Post by Vodmeister on Mar 14, 2014 23:47:43 GMT 5
Pseudorca crassidens
Discovery
False killer whales were first named Phocaena crassidens, a "Thick-toothed Grampus", based on a sub-fossil skeleton discovered in 1843 in the Lincolnshire Fens, near the ancient town of Stamford, England (Owen 1846). At the time it was considered the most complete example of a cetacean skeleton. The common name comes not from any similarity in external apperance to killer whales (Orcinus orca), but from similarities in skull shape and in the number, size and shape of the teeth.
Taxonomy
Order: Cetartiodactyla
Cetacea (unranked)
Odontoceti (unranked)
Family: Delphinidae
Genus: Pseudorca
Natural History
Size, shape and distinctive characteristics
False killer whales are one of the larger Delphinids. They are sexually dimorphic in body length, with adult males typically 75-85 cm longer than adult females. Maximum length documented for an adult male was 5.96 m, and for an adult female was 5.06 m. Maximum length does vary among populations; adult individuals off Japan are about 0.5 m longer than those off South Africa. Depending on the lighting, false killer whales generally appear black or dark grey in color; in good light or underwater a white blaze between the pectoral flippers and a white stripe running down center of the belly and expanding around the genital area is visible. The dorsal fin is falcate with a typically rounded tip. Compared to pygmy killer whales and melon-headed whales the fin is relatively small; compared to pilot whales the fin is much taller in relation to the length of the base of the fin. False killer whales have a rounded head and a gracile appearance. The pectoral flippers have a distinctive S-shape to the leading edge. This species often bowrides and exhibits more aerial activity than similar species.
Geographical distribution
False killer whales are found throughout the tropics in all oceans of the world. They range into the sub-tropics and occasionally into higher latitudes but densities drop dramatically above and below 15°N and 15°S latitude. They are primarily an open ocean species, regularly approaching close to shore mainly around oceanic islands.
maps.iucnredlist.org/map.html?id=18596
Ecology and Behaviour
False killer whales are one of several species known to mass strand, with the largest mass stranding documented of over 800 individuals. They are highly social and typically travel in groups of 10 to 20 individuals, although groups of several hundred individuals have been documented travelling together. Groups are often widely spread, with all individuals in a general area often traveling in the same direction and with sub-groups joining and diverging over time. This pattern probably reflects that false killer whales are cooperative hunters, and work together to capture dangerous and difficult-to-capture prey such as swordfish and large tuna. They frequently share prey, often passing intact prey back and forth among individuals prior to starting to consume it. This behavior may serve some symbolic purpose to build trust among regular hunting partners. Results from satellite tagging of multiple individuals within a group demonstrate individuals within a group may spread as much as 100 kilometers apart before rejoining. Based on association analyses of photo-identified individuals, social bonds can be both strong and enduring (at least 15 years). Around the main Hawaiian Islands individuals show long-term fidelity to the area, although movements of up to almost 500 kilometers have been documented. Little is known about the movements or range of individuals from open-ocean populations. Associations with other species, particularly common bottlenose dolphins and rough-toothed dolphins, are frequently documented. False killer whales are probably occasionally killed by large sharks, and there is one documented attack by killer whales on false killer whales in New Zealand.
Life History
Most of what is known about life history of false killer whales comes from analysis of individuals killed in drive fisheries in Japan or mass strandings in South Africa. In general false killer whales mature slowly, reproduce infrequently, and live a long time. First ovulation for females is between 8.25 and 10.5 years, gestation lasts 15.1-15.7 months, and thus age at first reproduction for females is from 9.5 to 11.75 years. Males are not sexually mature until their late teens. Average inter-birth interval from Japan was reported as 6.9 years. Females go through menopause in their early 40s, yet may live for 20 or more years after. Longevity has been reported as 57.5 years for males and 62.5 years for females based on examination of growth layer groups in teeth. However, aging older odontocetes with this method is problematic and both maximum longevity could be substantially greater for both sexes.
Diet
As a species false killer whales appear to have a diverse diet, including a wide range of squid and fish. Their tendency to share prey, and their habit of feeding on large prey (at least in some areas) has resulted in numerous observations of predation. In Hawaii they feed on large fish, including yellowfin tuna, skipjack tuna, albacore tuna, mahimahi, ono (or wahoo), threadfin jack, and broadbill swordfish, among others. In the eastern tropical Pacific false killer whales they feed on both fish and squid but have been reported feeding on smaller dolphins as they were being released from tuna purse-seine nets. Although one case of predation on a humpback whale calf has been reported in the literature, this has not been well-document
Global Abundance
No estimates of global abundance exist, although there are estimates for several areas within the tropical Pacific. In an area of the eastern tropical Pacific covering a total of 19,148,000 km2, there is an estimate of 39,800 individuals (CV = 0.636), which translates to 0.0021 individuals per km2. However density across this area is not constant; density is relatively high from 5°S to about 15°N, and is an order of magnitude lower north of 15°N. In the U.S. EEZ surrounding the Hawaiian archipelago there are two recognized stocks, the "Hawaii pelagic stock" and the "Hawaii Insular stock". The estimated abundance of the Hawaii pelagic stock is 484 individuals (CV = 0.93), while the abundance of the Hawaii insular stock is estimated at 123 individuals (CV = 0.72). In the US EEZ surrounding Palmyra Atoll the estimated abundance is 1,329 individuals (CV=0.65).
