Spiny Dogfish, Squalus acanthias, are small coastal sharks that can be found swimming in large packs. Their genus name translates as “a kind of sea-fish,” an appropriate fit for a species that can be found exclusively in seawater. Their species name is Latin for “a prickly thing,” referring to the characteristic spines found on their dorsal fins. The Spiny Dogfish is considered a vulnerable species by the IUCN due to a number of issues, including commercial fishing practices, the species’ advanced age at sexual maturity, and a long gestational period.
Spiny Dogfish are small coastal sharks. Females tend to be larger than males, reaching average mature lengths of 30” to 42”. Mature males measure 24” to 35” on average. Adult Spiny Dogfish weigh 6.5 to 20 lbs. The maximum recorded weight of a female was 21.6 lbs.1 Dogfish living in northern parts of their range tend to be larger on average than those in southern regions.2
Spiny Dogfish have a slate grey to brown dorsal surface and white or pale grey ventral surface. Immature fish have a row of small white spots on each side of their body that reaches from above their pectoral fins to above their pelvic fins. These spots usually fade away with maturity. The edges of the dorsal and caudal fins are pale in color at birth. Spiny Dogfish are covered in placoid scales that are similar in structure to teeth. Each scale terminates in a rear-pointing spine.
Spiny Dogfish have an elongated and slender appearance. Their long, pointed, flat snouts are lined with nearly continuous rows of small, flat, smooth teeth (28 upper and 22 to 24) with deeply notched cusps and oblique points that angle toward the corners of the mouth.3 They have large eyes and narrow anterior nasal flaps.
Spiny Dogfish have two dorsal fins, each with a prominent anterior spine that is mildly venomous. These spines are used as defense mechanisms. The second dorsal fin is approximately two-thirds the side of the first. The pectoral fins are triangular in shape, with curved margins and rounded rear tips.4 They control direction during swimming. The pelvic fins (which aid in stabilization along with the dorsal fins,) are located nearer to the second/rear dorsal fin than the first. Pectoral and pelvic fins are located on the sides of the body. Spiny Dogfish, like other Squaliform sharks, lack anal fins.5 The caudal peduncle has low lateral keels. The body of a Spiny Dogfish terminates in an asymmetrical caudal fin.
The name Dogfish came from fishermen who observed this species swimming in large “packs” chasing schools of smaller fish. Spiny Dogfish are gregarious and form schools of hundreds, or even thousands, of fish. These schools consist of fish of one sex that stay together as they grow (although small, immature males and females may be found in the same school.) Schools of immature fish are typically found offshore. Mature females usually school closer to shore.6
Female Spiny Dogfish reach sexual maturity at 18 to 21 years of age and can live to a maximum age of 40 years. Males reach sexual maturity by 11 years old and can live to a maximum age of 35 years.7 The advanced age of sexual maturity combined with a long gestational period make this species especially vulnerable to over-fishing.
Spiny Dogfish can be found on the continental shelf in shallow, northern inshore waters during the summer and deep, southern offshore waters in the winter. They inhabit temperate and boreal salt waters that are ideally 6° to 11°C. Although they prefer seawater, dogfish can tolerate brackish water.
Within their habitats, Spiny Dogfish are prey to by cod, goosefish, red hake, larger species of sharks including larger Spiny Dogfish, seals, and killer whales.
The Spiny Dogfish is an abundant species that can be found on the continental shelf in temperate and boreal waters around the world. They can be found as far north as Greenland, Iceland and the Bering Sea and as far south as Argentina, South Africa, Chile, and New Zealand8 at depths exceeding 2400’.9 The Spiny Dogfish is a highly migratory species whose movements are determined by water temperature. They move to inshore northern waters for the spring and summer and to deep, southern offshore waters for the winter.
Spiny Dogfish are voracious and opportunistic eaters that have become unpopular with fisherman due to their tendency to chew through commercial fishing nets, releasing and driving away schools of fish. They swim in predatory packs and attack schools of smaller fish. Their diets consist of deep-sea fish species including herring, mackerel, haddock, capelin, squid, jellyfish, octopus, smaller sharks and shark egg cases, shrimp, and crabs. Because of their slender appearances in the spring, it is believed that Spiny Dogfish rarely feed during winter months.10
Internal fertilization occurs in offshore waters during the winter months. Early on in development, the female’s body secretes thin, transparent, horny shells. Each shell, called a candle, surrounds several ova within the ovaducts.11 Litters may contain between 1 and 15 pups (but on average 6 to 7,) with an average male to female ration of 1:1. The embryos develop internally for 22 to 24 months, known as ovoviviparous development. This is the longest gestational period of any vertebrate. For the first 4 to 6 months of development, the embryos receive nourishment from surrounding membranes. After these membranes break down, the embryos no longer have a placental attachment and receive nourishment from yolk-sacs for the duration of the gestational period. The young, referred to as pups, are born offshore during the winter months. The pups are born headfirst, measuring 8” to 13” in length. Dorsal spines are present at birth. To protect the mother from being injured, the pups’ spines are covered in sheaths of cartilage during birth.12
Notes of interest
Spiny Dogfish are used to make oil and fish meal and are commonly used to make fish and chips.13
Spiny Dogfish may also be referred to as: Blue Dogs, Common Spinyfish, Grayfish, Rock Salmon, Spiky Dogs, Spur Dogs, and White-spotted Spurdogs.14
Spiny Dogfish are caught with longline, troll, trawl, sink gill nets, and jig handline gear.15
Spiny Dogfish are a popular species in the commercial fishing industry. The industry experienced a peak in 1974 when 27,400 metric tons collected. This was followed by a sharp decline in the 1980’s, when only 5900 metric tons were collected. Commercial fishing of this species regained its strength in the 1990’s and in 1996 over 28,000 metric tons were collected.16
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