Japanese Spurdog, Japanese Dogfish.
A slender squaloid shark with a long, narrow pointed snout. Snout length approximately 2x mouth width. Anterior nasal flaps split into two points. Two dorsal fins with anterior spines. First dorsal spine roughly half the length of fin anterior margin. First dorsal fin origin slightly posterior to free pectoral fin insertion. Second dorsal approximately .7x height of first dorsal fin. Spine on second dorsal fin longer and more curved than spine on first dorsal fin. Pectoral fins have rounded free rear tips.
Dorsal coloration grey, reddish grey, or brownish-grey and unmarked but fin tips may be somewhat dusky. Ventrum pale.
Maximum length 95cm. Size at birth 19-30cm.
The Japanese Spurdog is a deepwater species occuring on the continental shelf and upper slope. From 52m to at least 400m depth.
Northwest Pacific Ocean. The Japanese spurdog is restricted to Japan, Korea, China, and Taiwan. Records from the Philippines are probably misidenfications of the Western longnosed spurdog Squalus nasutus.
The Japanese Spurdog (Squalus japonicus) is a bycatch of demersal trawl, longline, and gillnet fisheries and is retained for human consumption and fish meal. Species-specific catch rates from Japan indicate a relatively stable population over the past 36 years (~one and a half generation lengths). Landings data of all sharks combined from the Taiwan Exclusive Economic Zone (EEZ) and reconstructed catches of all sharks, skates, and rays from the China and South Korea EEZs indicate declines of 52–99% over the past three generation lengths (63 years). These levels of declines are not species-specific but are informative for understanding the broader levels of decline in sharks in the region. Most of the distribution of the species is in Taiwan, China, and South Korea where historic and current fishing pressure is high. The inshore trawl and gillnet bans in Taiwan and China, respectively, may afford the species some refuge from fishing pressure. In Japan, where the population appears stable, fishing pressure and bottom trawl effort is decreasing and continuing to decline. It is inferred that the Japanese Spurdog has undergone a population reduction of 50–79% over the past three generation lengths (63 years) due to levels of exploitation, and it is assessed as ENDANGERED.
Citations and References
Rigby, C.L., Chen, X., Ebert, D.A., Herman, K., Ho, H., Hsu, H. & Zhang, J. 2020. Squalus japonicus. The IUCN Red List of Threatened Species 2020: e.T161433A124484752. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T161433A124484752.en. Downloaded on 09 January 2021.
An aplacental viviparous species. Litter size 2-8.
Probably feeds on bony fishes and invertebrates.
Reaction to divers
Unknown but probably very shy. The Japanese spurdog occurs in water deeper than 50m. Therefore, encounters with recreational divers are unlikely.
There are no records of Japanese divers seeing this species while on scuba but it may be possible to attract this species into recreational depths by baiting near drop offs close to recreational limits.
North Pacific Spiny Dogfish Distinguished by white spots on lateral line, shorter snout, and longer fin spines.
Mandarin Dogfish Distinguished by extremely long nasal barbells.
Taiwan Spurdog Distinguished by black posterior margins on dorsal fins.
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