IUCN status
False killer whales as a species were last assessed by the IUCN in 2006. They are globally widespread but naturally uncommon. Global trend data are not available. The primary threats that could cause declines are bycatch in fisheries and competition with fisheries, but insufficent information is available to assess whether such declines have occurred, thus they are listed as Data Deficient.
Conservation Issues
False killer whales face a number of serious conservation threats. As upper-trophic level predators they are naturally rare, and their life history puts them at risk - they are long-lived and slow to reproduce. In the one area where population structure has been well-studied, the central and eastern tropical Pacific, there is evidence of a genetically-isolated population (around the main Hawaiian Islands). If such population isolation is typical for the species there may be multiple small populations of false killer whales world-wide, implying that areas may not be rapidly recolonized if local populations are extirpated. Feeding high on the food web they accumulate high levels of persistent organic pollutants. They are well-known to take fish off lines, and in some areas they may be shot by fishermen as a deterrent or in retaliation. Evidence of a decline in the size of the Hawaii island-associated population over the last 20 years has resulted in a petition to list that population under the U.S. Endangered Species Act. In the Hawaii-based longline fishery false killer whales are the most frequently bycaught cetacean; more false killer whales are bycaught than the next three species of cetaceans combined. In 2010 a Take Reduction Team was formed by the U.S. National Marine Fisheries Service to reduce bycatch in the Hawaii-based longline fishery. At least in some areas their diet overlaps almost entirely with species that are targeted by humans both commercially and recreationally, and large-scale reductions in prey species biomass is likely influencing some populations. Large numbers have been killed off Japan in drive and harpoon fisheries, both for human consumption and as a cull to reduce perceived competition with fisheries. Smaller numbers have been deliberately killed in harpoon or other fisheries in the West Indies, the Indian Ocean, and the western Pacific. Because of their low density, monitoring anthropogenic impacts, and population trends, is difficult.
Robin Baird, Cascadia Research Collective
rwbaird@cascadiaresearch.org
www.marinemammalscience.org/index.php?option=com_content&view=article&id=463&Itemid=299
Discovery
False killer whales were first named Phocaena crassidens, a "Thick-toothed Grampus", based on a sub-fossil skeleton discovered in 1843 in the Lincolnshire Fens, near the ancient town of Stamford, England (Owen 1846). At the time it was considered the most complete example of a cetacean skeleton. The common name comes not from any similarity in external apperance to killer whales (Orcinus orca), but from similarities in skull shape and in the number, size and shape of the teeth.
Taxonomy
Order: Cetartiodactyla
Cetacea (unranked)
Odontoceti (unranked)
Family: Delphinidae
Genus: Pseudorca
Natural History
Size, shape and distinctive characteristics
False killer whales are one of the larger Delphinids. They are sexually dimorphic in body length, with adult males typically 75-85 cm longer than adult females. Maximum length documented for an adult male was 5.96 m, and for an adult female was 5.06 m. Maximum length does vary among populations; adult individuals off Japan are about 0.5 m longer than those off South Africa. Depending on the lighting, false killer whales generally appear black or dark grey in color; in good light or underwater a white blaze between the pectoral flippers and a white stripe running down center of the belly and expanding around the genital area is visible. The dorsal fin is falcate with a typically rounded tip. Compared to pygmy killer whales and melon-headed whales the fin is relatively small; compared to pilot whales the fin is much taller in relation to the length of the base of the fin. False killer whales have a rounded head and a gracile appearance. The pectoral flippers have a distinctive S-shape to the leading edge. This species often bowrides and exhibits more aerial activity than similar species.
Geographical distribution
False killer whales are found throughout the tropics in all oceans of the world. They range into the sub-tropics and occasionally into higher latitudes but densities drop dramatically above and below 15°N and 15°S latitude. They are primarily an open ocean species, regularly approaching close to shore mainly around oceanic islands.
maps.iucnredlist.org/map.html?id=18596
Ecology and Behaviour
False killer whales are one of several species known to mass strand, with the largest mass stranding documented of over 800 individuals. They are highly social and typically travel in groups of 10 to 20 individuals, although groups of several hundred individuals have been documented travelling together. Groups are often widely spread, with all individuals in a general area often traveling in the same direction and with sub-groups joining and diverging over time. This pattern probably reflects that false killer whales are cooperative hunters, and work together to capture dangerous and difficult-to-capture prey such as swordfish and large tuna. They frequently share prey, often passing intact prey back and forth among individuals prior to starting to consume it. This behavior may serve some symbolic purpose to build trust among regular hunting partners. Results from satellite tagging of multiple individuals within a group demonstrate individuals within a group may spread as much as 100 kilometers apart before rejoining. Based on association analyses of photo-identified individuals, social bonds can be both strong and enduring (at least 15 years). Around the main Hawaiian Islands individuals show long-term fidelity to the area, although movements of up to almost 500 kilometers have been documented. Little is known about the movements or range of individuals from open-ocean populations. Associations with other species, particularly common bottlenose dolphins and rough-toothed dolphins, are frequently documented. False killer whales are probably occasionally killed by large sharks, and there is one documented attack by killer whales on false killer whales in New Zealand.
