Spiny dogfish (Squalus acanthias)
Also known as: blue dog, common spiny fish, dogfish, grayfish, Darwen salmon, Pacific dogfish, picked dogfish, piked dogfish, rock salmon, white-spotted dogfish, spurdog, mud shark.
Morphology & Biology
The spiny dogfish is a shark species within the Squalidea family. The characteristic that sets apart the Squalus shark species from other similar looking shark species, is that these shark species do not have an anal fin. They are small, slim fish with a narrow, pointed snout and distinctive white spots that fade with age. They have a brownish slate color that fades to a pale underbelly. The name acanthias refers to the two spines that are found on the dorsal fins of the spiny dogfish. These spines contain venom that can be used to defend themselves from predators.
Spiny dogfish are amongst the longest living, latest maturing and slowest growing shark species. Female spiny dogfish can reach a length of 110-200 cm in the Mediterranean Sea and males can reach lengths of 70-160cm. Spiny dogfish can live to be up to 100 years old, however due to human activities and increased capture rates, many individuals of this species are found to be no older than 25-30 years. Maturation age depends on the biological sex of the fish; males maturing at 11 years old and females maturing anywhere between 18-21 years old. Spiny dogfish can live to up to 100 years. These sharks have some of the longest gestation periods recorded of any animal, lasting anywhere between 18-26 months until the embryo has grown into a pup. The eggs develop fully inside the mother and then when they are ready to hatch, they hatch within the uterus of the mother and are born without shells. The babies are fed by a yolk sac found within the egg, as opposed to a placenta. Between 1-20 pups are born with sizes ranging from 18-30cm. Mother spiny dogfish deliver the young head first with a protective layer to protect herself from injury in a series of rhythmic contractions, similar to birth in mammals.
The spiny dogfish prefers living in temperate seawater and brackish waters, preferring a minimum of 7-8C and a maximum of 12-15C. In the winter they spend time in deeper waters where they will eat less. In the summer they spend time in coastal warmer waters where they hunt in big groups on schools of bony fish such as mackerel and herring (the majority of their diet), as well as octopuses, smaller sharks, squid, crabs and shark egg cases, using their strong jaws to crack open the eggs. They even bite through fishing nets to get to the fish and drive off commercially caught fish like herring and mackerel, gaining themselves a bad reputation among fishermen. Besides being powerful and voracious predators they are also preyed on by larger sharks, some bony fish, seals and killer whales.
Distribution
The spiny dogfish can be found in the western Atlantic Ocean from Greenland to Argentina and in Eastern Atlantic from Iceland and Murmanski Coast (Russia) to South Africa, including the Mediterranean Sea and Black Sea. In the western Pacific Ocean they can be found from the Bering Sea to New Zealand and in the Eastern Pacific found from Bering Sea to possibly Chile. The distribution range of the spiny dogfish makes them likely to be one of the most abundant living shark species.
They swim in large schools with individuals of similar sizes staying together as they grow. Immature dogfish tend to swim in schools offshore, whereas mature females tend to swim inshore. The males are more likely to stay in shallower waters than females, except the heavily pregnant individuals.
Threats
According to Smith et al. (1998) the spiny dogfish has the lowest intrinsic rebound potential of the 26 shark species he analyzed due to the slow reproduction, low reproductive capacity and late maturity. Due to these reasons, they are highly vulnerable for overfishing and high fishing pressure. Therefore, the management of capturing this species is important to monitor. Population decline has been recorded and is only exemplified due to habitat destruction and intense fishing pressure. Therefore, according to the IUCN red list the worldwide stock is at this moment in a vulnerable state and in some parts even critically endangered.
Fisheries & Aquaculture
Fisheries can be found around the world due to how widespread this species is. In European waters (mainly North and Irish seas) spiny dogfish have been caught since the beginning of this century. They are commercially used for different purposes, an example being the popular English “fish and chips” but also for their oil and to be used as fish meal. Spiny dogfish are also frequently caught as by-catch within other fisheries. The most used fishing techniques are bottom trawlers and longline but other common fishing techniques include gill nets and seines.
The Marine Fisheries Commission (ASMFC) has, for a long while, recommended to fully cease fishing and capturing spiny dogfish in the North Atlantic Ocean. They argue that a world wide effort is important to encourage more sustainable population levels in the future. In some countries measurements have been taken over the years, such as a more restrictive catch quota in Europe or maximum landing size and number in the UK. Spiny dogfish are commercially not among the highest targeted and caught fish. However, due to the high biological vulnerability of the species, the measurements that were put in place, were not stringent enough, leading to many stocks still experiencing a high fishing pressure. In the NE and NW Atlantic Ocean, local spiny dogfish populations are on the brink of collapse. This is very dangerous, as sharks play a crucial role in the balance of the ecosystems, these shark species are still the most abundant shark species.
Fun Fact
The common name “dogfish” comes from fishers describing this fish as chasing fish in large dog-like “packs”. Spiny dogfish are amongst the longest living, latest maturing and slowest growing shark species.
