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Lamna ditropis:   (click for more)

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FAO Names
En - Salmon shark, Fr - Requin-taupe saumon, Sp - Marrajo salmón.
3Alpha Code: LMD     Taxonomic Code: 1060800303
Scientific Name with Original Description
Lamna ditropis  Hubbs and Follett, 1947. Holotype, Museum of Comparative Zoology, Harvard University, MCZ-36471, adult male (partial specimen, size uncertain); type locality, La Jolla, California, 92 to 107m off the La Jolla Beach Club in shallow water. Copeia 1947(3): 194.
Diagnostic Features
fieldmarks: Heavy spindle-shaped body, short conical snout, moderately large blade-like teeth with lateral cusplets, long gill slits, large first dorsal fin with dark free rear tip, minute, pivoting second dorsal and anal fins, strong keels on caudal peduncle, short secondary keels on caudal base, crescentic caudal fin, underside of preoral snout dark, often dusky blotches on ventral surface of body and white patches over pectoral bases.

Snout short and bluntly pointed, with preoral length 4.5 to 7.6% of total length (adults 4.5 to 5.0%), space from eye to first gill slit 1.3 to 1.9 times preorbital length.  First upper lateral teeth with oblique cusps.
  Total vertebral count 170, precaudal vertebral count 103. Cranial rostrum expanded as a huge hypercalcified knob which engulfs most of the rostral cartilages except bases in adults.
  Colour:Dark grey or blackish on dorsolateral surface of body, white below, with white abdominal colour extending anteriorly over pectoral bases as a broad wedge-shaped band; first dorsal fin without a white free rear tip; ventral surface of head dusky and abdomen with dusky blotches in adults but not in young. 
Geographical Distribution

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Coastal and oceanic. North Pacific: Japan (including Sea of Japan), North Korea, South Korea, and the Pacific coast of Russia (including Sea of Okhotsk) to Bering Sea and the eastern Pacific coast of the USAand Canada (Alaska south to British Columbia, Washington, Oregon, and southern California) and probably Mexico (northern Baja California).
Habitat and Biology
Habitat: A common coastal-littoral, offshore and epipelagic shark with a preference for boreal to cool temperate waters, found at depths from the surface to below 152 m. One was photographed at 255 m near the bottom in Monterey Canyon using an underwater camera, while a diver in a submersible saw one at 224 m off Alaska. Salmon sharks are common in continental offshore waters but range inshore to just off beaches; they also are abundant far from land in the North Pacific Ocean basin, along with their pelagic fish prey.

Salmon sharks are common and are often encountered by oceanic and coastal fisheries but are sketchily known biologically. Behaviour and sociobiology are little-known. They occur singly and in schools or feeding aggregations of several individuals and in some areas are seen at or near the surface.Water temperatures where salmon sharks were caught ranged from 2.5° to 24°C.  They are swift-swimming sharks, maintaining a body temperature well above ambient water temperature. Recent studies suggest that salmon sharks may have the highest body temperature of any shark. Body temperature elevations of 8° to 11°C above that of the surrounding water have been reported for smaller specimens, while elevations up to 13.6°C have been recorded in larger ones (Smith and Rhodes 1983; Goldman and Human, in press).Salmon sharks are migratory, with segregation by size and sex, and with larger sharks ranging more northerly than young. In the western North Pacific large sharks migrate from Japanese waters (where they breed) in the wintertime, move north to the Sea of Okhotsk and the western Bering Sea when the water warms, and return to Japan in the autumn or early winter (for a one-way distance of 3 220 km). In the eastern Pacific females apparently migrate south to pup in the spring off Oregon and California, USA, as suggested by commercial fish catch records, washed up (beached) young of the year and anecdotal information. A strong sexual segregation appears to exist across the Pacific Ocean basin, with males dominating the western North Pacific and females dominating the eastern North Pacific (K.J. Goldman and J. A. Musick, pers. comm.).


