Shortspine spurdog (Squalus mitsukurii)
|Also known as:||greeneye spurdog shark|
|Size||Length: up to 1 m (2)|
Classified as Data Deficient (DD) on the IUCN Red List (1).
The shortspine spurdog, belonging to the Squalidae, or dogfish family, is characterised by having two dorsal fins, each preceded by a large, solid spine from which it gets its name (2). The snout is fairly long, broad and rounded and, unlike many other fish, there is no anal fin. The stout body of the shortspine spurdog is pearl-grey on the back and white on the underside, and the fins are edged in white (2) (3).
The range of the shortspine spurdog is not entirely clear, due to confusion with other species and uncertainty regarding this species’ taxonomy (4). However, it appears to have a wide distribution in the Atlantic, Pacific and Indian Oceans (1).
The shortspine spurdog occurs in warm-temperate and tropical waters, where it is often found near, or on the bottom of, continental and insular shelves and the upper slopes of underwater ridges (2) (4), mostly between depths of 100 and 700 metres (4).
Shortspine spurdogs, which often occur in large aggregations or schools (4), feed on prey that also live on, or near the bottom, of the ocean, such as fishes, cephalopods and crustaceans (2) (5). These fish are ovoviviparous (2), a method of reproduction in which the young develop within eggs that remain inside the body until they hatch. Females give birth to litters of between four and ten pups, after a gestation of up to two years (2) (4).
Males tend to reach maturity at smaller sizes than females (5); shortspine dogfish from south-east Australia mature at around 37 to 70 centimetres for males and 80 to 82 centimetres for females (4). Female shortspine spurdog are estimated to live for a maximum of 27 years, while males are thought to live only up until 18 years of age (4) (5). However, data regarding biological characteristics of the shortspine spurdog, such as its size at maturity and size differences between male and females, vary widely between populations at different locations, and even within the same population (4).
Fisheries appear to pose the greatest threat to the shortspine spurdog, as it is apparently caught throughout much of its distribution (1). It is captured primarily in demersal trawl fisheries; a method of fishing in which a net is dragged along the ocean bottom by a trawler. Around Australia, the shortspine spurdog is fished at varying levels; for example, fishing pressure is intensive around south-east Australia, and in a heavily-trawled area off New South Wales, the shortspine spurdog declined by as much as 97 percent between the period 1976-77 and 1996-97. Elsewhere, such as Western Australia, fishing pressure is low and the shortspine spurdog is taken only as by-catch (4). While the World Conservation Union (IUCN) has classified the shortspine spurdog as Data Deficient due to taxonomic confusion and the lack of fisheries data in many parts (1), the expansion of deepwater demersal trawls in parts of its range has led some to conclude that this species is not able to withstand continued exploitation at current levels (4), and action is required to protect this shortspine spurdog’s future.
While listed globally as Data Deficient by the IUCN (1), populations of shortspine spurdogs in Australia and New Zealand are listed as Endangered and Near Threatened, respectively (4). Despite these worrying listings, there are no known conservation measures currently in place for this little-known species (1).
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- Anal fin: in fish, an unpaired fin on the under surface of a fish, behind the anus.
- By-catch: in the fishing industry, the part of the catch made up of non-target species.
- Cephalopods: from the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
- Continental: relating to continental shelves: the regions of relatively shallow water, not usually deeper than 200 metres, surrounding each of the continents.
- Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
- Dorsal fins: the unpaired fins found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Gestation: the state of being pregnant; the period from conception to birth.
- Insular shelves: the regions of relatively shallow water surrounding islands.
- Taxonomy: the science of classifying organisms, grouping together animals which share common features and are thought to have a common ancestor.
IUCN Red List (December, 2009)