The spurdog is a typical small bottom-dwelling shark and is thought to be the most abundant living shark due to its worldwide distribution in shallow (usually <200 metres) temperate seas. It is also known by other names including spiny dogfish, piked dogfish, rock salmon (especially in ‘fish and chip ’ shops), spiky dog, or white spotted dogfish. Spurdog were formerly very common in Northern Ireland waters but have declined considerably due to overfishing, both directed fishing and as a by-catch. It is also avidly fished for by anglers. Its late sexual maturity and slow rate of reproduction limits population recovery after exploitation.

In brief

• Found in the seas around Northern Ireland
• Normally found at <200 metres depth
• Can be seen at any time of year, especially in the main fishing harbours in County Down
• Has declined considerably in recent years but still present in substantial numbers
• Main threat is overfishing, especially of pregnant females
• Late maturity, slow rate of reproduction and long gestation period makes it vulnerable to exploitation.

Species description: A typical shark with five external gill slits and a tail with a larger upper lobe (heterocercal). Locally, any cartilaginous fish with spines in front of the dorsal fins (hence name ‘spur’ dog) is likely to be this species. The spines are defensive and their toxin can cause painful, often septic, wounds if incorrectly handled. The upper body is dark grey with distinct round white spots along the back and sides, while the underside is white. The head is narrow and the snout tapers to a pointed tip. The anal fin is absent. Locally it grows to about 1.3m in length and a mass of 10kg, with females being larger than males. It occurs in shoals of same sex and size at depths down to c.1000 metres, although normally in waters of 10-200 metres. Its diet consists mainly of fish, together with crustaceans, molluscs, and other invertebrates.

Life cycle: Spurdog are long-lived (up to 70 years) and do not reproduce until they are about 20 years old and there may be several years between subsequent breeding. Fertilisation is internal, the male using his claspers to introduce sperm. The developing eggs (3-10+ depending on female size) are retained within the mother’s body (ovoviviparous reproduction) for eighteen months to two years, one of the longest gestation periods for any animal. During this time the young feed on the egg yolk sac contents. When born the young are about 25cm in length. In the autumn females migrate into shallow water to give birth to their young, where they are often the target of commercial fisheries. Spurdog are frequently found in single sex shoals of similar size. Spurdog are highly migratory and it is thought that the north-east Atlantic fish comprise a single stock.

Similar species: Spurdog may be confused with other small sharks (for example, lesser-spotted dogfish and smooth hounds) but the dorsal fin spines and white spots serve to distinguish it.

How to see this species: Young fish are often to be seen in aquaria such as ‘Exploris’ at Portaferry. Spurdog can been seen during fish landings at the main fishing ports in County Down and are also frequently caught by anglers.

Current status: Spurdog was very abundant in the early twentieth century and was regarded as a pest by commercial fishermen. However, it is now the most commercially important shark species in the north-east Atlantic. As with other cartilaginous fishes it declined rapidly in the middle twentieth century as a result of overexploitation, both directly through targeted fisheries and as a by-catch. Local numbers are unknown but spurdog are still relatively frequently captured, albeit in much reduced quantity. Total biomass is thought to have declined by over 90 per cent although accurate assessments are not available. Numerous (35 in 2006) specimen-weight spurdog (>5.443kg / 12 lbs) are taken off the County Antrim coast, generally off Red Bay and Ballycastle each year, and submitted to the Irish Specimen Fish Committee.

Why is this species a priority in Northern Ireland?

• Declining.

The species overall is classified as Vulnerable on the IUCN Red List 2006 with the north-east Atlantic stock assessed as Critically Endangered.

Threats: The main cause of decline and continuing threat is overfishing, both target and by-catch fisheries. Its late maturity, slow rate of reproduction and long gestation period makes the species especially vulnerable to overfishing and limits population recovery. Segregation by size and sex and aggregation in groups makes pregnant females very vulnerable. Pollution, dredging and bottom trawling may have adverse impact on spurdog habitat, especially for juveniles inshore.

Action:

• The status of spurdog will be surveyed and monitored and appropriate conservation action taken if required
• Establish a suitable no-fishing refuge area, which would benefit many other marine species as well.

Objectives: Avoid eating this species (including shark fin soup) to discourage commercial capture. Anglers should practice catch-and-release with spurdog, especially larger female fish.

Conservation:

• The EU has set a Total Allowable Catch quota of 2828 tonnes for the north-east Atlantic (excluding the North Sea) for 2007
• Has been proposed for listing under the Convention on International Trade in Endangered Species (CITES)
• A maximum landing size has been proposed to deter fisheries from targeting larger pregnant females.

Distribution Map from NBN: Squalus acanthias at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Squalus acanthias at iNaturalist World Species Observations database.

WoRMS: Squalus acanthias at World Register of Marine Species. Accepted name: Squalus acanthias Linnaeus, 1758. AphiaID: 105923.

Literature:

Central Fisheries Board, Dublin. (2003). Irish Sport Fishes: A guide to their identification. Available to download at: http://www.cfb.ie/fishing_in_ireland/fish_id.htm.

Ellis, J.R., Cruz-Martínez, A., Rackham, B.D. and Rogers, S.I. (2005). The distribution of chondrichthyan fishes around the British Isles and implications for conservation. Journal of Northwest Atlantic Fishery Science 35: 195-213. Available to download at: http://journal.nafo.int/35/ellis/5-ellis.html.

Ellis, J.R., Dulvy, N.K., Jennings, S., Parker-Humphreys, M. and Rogers, S.I. (2005). Assessing the status of demersal elasmobranchs in UK waters: a review. Journal of the Marine Biological

Association UK 85: 1025-1047.

Classification: Biota; Animalia; Chordata; Vertebrata; Gnathostomata; Chondrichthyes; Elasmobranchii; Neoselachii; Selachii; Squalomorphi; Squaliformes; Squalidae; Squalus

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Professor Andrew Ferguson (2026). Squalus acanthias. Linnaeus, 1758. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZF1040. Accessed on 2026-07-28