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Lamna ditropis

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA CHONDRICHTHYES LAMNIFORMES LAMNIDAE

Scientific Name: Lamna ditropis
Species Authority: Hubbs & Follett, 1947
Common Name(s):
English Salmon Shark
French Requin-taupe Saumon
Spanish Marrajo Salmón

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2009
Date Assessed: 2008-12-01
Assessor(s): Goldman, K., Kohin, S., Cailliet, G.M. & Musick, J.A.
Reviewer(s): Cavanagh, R.D., Fowler, S.L. & Valenti, S.V. (Shark Red List Authority)
Justification:
The Salmon Shark occurs in the eastern and western North Pacific and its population appears to be stable and at relatively high levels of abundance. Currently there is no directed fishery in the Northeast Pacific, apart from a small sport fishery for the species in Alaska. Bycatch in the Northeast and Eastern Central Pacific appears to be at low levels and is not increasing at this point-in-time. Additionally, with the current ban on commercial fishing in Alaska state waters and fairly conservative sport fishing limits, it appears that the population is stable. In the Northwest Pacific, a small directed fishery still exists, but typically takes no more than ~5,000 animals per year. Bycatch in the Eastern and Western Central Pacific has been significantly reduced since the elimination of the drift gillnet fishery and the population appears to have rebounded to its former levels. In addition, the most recent demographic analysis supports the contention that salmon shark populations in the Northeast and Northwest Pacific are stable at this time and it is assessed as Least Concern. Nevertheless, there are very little data on catch in other fisheries, discards and potential finning from the major pelagic fisheries in the North Pacific. Bycatch in U.S. State and Federal waters should be documented in order to foster responsible management and catch records should be obtained from the Northwest and Central Pacific.
History:
2000 Data Deficient

Geographic Range [top]

Range Description: The Salmon Shark is a coastal and oceanic inhabitant of the north and central Pacific Ocean, ranging between 35°N-65°N in the Northwest Pacific (greatest densities between 42°N-52°N) and 30°N-65°N in the northeast and eastern central Pacific (appear to be most concentrated between 50°N-60°N) (Neave and Hanavan 1960, Farquhar 1963, Compagno 1984, Blagoderov 1994, Nakano and Nagasawa 1996). They have been caught as far as 67°N and a few specimens have been taken at around 10°N (Nagasawa 1998). In general, the southern boundary of this sharks' range in the western Pacific is the transitional domain separating the subarctic current from the central Pacific current (McKinnell and Waddell 1993, Nakano and Nagasawa 1996, Nagasawa 1998). In the eastern central Pacific, Salmon Sharks occur as far south as 30°N (Croker 1942, Strasburg 1958, Hart 1973) to Baja California, Mexico (Compagno 2001).

Recent electronic tagging studies have demonstrated that the salmon shark is extremely wide ranging throughout the north Pacific with individuals traveling from Prince William Sound, Alaska into waters as far south as Baja California, Mexico and Hawaii (Weng et al. 2003, Weng et al. 2005). Bycatch of Salmon Sharks are reported in the equatorial central Pacific (Nakano and Bayliff 2003), although the authors are convinced that the reports are misidentifications since the shark is otherwise not reported in waters south of 20°N. An improved set of criteria will be used for fishery observers in the Eastern Central Pacific starting in 2004, and should help to resolve the misidentifications.

This species is abundant in water temperatures ranging from 5-18°C, and high catches have been recorded in sea surface temperatures (SST) of 9-16°C (Nakano and Nagasawa 1996).
Countries:
Native:
Canada (British Columbia, Yukon); United States (Alaska, California, Oregon, Washington)
FAO Marine Fishing Areas:
Native:
Pacific – eastern central; Pacific – northeast; Pacific – northwest; Pacific – western central
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: No detailed information exists on salmon shark abundance, and nothing is known about stock structure. No abundance estimates are available for salmon sharks in the Northeast Pacific. Minimum stock size for the Northwest and Western Central Pacific has been estimated via catch data from several Japanese drift/gillnet fisheries to range from 1.66 x 106 to 2.19 x 106 (Shimida and Nakano unpublished data, cited in Nagasawa 1998), however no detailed information was given as to how these estimates were obtained.

