The Portuguese dogfish (Centroscymnus coelolepis) is your quintessential “deep-water fish”, being one of the deepest dwelling shark species. It likes to stay way down in the ocean depths, most commonly around 1000 m (3300 ft). However it has been recorded down to 3.7 km (2.3 mi) beneath the surface!
This common and heavily exploited species is also referred to as the Portuguese shark. While the original species description was based on a specimen caught off of Portugal, this is a widespread shark that occurs around the globe.
The stocky body is dark brown, and has a shape that is more like a typical bony fish than a shark. It has large dermal denticles, no anal (bottom) fin, and the body reaches a maximum length of 1.58 m (5.2 ft).
Portuguese Dogfish Facts
This is a slow growing shark that is thought to live up to 70 years of age.
It belongs to the family known as “Sleeper Sharks” (Somniosidae), so named because they were originally thought to be sluggish bottom-dwellers. Sleepers? Well as it turns out, not so much. These sharks are in fact active, stealth predators.
Habitat and Range
This shark has a wide but patchy global distribution, encompassing the western North Atlantic Ocean, the eastern Atlantic, the western Mediterranean Sea, and the western Pacific Ocean.
While much information is available about this species because it has been the target of fisheries for so long, little data are available on its general biology, population sizes, or changes over time.
A recent genetic study looked at population structure of this species in the eastern Atlantic, off the coasts of Ireland, Portugal, Madeira, Mauritania, South Africa, and the Azores (Mid-Atlantic Ridge). The researchers found high levels of genetic diversity in the fish examined, however this was spread evenly across sampling locations. They found no evidence of distinct populations, indicating that there is a large amount of dispersal happening throughout the eastern Atlantic.
The preferred habitat of the Portuguese dogfish is the bottom of continental slopes and deep ocean plains. However, the depth at which individuals tend to reside depends on their sex and size. Younger sharks are generally found at greater depths than adults, and pregnant females in particular stay in shallower waters. The reason for this difference has not been demonstrated.
The upper teeth of this dogfish are spearlike: each has a long narrow cusp, or point. The lower teeth are blade-like and slightly larger than the uppers, each with a cusp that is slanted to the side.
Like all dogfish, this species is a successful predator. It seems to have a low diet diversity and feeds mainly on cephalopods, although fish (including some other sharks) and benthic invertebrates may also be consumed. Occasionally it acts as an opportunistic scavenger, meaning that when it’s really hungry it will take whatever it can get!
It is thought that this shark’s preference for deep waters may be explained by avoidance of competition with other shark species that have a similar trophic position (i.e. feeding habits), such as the Velvet belly lanternshark (Etmopterus spinax).
This is a species with late reproductive maturity, a long gestation period of possibly more than a year, and a relatively low rate of reproduction. Body size varies with geographic location, but in general males mature at about 85 cm (2.8 ft), and females at about 1m (3.3 ft) in length.
The breeding system is ovoviviparous, with females bearing anywhere from 5 to 29 young per litter. Pups are about 30 cm (11.8″) at birth. The nursery areas remain undiscovered.
In addition to the depth segregation described above, individuals of different sexes and ages are also found in different spatial locations. This is possible evidence of a large-scale migration timed with the reproductive cycle of this species.
Humans and Conservation
The Portuguese dogfish is an important species in commercial fisheries of the eastern Atlantic and Japan. It is caught by trawl, hook and gillnet. This species has been taken for human consumption over a long period of time. Catches in Japan peaked during World War II, but then declined due to overfishing. The shark meat is dried or salted for storage, and used to make fishmeal.
Another reason for the active pursuit of this species is the high squalene content of the liver. Squalene is an organic compound found in plants and animals, but particularly in shark livers. It is used as an additive to foods, cosmetics, medication and health supplements, and in vaccines to help boost immune responses.
Information about the current status of Portuguese dogfish populations is sketchy, because it is difficult to separate data about this species from other dogfish sharks in fisheries records. However there is evidence that numbers are declining, and some stocks are considered to be depleted. The ongoing removal of this species is worrisome to begin with – having such a low rate of reproduction, it cannot logically withstand such fishing pressure.
Accordingly, the International Council for the Exploration of the Sea(ICES) recommends a zero catch of Portuguese dogfish. The IUCN Red List assesses this species as “Near Threatened”, based on its low fecundity and the lack of relevant data on population trends.
At the very least, available scientific information indicates that commercial exploitation of the Portuguese dogfish should proceed with caution.
Written By: Kara Lefevre
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