This shark reproduces by aplacental viviparity, with uterine cannibalism (oophagy); litter size is 2 to 5 young. Length of gestation period might be nine months; length of entire reproductive cycle unknown. Breeding occurs in late summer and into autumn, and females bear young in spring. Breeding and nursery areas may be localized in the offshore western North Pacific between about 156° and 180°E in the open ocean, off the southern Kuril region, and in the Sea of Okhotsk, where young below 60 cm (possibly newborn) occur and juveniles up to 110 to 120 cm long also are found. Age 0 and 1 salmon sharks occur off California, USA, suggesting that a breeding and nursery ground might exist in the eastern North Pacific (K.J. Goldman and J.A. Musick, pers. comm.).  Males may mature at 5 years and about 182 cm TL, and females at 8 to 10 years and about 221 cm TL (Tanaka, 1980). Females in the eastern North Pacific live to at least 20 years of age, males to at least 27 years; preliminary indications are that female salmon sharks mature at an earlier age and are heavier in the eastern North Pacific relative to those living in the western North Pacific (K.J. Goldman and J.A. Musick, pers. comm.).

Salmon sharks are opportunistic feeders and eat a variety of pelagic and demersal bony fishes including Pacific salmon and steelhead trout (Salmonidae), herring and sardines (Clupeidae), pollock, Alaska cod and tomcod (Gadidae), lancetfishes (Alepisauridae), daggerteeth (Anotopteridae), sauries (Scombresocidae), lanternfishes (Myctophidae), pomfrets (Stromateidae), mackerel (Scomber, Scombridae), lumpfishes (Cyclopteridae), sculpins (Cottidae), possibly rockfish (Sebastes, Scorpaenidae), possibly sablefish (Anoplopomatidae), and Atka mackerel (Pleurogrammus, Hexagrammidae). Salmon sharks also feed on spiny dogfish (Squalus acanthias, Squalidae) and several species of pelagic squid, and have been attracted to bycatch offal dumped by shrimp trawlers.

The salmon shark is generally considered to be one of the principal predators of Pacific salmon (Oncorhynchus) apart from humans and is depicted as voraciously feeding on salmon. This is apparently the case around the Aleutians and the Gulf of Alaska, where peaks in abundance in salmon sharks follow maximum catches of salmon and the distribution and migrations of the two appear to be strongly correlated as predator and prey. Salmon sharks caught by Japanese pelagic salmon gill netters in this area have had salmon in their stomachs and little else. However Blagoderov (1994) suggested that this relationship is highly unlikely, and cited major differences in areal distribution between salmon and salmon sharks in the western North Pacific, with most salmon sharks concentrated south of the main migration path of salmon and very few within it. In the western North Pacific these sharks congregate in areas with breeding aggregations of herring and sardines and may be selecting these fishes rather than salmon.
Size
Maximum total length about 305 cm; anecdotal accounts mention sizes of 3.7 to 4.3 m TL but cannot be confirmed, and confusion with the larger white shark is possible and has happened. Size at birth between 40 and 50 cm and 85 cm TL, with the largest foetuses at least 70 cm long and the smallest free-ranging young between 40 and 50 cm. Males maturing at about 182 cm TL (5 years) and females at about 221 cm TL (8 to 10 years); both sexes adult over about 210 to 220 cm TL.
Interest to Fisheries
This species has been fished in the North Pacific in the past by Japanese coastal and oceanic longliners. Salmon sharks are commonly caught by Japanese, United States and Canadian offshore salmon gill netters as bycatch but are generally discarded (except for fins). They are also caught in salmon seines, by salmon trollers towing hooks, and possibly by bottomtrawlers off Alaska; Russian research vessels have regularly caught them in pelagic trawls in the western North Pacific. They are occasionally trammel-netted by halibut fishermen off California and have showed up in numbers as bycatch in gillnets set for swordfish and threshers off California but have usually not been marketed there. Sports anglers in Alaska and Canada catch salmon sharks using rod and reel much like porbeagle anglers in the North Atlantic.


The flesh of the salmon shark is used fresh for human consumption in Japan, where it is processed into various fish products, and to a lesser extent in Alaska and California, United States, where it is seldom marketed and has in the past (California) been occasionally sold as swordfish. Presumably its flesh is less desirable than that of the shortfin mako. Its oil, skin (for leather), and fins (for shark fin soup) are utilized also. The heart of the salmon shark is highly appreciated in a local sashimi dish in the northern fishing port of Kesennuma, where most of the landings of salmon sharks occur in Japan (R. Bonfil, pers. comm.).
Salmon sharks are generally considered a nuisance for the damage they do to salmon nets and other fishing gear. A commercial fishery was initiated off Alaska but this did not succeed. FAO catch statistics for recent salmon shark landings were not available (FAO FishStat Plus database, 2000) but available data (Makihara, 1980) indicates that Japanese fishers landed 100 to 41 000 t during 1952-1978 (with one very high catch year, 41 000 t in 1954, but mostly below 10 000 t and averaging about 6 900 t). Bycatch of salmon sharks in the flying squid and large-mesh driftnet fisheries of the North Pacific in 1990, just before high-seas driftnets were internationally banned was estimated to be about 5 400 t and 71 t respectively.