While sexual segregation is relatively common in sharks, a remarkable sex ratio difference occurs in salmon sharks across the North Pacific basin. The western side is male dominated and the eastern side is female dominated, with dominance increasing with latitude (Sano 1962, Nagasawa 1998, Goldman 2002, Goldman and Musick in press). Larger sharks range farther north than smaller individuals, and southern catches generally occur in deeper waters (Nagasawa 1998, Goldman and Musick unpublished data).
Population Trend: Stable

Habitat and Ecology [top]

Habitat and Ecology: Salmon Sharks occur in both the nearshore and oceanic environments. Typical vertical distribution is from surface waters to 150 m (Farquhar 1963, McKinnell and Waddell 1993, Nakano and Nagasawa 1996, Robinson and Jamieson 1984), with recent tagging studies showing they penetrate depths >600 m (Weng et al. 2003, Weng et al. 2005). Adult Salmon Sharks typically range in size from 180-210 cm PCL (where TL = 1.1529*PCL + 15.186, from Goldman and Musick 2006) for northeast Pacific - no conversions are given in the literature for Salmon Sharks in the northwest Pacific), and can weigh upwards of 220 kg. Reported lengths of 260 cm PCL (>300 cm TL) and greater with weights exceeding 450 kg are unsubstantiated. Length-at-maturity in the northwest Pacific has been estimated to occur at approximately 140 cm PCL (age five) for males and between 170 and 180 cm PCL (ages 8-10) for females (Tanaka 1980), while length-at-maturity in the northeast Pacific has been estimated to occur between 125 and 145 cm PCL (age 3-5) for males and between 160 and 180 cm PCL (ages 6-9) for females (Goldman 2002, Goldman and Musick 2006).

In addition to length and age-at-maturity, growth rates and weight-at-length of L. ditropis also differ between males and females from northeast Pacific and the northwest Pacific. Tanaka (1980), (also see Nagasawa 1998) states that maximum age from vertebral analysis for northwest Pacific L. ditropis is at least 25 years for males and 17 for females, and that the growth coefficient (k) for males and females are 0.17 and 0.14 respectively. Goldman (2002) and Goldman and Musick (2006) gave maximum ages for Northeast Pacific L. ditropis (also from vertebral analysis) of 17 years for males and 20 years for females, with growth coefficients of 0.23 and 0.17 for males and females, respectively. Longevity estimates are similar (20-30 years) for the northeast and northwest Pacific. Salmon sharks in the northeast and northwest Pacific attain the same maximum length (approximately 215 cm PCL for females and about 190 cm PCL for males). However, past approximately 140 cm PCL for males and 110 cm PCL for females, salmon sharks in the northeast Pacific are of a greater weight-at-length than their same-sex counterparts in the northwest Pacific (Goldman 2002, Goldman and Musick 2006).

The reproductive mode of salmon sharks is aplacental viviparity and includes an oophagous stage (Tanaka 1986, cited in Nagasawa 1998). Litter size in the western Pacific is three to five pups and litters are reportedly male dominated 2.2:1 (Nagasawa 1998). Data from a single pregnant female taken in Alaska waters in December 2006 provide the only insight to litter size and sex ratio of pups in the northeast Pacific. That shark had four pups with a 1:1 sex ratio (Gallucci et al. unpub. data). Gestation times throughout the north Pacific appear to be nine months with mating occurring during the late summer and early fall, and parturition occurring in the spring (Tanaka 1980, Nagasawa 1998, Goldman 2002, Goldman and Human 2005, Goldman and Musick 2006, Goldman and Musick in press, Tribuzio unpub. data). Size at parturition is between 60-65 cm PCL in both the northeast and northwest Pacific (Tanaka 1980, Goldman 2002, Goldman and Musick 2006).

