive fishery at Izu on the east coast of Honshu from 1978 to 1996 with 123 taken in 1978 (Kasuya 2017), and 79 were killed in a crossbow fishery in Okinawa between 1993 and 2011 (Kasuya 2017). False Killer Whales have also been killed in the drive fishery at Taiji on the west coast of Honshu, with 586 killed between 1982 and 2011 and up to 91 killed in a single year (Kasuya 2017). Quotas are in place in Japan for drive and crossbow fisheries although those quotas have only rarely been reached in recent years (Kasuya 2017). Some of the animals caught in the Japanese and Taiwanese drive fisheries have been kept alive and sold to oceanaria (Reeves et al. 2003). Deliberate shooting in response to depredation may also occur in a number of areas. Oleson et al. (2010) noted, for example, that in Hawaii fishermen have reported shooting at False Killer Whales or other dolphins in response to depredation. Anderson (2014) noted that False Killer Whales are likely being shot by tuna longline fishermen in the Indian Ocean.
Although there is considerable controversy regarding the absolute extent of declines, there is good evidence of large-scale reductions in many predatory fish populations (e.g., Baum et al. 2003, 2005, Sibert et al. 2006, Polacheck 2006), including Yellowfin and Bigeye Tuna in the western and central Pacific (Harley et al. 2009; Langley et al. 2009). In addition to overfishing, expansion in the extent of unproductive areas and other ecosystem changes (e.g., Polovina et al. 2008, Coll et al. 2008, Koslow et al. 2015) have the potential to influence fish populations at multiple trophic levels. The effects of such reductions of fish populations and subsequent ecosystem changes on populations of False Killer Whales worldwide are unknown but could have resulted in population declines.
Studies of persistent organic pollutants in the main Hawaiian Islands insular population have revealed that all adult males and a number of adult females sampled from the population have levels of PCBs that exceed thresholds thought to cause immunosuppression (Ylitalo et al. 2009, Foltz et al. 2014). Evidence from stranded individuals of several similar species indicates that they have swallowed discarded plastic items, which may eventually lead to death (e.g. Scott et al. 2001); False Killer Whales may also be at risk from this threat.