Recently (1997) there has been numerous strandings of small salmon sharks, ca. 1 m long, off north-central and southern California (R. Lea, pers. comm.), which was of rare occurrence in the 1970s and 1980s. Whether this has to do with human-induced environmental problems such as pollution or unusual water conditions is not known.

Conservation Status : The conservation status of this species is of concern because it is heavily fished as largely discarded but complementary bycatch (with finning) in major pelagic fisheries in the North Pacific. Unlike Lamna nasus, this species has limited fisheries statistics (with no country reporting catch statistics to FAO in 1997) and no regulation of the largely pelagic fishery in international waters, so that trends in abundance are unknown. It also has a negative image as an abundant and low-value pest that avidly eats or damages valuable salmon and wrecks gear, which encourages fishers to kill it and add to mortality from finning and capture trauma. Knowledge of its biology is limited despite its abundance, which invites neglect, but its fecundity is very low and probably cannot sustain current fishing pressure for extended periods.
Local Names
Japan : Nezumizame ,  Mokazame ,  Radukazame ,  Sakezame ,  Japanese mackerel shark .
U.K. : Salmon shark ,  Mackerel shark .
Remarks
The Alaska Board of Fisheries has closed all commercial shark fishing and has heavily regulated the sport fishery in Alaska state waters since 1997 in view of the lack of biological information at a time when there was a small amount of commercial fishing and a large increased interest in sport fishing for salmon sharks (Goldman and Human, in press). The North Pacific Fishery Management Council is currently considering closure of commercial fishing for sharks in Federal waters as no Federal Management plan exists specifically for sharks in the Gulf of Alaska and the Aleutian Islands. Currently, salmon sharks are allowed as bycatch, and are included in the commercial bycatch TAC (Total Allowable Catch) for Alaska Federal waters. Sport fishing regulations in Alaska include EEZ waters and are two sharks per person per year, one in possession at any time (one per day).
Threat to humans: The salmon shark has been regarded as potentially injurious to people because of its large size and relationship to other species that occasionally bite people, but has never been positively identified in shark-bite incidents. There are a few unsubstantiated incidents reported for the species, but possibly by confusion with the white shark. Salmon sharks are reported as occasionally circling, approaching and bumping fishing vessels and sports boats off southeastern Alaska (Paust and Smith, 1985), but their identity needs to be confirmed to eliminate white sharks as being involved in such activities. Divers have seen and photographed schools of adult salmon sharks underwater, with no agonistic overtures on the part of the sharks (R. Lea, pers. comm.). An ecotouristic dive site at Roca Partida, Socorro Islands, Mexico has reported "Lamna nasus ", but it is not known if salmon sharks or other lamnids are being viewed there. The salmon shark is not currently held in captivity in large oceanaria, nor does the writer know of any attempts to keep salmon sharks in the past.
Source of Information
Sharks of the world An annotated and illustrated catalogue of shark species known to date. Volume 2 Bullhead, mackerel and carpet sharks (Heterodontiformes, Lamniformes and Orectolobiformes). Leonard J.V. Compagno 2001.  FAO Species Catalogue for Fishery Purposes. No. 1, Vol. 2. Rome, FAO. 2001. p.269.
Bibliography
Applegate et al., 1989
Bigelow & Schroeder, 1948
Blagoderov, 1994
Bonfil, 1994
Brodeur, 1988
Castro, Woodley & Brudek, 1999
Compagno, 1984
Farquhar, 1963
Goldman & Human, (in press).
H. Mollet, (pers. comm.)
Hanan, Holts & Coan, 1993
Hart, 1973
Hubbs & Follett, 1947
K.G. Goldman & J. A. Musick, (pers. comm.)
Kato, Springer & Wagner, 1967
Larkins, 1964
Lindberg & Legeza, 1959
Makihara, 1980
Miller & Lea, 1972
Nagasawa, 1998
Nakaya, 19711984
Paust, 1987
Paust & Smith, 1986
R. Lea, (pers. comm.)
Roedel & Ripley, 1950
S. Kato, (pers. comm.)
Sano, 1962
Smith & Rhodes, 1983
Strasburg, 1958
T. Neal, (pers. comm.)
Tanaka, 1980
Urquhart, 1981
 
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