In the northwest Pacific, a salmon shark pupping and nursery ground may exist just north of the transitional domain in oceanic waters. According to Nakano and Nagasawa (1996), larger juveniles than term (70-110 cm PCL) were caught in waters with SSTs of 14-16°C with adults occurring in colder waters further north. Another pupping and nursery area appears to range from southeast Alaska to northern Baja California, Mexico, in the northeast Pacific (Goldman and Musick in press).

Salmon Sharks are opportunistic feeders, sharing the highest trophic level of the food web in subarctic Pacific waters with marine mammals and seabirds (Brodeur 1988, Nagasawa, 1998, Goldman and Human 2005). They feed on a wide variety of prey including salmon (Oncorhynchus), rockfishes (Sebastes), Sablefish (Anoplopoma fimbria), lancetfish (Alepisaurus), daggerteeth (Anotopterus), lumpfishes (Cyclopteridae), sculpins (Cottidae), atka mackerel (Pleurogrammus), mackerel (Scomber), pollock and tomcod (Gadidae), herring (Clupeidae), Spiny Dogfish (Squalus acanthias), tanner crab (Chionocetes), squid and shrimp (Sano 1960, 1962; Farquhar 1963; Hart 1973; Urquhart 1981; Compagno 1984, 2001, Nagasawa 1998).

As with all members of the family Lamnidae, this species is endothermic, retaining heat created by its own oxidative metabolism (Carey et al. 1985, Goldman 1997). Body temperatures from moribund or recently dead specimens have shown elevations (over water temperature) of 8-11°C in smaller specimens and up to 13.6°C in larger specimens (Smith and Rhodes 1983, Anderson and Goldman 2001). Body temperature can exceed ambient water temperature in free-swimming salmon sharks by as much as 21.2°C (Goldman 2002, Goldman et al. 2004).
Systems: Marine

Threats [top]

Major Threat(s): Historical records of commercial catch and bycatch of salmon sharks are sparse. Japanese commercial catch was reported to the FAO between 1952 and 1965 totaling 110.4 metric tons (mt), with 40.1 mt being the highest amount taken in any single year (Compagno 1990). The current fishing mortality on salmon sharks comes mainly from their being taken as bycatch in purse seine fisheries (e.g. for salmon), but they are also taken in longline fisheries for halibut and sablefish. There is also a small amount of directed commercial and sport fishing taking place off Japan and in Alaska, respectively.

Historically, salmon sharks were commonly caught in gillnets set for salmon (Oncorhynchus species) and flying squid (Ommastrephes) primarily by Canadian, Japanese and Russian fisheries, with smaller numbers taken by Taiwanese and North Korean fisheries (Blagoderov 1994, McKinnell and Waddell 1993, Nakano and Nagasawa 1996, Robinson and Jamieson 1984). Based on the number of boats, average number of sets and the average CPUE reported in Robinson and Jamieson (1984), flying squid fisheries may have taken between 105,000 and 155,000 salmon sharks as bycatch over a four-month period each year (Goldman and Human 2005).

With the elimination of open ocean driftnet fishing and the cessation of the Japanese open ocean salmon fishery, it is likely that there is considerably less salmon shark bycatch in the open north Pacific and the population appears to have rebounded to its former levels (Nagasawa et al. 2002, H. Nakano pers. comm.). However, Salmon Sharks are still taken in U.S. waters (particularly the Gulf of Alaska and Prince William Sound, PWS) as bycatch in trawl, gillnet, and seine fisheries, but this bycatch has been poorly documented (Camhi 1999), and salmon shark bycatch from other countries fisheries is not reported. However, according to the Alaska Department of Fish and Game (ADF&G), trawl, gillnet, and (particularly) seine fisheries are probably responsible for a large number of shark interactions. The state extended the sport fishing regulations to include the exclusive economic zone (EEZ) (to 200 miles). Anecdotal reports from fishermen are that there are less salmon sharks in PWS in recent years, and that there is more time between hook-ups. However, there are no abundance estimates to support a decrease in the number of salmon sharks at this time.

