Mottled skate

Mottled skate
Conservation status
Vulnerable (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Rajiformes
Family:	Rajidae
Genus:	Raja
Species:	R. pulchra
Binomial name
Raja pulchra F. H. Liu, 1932

The mottled skate (Raja pulchra) is a species of skate, family Rajidae. An inhabitant of shallow coastal waters, it is found in the northwestern Pacific Ocean off Korea, Japan, and China. This species grows to 1.12 m (3.7 ft) long and has a diamond-shaped pectoral fin disc with a long snout. It is characterized by a covering of prickles above and below its snout but not elsewhere on its body, and a dark ring in the middle of each "wing" (though this may be indistinct in adults).

The diet of the mottled skate consists of shrimps, cephalopods, bony fishes, and crabs. It is oviparous, with females producing egg capsules almost year-round. With the similar big skate (R. binoculata), it is one of only two skate species that regularly places multiple embryos (up to five) into a single egg case. The mottled skate is valued for food in South Korea and Japan. It is subject to heavy fishing, both intentional and otherwise, throughout its range, and its population has declined substantially since the 1980s. As a result, the International Union for Conservation of Nature (IUCN) has assessed it as Vulnerable.

Taxonomy

The mottled skate was scientifically described by ichthyologist Liu Fah-Hsuen in a 1932 issue of The Science Reports of National Tsing Hua University.^[2] The type specimen has since been lost.^[3] Systematic studies have shown that this and five other species of Raja in the northern Pacific belong to a separate genus, which has yet to be named.^[4]

Distribution and habitat

The mottled skate is found in the temperate waters of the northwestern Pacific Ocean, in the Sea of Okhotsk, the Sea of Japan, the Yellow and Bohai Seas, and the East China Sea, to as far south as Taiwan. Historical records from the 1980s indicate that it was highly abundant around the Korean islands of Daecheong and Heuksando, and the Japanese main island of Hokkaidō. This bottom-dwelling species generally inhabits shallow coastal waters: it is found at depths of 5–30 m (16–98 ft) in the Sea of Okhotsk, and at 5–15 m (16–49 ft) in the Yellow Sea.^[1] However, it has been recorded from as far down as 120 m (390 ft).^[5]

Description

Reaching a maximum known length of 1.12 m (3.7 ft),^[1] the mottled skate has a diamond-shaped pectoral fin disc with a broad, elongated snout that tapers to a point. The length of the head from above is less than four times the distance between the eyes. There is a notch in the posterior margin of each pelvic fin. The tail bears a fold running along each side and two small dorsal fins positioned near the tip; the caudal fin is reduced to a low fold no deeper than the lateral tail folds. There is usually only a single thorn on the "nape". Prickles cover the dorsal and ventral sides of the snout, but they do not extend onto the back or belly as in the similar big skate (R. binoculata). This species is brownish above and lighter below. Young rays have a pair of large, dark rings on the disc, which fade or change into light blotches with age; adults also gain darker reticulations over upper surface of the disc.^[3]

Biology and ecology

The mottled skate feeds primarily on shrimps, as well as cephalopods, bony fishes, and crabs. In the Yellow Sea, by far the most important prey species is the shrimp Crangon affinis, while the shrimp Trachypenaeus curvirostris and the sandlance Ammodytes personatus also contribute substantially to its diet.^[1] The tail contains a pair of lateral electric organs, each consisting of a column of disc-shaped cells. These organs produce a weak electric field that may function in communication.^[6]

Like other members of its family, the mottled skate is oviparous. Breeding occurring almost year-round, peaking from April to June and from November to December, and pausing only in mid-summer. Females produce 98 to 556 eggs per year (average 240). The eggs are generally deposited on sandy or muddy flats; off Hokkaidō, they are commonly laid inside culturing cages used by scallop farms.^[1][5] The egg capsule is rectangular, measuring 14–18.8 cm (5.5–7.4 in) long and 7–9.4 cm (2.8–3.7 in) across; there is a deep notch on each side and short, flattened horns at the four corners. The capsule usually contains multiple embryos, up to five; the big skate is the only other skate species known to regularly place more than one embryo into a capsule.^[1][7] Newly emerged skates measure 9.5 cm (3.7 in). Males reach sexual maturity at 47.3 cm (18.6 in) across, and females at 68.5 cm (27.0 in) across.^[1]

Human interactions

The mottled skate is a commercially significant food fish in South Korea and Japan. In South Korea, it is the predominant species of skate (known as hong-eo) consumed and one of the most expensive fishes on the market at $10–30/kg; it is popularly served at wedding feasts.^[1] This species is caught intentionally by targeted gillnet fisheries, and incidentally by flounder gillnet fisheries. In Japan, this species is less valued at around $5/kg, and is targeted by a Hokkaidō fishery. It is not sought after by Chinese fisheries, but is taken as bycatch.^[1]

The International Union for Conservation of Nature (IUCN) has assessed the mottled skate as Vulnerable. Annual catches by South Korea averaged 2,700 tons in 1991–1993 but only 220 tons in 2001–2003, indicating a 90% population decline over a ten-year period. Japanese fishermen have also reported declines in recent years, while off China the numbers of all fish species are diminishing due to over-exploitation and habitat degradation. The overall decline in the mottled skate population has been estimated at over 30% since the 1980s, and may in fact be severe enough to warrant a status of Endangered. This species is not currently the subject of any conservation or management schemes.^[1]

References

1. ^ ^{a b c d e f g h i j} Ishihara, H., Y. Wang, S. Tanaka, K. Nakaya and C.H. Jeong (2004). Raja pulchra. In: IUCN 2008. IUCN Red List of Threatened Species. Downloaded on April 3, 2010.
2. ^ Eschmeyer, W.N. (ed.) pulchra, Raja. Catalog of Fishes electronic version (February 19, 2010). Retrieved on April 3, 2010.
3. ^ ^{a b} Ishihara, H. (1987). "Revision of the Western North Pacific species of the genus Raja". Japanese Journal of Ichthyology 34 (3): 241–285. 
4. ^ Ebert, D.A. and L.J.V. Compangno (October, 2007). "Biodiversity and systematics of skates (Chondrichthyes: Rajiformes: Rajoidei)". Environmental Biology of Fishes 80 (2–3): 111–124. doi:10.1007/s10641-007-9247-0. 
5. ^ ^{a b} Froese, Rainer, and Daniel Pauly, eds. (2010). "Raja pulchra" in FishBase. April 2010 version.
6. ^ Ishiyama, R. and S. Kuwabara (1954). "The electric fish of Japan. 1. Some observations on the structure of the electric organ in the skate, Raja pulchra". Journal of the Shimonoseki College of Fisheries 3: 275–282. 
7. ^ Ebert, D.A. and M.V. Winton (2010). "Chondrichthyans of High Latitude Seas". In Carrier, J.C., M.R. Heithaus and J.A. Musick. Sharks and Their Relatives: Physiological Adaptations, Behavior, Ecology, Conservation, and Management. CRC Press. pp. 115–158. ISBN 1420080474.