Justification of Red List Category
This species has a very large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The global population is estimated at 230,000-240,000 individuals (Wetlands International 2015). The European population is estimated at 76,300-78,500 pairs, which equates to 153,000-157,000 mature individuals (BirdLife International 2015).
The overall trend is decreasing, although some populations may be stable (Wetlands International 2015). The European population is estimated to be decreasing by less than 25% in 26.4 years (three generations) (BirdLife International 2015).
This species can be found along the entire Atlantic coast of Europe as far north as Finland and Iceland and as far south as the coast of Morocco. It ranges in the entire Mediterranean, nesting on parts of the coastline of most European (e.g. Italy, Turkey) and north African countries (e.g. Algeria, Libya), as well as parts of the Black Sea coast (e.g. Ukraine) (del Hoyo et al. 1992).
Behaviour This is a coastal species that shows high nesting site fidelity. It feeds exclusively diurnally, and one bird is always present with the clutch or brood during the breeding season. The species breeds in colonies that can hold more than a thousand well-spaced pairs (del Hoyo et al. 1992, Snow and Perrins 1998, Nelson 2005). It is largely sedentary, although immatures may undergo post-breeding dispersive movements over short distances (del Hoyo et al. 1992). Some birds undergo short-distance migrations during winter. Individuals often forage alone when away from nesting colonies and in winter, but may follow dense shoals of fish in flocks of several hundred individuals (del Hoyo et al. 1992, Snow and Perrins 1998, Nelson 2005).
Habitat It occupies marine habitats, but does not usually occur far from land (del Hoyo et al. 1992). It shows a strong preference for rocky coasts and islands with adjacent deep, clear water, and forages over sandy and rocky seabeds (del Hoyo et al. 1992, Nelson 2005). It also prefers sheltered fishing grounds such as bays and channels, although it generally avoids estuaries, shallow or muddy inlets and fresh or brackish waters (Wanless and Harris 1997).
Diet The species feeds on a wide range of benthic, demersal and schooling, pelagic fish. Sandeels (Ammodytidae) are the dominant prey of birds in British and some Spanish populations (Wanless and Harris 1997, Velando and Freire 1999, BirdLife International 2000, Velando et al. 2005), and are consistently present in the species's diet in most other locations studied. These are usually caught at, or near, the sea bed (Wanless and Harris 1997). Other prey species include fish of the families Gadidae, Clupeidae, Cottidae, Labridae, and Trisopterus spp., although birds also take small numbers of polychaetes, cephalopods, other molluscs and small benthic crustaceans (del Hoyo et al. 1992, Wanless and Harris 1997). Adults provision their chicks with sandeels, but consume a broader variety of prey for themselves (BirdLife International 2000). The Mediterranean subspecies feeds mainly on coastal fish, caught from the bottom or mid water over rocky or sandy seabeds, but economically important fish seem to form a very small part of the diet (Aguilar and Fernandez 1999).
Breeding site The nest is constructed of marine vegetation and flotsam, from just above the high water level to over 100 m high on ledges, in crevices or in caves on sea cliffs, rocks and stacks, and at the base of sea cliffs amongst boulders (del Hoyo et al. 1992, Snow and Perrins 1998, Nelson 2005).
Foraging range At Islas Cíes, Spain, birds foraged within 20 km of the colony all year round (Velando et al. 2005). During the breeding season, the foraging range was typically within 4 km of the colony, and birds foraged in groups of 300-1000 individuals (Velando et al. 2005). Foraging areas tend to coincide with areas of sandy benthic sediment, and occur where depth is less than 80 m (Wanless et al. 1991, Velando and Friere 1999, BirdLife International 2000, Velando et al. 2005). At the Isle of May, Scotland, over 90% of foraging occurred within 13 km of the colony, and the maximum distance recorded was 17 km (Wanless et al. 1991). Foraging individuals visited more than one area during a trip, often feeding at sites several kilometres apart (Wanless et al. 1991). Birds were often found feeding in areas of strong tidal flow (Wanless et al. 1991). The available data on European Shag feeding habitat suggest that, within the inshore zone as a whole, the species is fairly plastic in its habitat requirements. In some areas, the birds' foraging range is considerably less than 20 km; the small number of birds breeding at Hirta, Scotland, all appeared to forage within a 2 km radius (BirdLife International 2000). Similarly, birds were only present within 3 km of North Rona, Scotland (BirdLife International 2000).
The species is highly dependent on herring and sand eel stocks. Failure of these food supplies results in extensive non-breeding and increased mortality (between 1974 and 1976, sand eel abundance decreased by 48%, and the Illas Cíes shag population halved in response) (Aebischer 1986). Climate change is one factor threatening prey availability, including shifts of currents that are important for supply of larval fish (Aebischer 1986). Climate change and increased frequency of severe weather events is likely to be responsible for the major population declines recorded over the past decades (Joint Nature Conservation Committee 2016), including wrecks (mass mortality events with no obvious cause). In 2012/13, wreck resulted in up to 53% decreases in study populations in Scotland (Gunn et al. 2014). All causes combined, the species shows unusually large variability in population size and more frequent cases of extreme weather (due to climate change) is likely to continue threatening the population in the future (Frederiksen et al. 2008).
Coastal oil pollution poses another threat to the species through direct mortality (e.g. Prestige Oil Spill in 2003) and more significantly, through indirect ecosystem effects reducing prey availability (Wanless and Harris 1997, Velando et al. 2005). Local habitat loss to commercial developments (construction of ports, marinas and sea walls, uncontrolled anchoring of boats and sand extraction for beach generation) and illegal trawling strongly affects the Mediterranean subspecies Gulosus aristotelis desmarestii (c. 10,000 individuals) and the benthonic communities on which it feeds (Aguilar and Fernandez 1999). Bycatch is major cause of mortality, with significant numbers being trapped in gillnets annually (Wanless and Harris 1997, Velando and Freire 2002). Overfishing is another threat to critical prey populations, particularly in the Mediterranean (Velando and Freire 2002). In the past, paralytic shellfish poisoning has caused mass mortality, notably in Shag colonies on the Farne Islands in 1968 when most individuals died and 90% of nests were deserted over the course of one week (Coulson et al. 1968).
Conservation Actions Underway
Listed on Appendix II of the Convention for Migratory Species, and Annex II in the Bern Convention and under Annex I of the EU Birds Directive. The following information refers to the species's European range only: There are 212 Important Bird Areas for this species across the European region. The species occurs within 96 Special Protection Areas in the EU. Work is underway to identify new Important Bird Areas and Special Protection Areas in Slovenia for the Mediterranean sub-species.
Conservation Actions Proposed
The following information refers to the species's European range only: Identification of important sites at sea for this species; and subsequent designation as Marine Protected Areas. Management of key breeding and wintering roosting sites, including eradication of invasive predators. On-board monitoring programmes of fishing vessels to determine high risk areas for gillnet bycatch, and where relevant implement mitigation and protection measures.
Text account compilers
Martin, R., Calvert, R., Butchart, S., Ekstrom, J., Fjagesund, T., Hatchett, J., Ashpole, J, Hibble, R., Malpas, L.
BirdLife International (2023) Species factsheet: Gulosus aristotelis. Downloaded from http://www.birdlife.org on 25/03/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 25/03/2023.