The most recent demographic analysis support the contention that salmon shark populations in the eastern and western north Pacific are stable at this time (Goldman 2002).

The State of Alaska and Alaska Department of Environmental Conservation has also noted high levels of methyl mercury content in Salmon Shark muscle tissue samples provided by the ADF&G, suggesting that pollution is having an affect on this species.

The conservation status of this species is still of some concern because there is so little data on catch in other fisheries, discards and potential finning from the major pelagic fisheries in the north Pacific. There is a great need to document the bycatch in U.S. State and Federal waters in order to foster responsible management of this species, as well as to obtain catch records from the northwest and central Pacific.

Conservation Actions [top]

Conservation Actions: Sharks are currently listed in the Federal Groundfish Management Plans for the Gulf of Alaska, Bering Sea and Aleutian Islands as "other species" and are allowed as bycatch. They are included in the commercial bycatch TAC (Total Allowable Catch) for Alaska Federal waters. How much of this bycatch is salmon shark (vs. Spiny Dogfish (Squalus acanthias) and Sleeper Shark (Somniosus pacificus)) is unknown. The North Pacific Fishery Management Council (NPFMC) is considering closure of directed commercial fishing for sharks in Federal waters as no Federal Management plan exists specifically for sharks in the Gulf of Alaska, Aleutians and the Bering Sea (J. DiCosimo, personal communication 2007).

Commercial fishing for all shark species in Alaska State waters has been illegal since 1997. State of Alaska regulations prohibit directed commercial fishing of sharks statewide except for a spiny dogfish permit fishery adopted by the Alaska Board of Fisheries for the Cook Inlet area in 2005. Sharks taken incidentally to commercial groundfish and salmon fisheries may be retained and sold provided that the fish are fully utilized. The state limits the amount of incidentally taken sharks that may be retained to 20% of the round weight of the directed species on board a vessel except in the Southeast District where a longline vessel or a troller may retain up to 35% round weight of sharks to round weight of the target species on board. All sharks landed must be recorded on an Alaska Department of Fish and Game (ADF&G) fish ticket.

The ADF&G manages the recreational shark fishery in state and federal waters with a daily bag limit of one shark of any species per person per day, and an annual limit of two sharks of any species per person. There is a directed fishery for salmon sharks in Prince William Sound involving a small number of charter boats. There have been no reported incidents of sport-caught sharks being finned and discarded, and state regulations prohibit the intentional waste or destruction of any sport-caught species. Recreational harvest of all shark species combined is estimated through a mail survey of sport fishing license holders. In addition, harvest of salmon sharks by guided anglers is required to be reported in charter logbooks. About 1,600 sharks of all species were harvested by the sport fishery in state and federal waters of Southeast and Southcentral Alaska in 2005 (most recent estimate). No sport harvest of sharks was reported in western Alaska. Charter boats reported a salmon shark harvest of about 240 fish statewide in 2005. Although estimates of salmon shark harvest are not available for unguided anglers, the charter fleet is believed to account for the majority of salmon shark harvest. In addition to the mail survey and logbook, shark fisheries are monitored in Southcentral Alaska through biological sampling for species, size, age, and sex composition, as well as spatial distribution of the harvest.

Family Lamnidae is listed on Annex I, Highly Migratory Species, of the UN Convention on the Law of the Sea, which urges States to cooperate over the management of these species. No such management yet exists. Precautionary adaptive collaborative management of target and bycatch fisheries is needed for this biologically vulnerable shark. It is also essential to improve data collection and develop stock assessments for this species.

The adoption of shark finning bans by fishing states (e.g. USA), regional entities (EU) and regional fisheries organisations (IATTC) is accelerating and should increasingly prevent the harvesting of salmon shark bycatch on the high seas for their fins alone.

Citation: Goldman, K., Kohin, S., Cailliet, G.M. & Musick, J.A. 2009. Lamna ditropis. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 25 October 2014.
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