Monach Isles and Outer Hebrides Western Continental Shelf IMMA

Size in Square Kilometres

35,538 km2

Qualifying Species and Criteria

Grey seal – Halichoerus grypus

Criterion B (2), C (1,2)

Sperm whale – Physeter macrocephalus

Criterion A

Sei Whale – Balaenoptera borealis

Criterion A

Fin whale – Balaenoptera physalus

Criterion A

Marine Mammal Diversity 

Criterion D (2)

Physeter macrocephalus, Hyperoodon ampullatus, Ziphius cavirostris, Mesoplodon bidens, Globicephala melas, Balaenoptera borealis, Balaenoptera musculus, Balaenoptera physalus, Lagenorhynchus acutus, Tursiops truncatus



This IMMA encompasses the continental shelf waters between the Outer Hebrides and the continental shelf edge, and includes the Monach Isles. This area is particularly important for grey seals (Halichoerus grypus), with the Monach Isles supporting the largest breeding colony in Europe, and ~6.5% of global pup production. These rich and productive waters also support a diverse range of additional marine mammal species. These include species associated with the continental shelf slope (sei whales (Balaenoptera borealis), fin whales (Balaenoptera physalus), sperm whales (Physeter macrocephalus), northern bottlenose whales (Hyperoodon ampullatus), long-finned pilot whales (Globicephala melas), Atlantic white-sided dolphins (Lagenorhyncus acutus), common dolphins (Delphinus delphis)), as well as species associated with the shelf itself (humpback whales (Megaptera novaeangliae), white-beaked dolphins (Lagenorhyncus albirostris), harbour porpoises (Phocoena phocoena), bottlenose dolphins (Tursiops truncatus), grey seals, and harbour seals (Phoca vitulina)). Minke whales (Balaenoptera acutorostrata), killer whales (Orcinus orca), and Risso’s dolphins (Grampus griseus)). The shelf slope is an important habitat for migrating whales, and a century ago, supported a major whaling industry.

Description of Qualifying Criteria

Criterion A – Species or Population Vulnerability

Both the sperm whale (Physeter macrocephalus) and the North Atlantic fin whale (Balaenoptera physalus) are classified as Vulnerable on the IUCN Red List of Threatened Species (Taylor et al. 2019, Cooke, 2018a), whilst the sei whale (Balaenoptera borealis) is classified as Endangered (Cooke, 2018b).

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

This area encompasses important terrestrial and offshore habitat for grey seals (Halichoerus grypus) hauling out at the Monach Isles Special Area of Conservation (SAC): the largest breeding aggregation of grey seals in the Northeast Atlantic (Russell et al., 2022). Approximately 6,000 grey seals regularly haul out on the Monach Isles, making regular foraging trips to sea (Russell et al, 2022; Russell & Carter, 2021). Compared to the foraging season, a higher proportion of the UK grey seal population aggregates on the Monach Isles to breed (Russell et al. 2022).

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

The Monach Isles is the largest grey seal breeding colony in the Northeast Atlantic, with ~12,500 pups born annually (approx. 18.5% of UK pup production; Russell et al., 2022), and ~ 6.5% of the global pup production (SCOS, 2022).

Sub-criterion C2: Feeding Areas

At-sea distribution models based on tracking data (Carter et al., 2022) demonstrate that the shelf waters west of the Outer Hebrides out to the shelf break are important foraging areas for grey seals hauling out at the Monach Isles and elsewhere. Analysis of scats has shown several fish species that occur commonly in the region (cod (Gadus morhua), plaice (Pleuronectes platessa), poor cod (Trisopterus minutus), sand eel (Ammodytes spp.), sea scorpion (Taurulus bubalis), dragonet (Callionymus lyra), ling (Molva molva), herring (Clupea harengus), and mackerel (Scomber scombrus)) are prevalent in the diet of grey seals (Hammond and Wilson, 2016; Wilson and Hammond, 2019).

Criterion D: Special Attributes

Sub-criterion D2: Diversity

Several surveys have been conducted along the continental shelf edge to the north and west of the Western Isles (e.g., Weir et al. 2001, Waggitt et al. 2020). These have shown a high diversity of marine mammal species, with the area suggested to be one of the most species diverse of any in the British Isles (Evans & Waggitt, 2020). Using systematic conservation planning to identify priority areas of cetacean biodiversity in the Northeast Atlantic, the shelf edge was clearly identified as important, and a consistent hotspot across seasons at a community level (Giménez et al. 2023).

The rich and productive waters support several regularly sighted cetacean species. These include continental shelf slope species (fin whales (Balaenoptera physalus), humpback whales (Megaptera novaeangliae), long-finned pilot whales (Globicephala melas), Atlantic white-sided dolphins (Lagenorhyncus acutus), and common dolphins (Delphinus delphis)), and continental shelf species (white-beaked dolphins (Lagenorhyncus albirostris), harbour porpoises (Phocoena phocoena), bottlenose dolphins (Tursiops truncatus), grey seals, and harbour seals (Phoca vitulina)). Minke whales (Balaenoptera acutorostrata), killer whales (Orcinus orca), and Risso’s dolphins (Grampus griseus) are regularly seen (Evans & Waggitt, 2020). Even deep-water species (northern bottlenose whales (Hyperoodon ampullatus), and sperm whales) are sighted (Evans & Waggitt, 2020).