Life History
Most of what is known about life history of false killer whales comes from analysis of individuals killed in drive fisheries in Japan or mass strandings in South Africa. In general false killer whales mature slowly, reproduce infrequently, and live a long time. First ovulation for females is between 8.25 and 10.5 years, gestation lasts 15.1-15.7 months, and thus age at first reproduction for females is from 9.5 to 11.75 years. Males are not sexually mature until their late teens. Average inter-birth interval from Japan was reported as 6.9 years. Females go through menopause in their early 40s, yet may live for 20 or more years after. Longevity has been reported as 57.5 years for males and 62.5 years for females based on examination of growth layer groups in teeth. However, aging older odontocetes with this method is problematic and both maximum longevity could be substantially greater for both sexes.
Diet
As a species false killer whales appear to have a diverse diet, including a wide range of squid and fish. Their tendency to share prey, and their habit of feeding on large prey (at least in some areas) has resulted in numerous observations of predation. In Hawaii they feed on large fish, including yellowfin tuna, skipjack tuna, albacore tuna, mahimahi, ono (or wahoo), threadfin jack, and broadbill swordfish, among others. In the eastern tropical Pacific false killer whales they feed on both fish and squid but have been reported feeding on smaller dolphins as they were being released from tuna purse-seine nets. Although one case of predation on a humpback whale calf has been reported in the literature, this has not been well-document
Global Abundance
No estimates of global abundance exist, although there are estimates for several areas within the tropical Pacific. In an area of the eastern tropical Pacific covering a total of 19,148,000 km2, there is an estimate of 39,800 individuals (CV = 0.636), which translates to 0.0021 individuals per km2. However density across this area is not constant; density is relatively high from 5°S to about 15°N, and is an order of magnitude lower north of 15°N. In the U.S. EEZ surrounding the Hawaiian archipelago there are two recognized stocks, the "Hawaii pelagic stock" and the "Hawaii Insular stock". The estimated abundance of the Hawaii pelagic stock is 484 individuals (CV = 0.93), while the abundance of the Hawaii insular stock is estimated at 123 individuals (CV = 0.72). In the US EEZ surrounding Palmyra Atoll the estimated abundance is 1,329 individuals (CV=0.65).
IUCN status
False killer whales as a species were last assessed by the IUCN in 2006. They are globally widespread but naturally uncommon. Global trend data are not available. The primary threats that could cause declines are bycatch in fisheries and competition with fisheries, but insufficent information is available to assess whether such declines have occurred, thus they are listed as Data Deficient.
Conservation Issues
False killer whales face a number of serious conservation threats. As upper-trophic level predators they are naturally rare, and their life history puts them at risk - they are long-lived and slow to reproduce. In the one area where population structure has been well-studied, the central and eastern tropical Pacific, there is evidence of a genetically-isolated population (around the main Hawaiian Islands). If such population isolation is typical for the species there may be multiple small populations of false killer whales world-wide, implying that areas may not be rapidly recolonized if local populations are extirpated. Feeding high on the food web they accumulate high levels of persistent organic pollutants. They are well-known to take fish off lines, and in some areas they may be shot by fishermen as a deterrent or in retaliation. Evidence of a decline in the size of the Hawaii island-associated population over the last 20 years has resulted in a petition to list that population under the U.S. Endangered Species Act. In the Hawaii-based longline fishery false killer whales are the most frequently bycaught cetacean; more false killer whales are bycaught than the next three species of cetaceans combined. In 2010 a Take Reduction Team was formed by the U.S. National Marine Fisheries Service to reduce bycatch in the Hawaii-based longline fishery. At least in some areas their diet overlaps almost entirely with species that are targeted by humans both commercially and recreationally, and large-scale reductions in prey species biomass is likely influencing some populations. Large numbers have been killed off Japan in drive and harpoon fisheries, both for human consumption and as a cull to reduce perceived competition with fisheries. Smaller numbers have been deliberately killed in harpoon or other fisheries in the West Indies, the Indian Ocean, and the western Pacific. Because of their low density, monitoring anthropogenic impacts, and population trends, is difficult.
Robin Baird, Cascadia Research Collective
rwbaird@cascadiaresearch.org
www.marinemammalscience.org/index.php?option=com_content&view=article&id=463&Itemid=299