The shelf slope is important habitat for migrating whales, and a century ago, supported a major whaling industry (Thompson,1928; Brown, 1976; Ryan et al., 2022). Species such as fin whales, sei whales and humpback whales (Megaptera novaeangliae) were taken in great numbers and are slowly recovering.

Supporting Information

Brown, S.G. 1976. Modern whaling in Britain and the north-east Atlantic Ocean. Mammal Review, 6: 25-36.

Carter, M. I. D., Boehme, L., Cronin, M. A., Duck, C. D., Grecian, W. J., Hastie, G. D., Jessopp, M., Matthiopoulos, J., Mcconnell, B. J., Miller, D. L., Morris, C. D., Moss, S. E. W., Thompson, D., Thompson, P. M., & Russell, D. J. F. 2022. Sympatric seals, satellite tracking and protected areas: habitat-based distribution estimates for conservation and management. Frontiers in Marine Science, 9, 875869.

Cooke, J.G. 2018a. Balaenoptera physalus. The IUCN Red List of Threatened Species 2018: e.T2478A50349982. Accessed on 11 April 2023

Cooke, J.G. 2018b. Balaenoptera borealis. The IUCN Red List of Threatened Species 2018: e.T2475A130482064. Accessed on 25 October 2023.

Cox, S.L., Embling, C.B., Hosegood, P.J., Votier, S.C., & Ingram, S.N. 2018. Oceanographic drivers of marine mammal and seabird habitat-use across shelf-seas: a guide to key features and recommendations for future research and conservation management. Estuarine, Coastal and Shelf Science, 212, 294–310.

Ellett D.J., Kruseman, P., Prangsma, G.J., Pollard, R.T., Vanaken, H.M., Edwards, A., Dooley, H.D., Gould, W.J. 1983. Water masses and mesoscale circulation of North Rockall Trough waters during JASIN 1978. Phil Trans R. Soc. Lond A., 308: 231-252.

Evans, P.G.H. 1990. European cetaceans and seabirds in an oceanographic context. Lutra 33:95-125

Evans, P.G.H and Waggitt, J.J. 2020. Impacts of climate change on marine mammals, relevant to the coastal and marine environment around the UK. MCCIP Science Review 2020, 421–455. doi: 10.14465/2020.arc19.mmm

Evans, P.G.H., Anderwald, P., and Baines, M.E. 2003. UK Cetacean Status Review. Report to English Nature and the Countryside Council for Wales. Sea Watch Foundation, Oxford. 160pp.

Evans, P.G.H., Carrington, C., and Waggitt, J. 2021. Risk Assessment of Bycatch of Protected Species in Fishing Activities. European Commission, Brussels. 213pp.

Giménez J., Waggitt J.J., and Jessopp M. 2023. Identification of priority cetacean areas in the north-east Atlantic using systematic conservation planning. Aquat Conserv-Mar Freshw Ecosyst

Hammond, P.S. and Wilson, L.J. 2016. Grey Seal Diet Composition and Prey Consumption. Scottish Marine and Freshwater Science Vol 7 No 20, 47pp.

Russell, D.J.F. & Carter, M.I.D. 2021. SCOS Briefing Paper. Estimating the proportion of grey seals hauled out during August surveys.

Russell, D.J.F. et al. 2022. Trends in seal abundance and grey seal pup production. Special Committee on Seals Briefing Paper. SCOS-BP 22/02

Ryan, C., Calderan, S., Allison, C., Leaper, R. and Risch, D. 2022. Historical occurrence of whales in Scottish Waters inferred from whaling records. Aquatic Conserv: Mar Freshw Ecosyst., 32:1675–1692.

SCOS. 2022. Scientific Advice on Matters Related to the Management of Seal Populations: 2022. Natural Environment Research Council Special Committee on Seals, UK.

Taylor, B.L., Baird, R., Barlow, J., Dawson, S.M., Ford, J., Mead, J.G., Notarbartolo di Sciara, G., Wade, P. & Pitman, R.L. (2019). Physeter macrocephalus (amended version of 2008 assessment). The IUCN Red List of Threatened Species 2019: e.T41755A160983555. Accessed on 11 April 2023

Thompson, D’A.W. 1928. On Whales Landed at the Scottish Whaling Stations during the Years 1908-1914 and 1920-1927. Scientific Investigations Fishery Board of Scotland. 1928. No. 3: 1-40.

Waggitt J.J., Evans P.G.H., Andrade J., Banks A.N., Boisseau O., Bolton M., Bradbury G., Brereton T., Camphuysen C.J., Durinck J., Felce T., Fijn R.C., Garcia-Baron I., Garthe S., Geelhoed S.C.V., Gilles A., Goodall M., Haelters J., Hamilton S., Hartny-Mills L., Hodgins N., James K., Jessopp M., Kavanagh A.S., Leopold M., Lohrengel K., Louzao M., Markones N., Martínez-Cedeira J., Cadhla O.O., Perry S.L., Pierce G.J., Ridoux V., Robinson K.P., Santos M.B., Saavedra C., Skov H., Stienen E.W.M., Sveegaard S., Thompson P., Vanermen N., Wall D., Webb A., Wilson J., Wanless S., Hiddink J.G. 2020. Distribution maps of cetacean and seabird populations in the North-East Atlantic. J Appl Ecol 57:253-269

Weir C.R., Pollock C., Cronin C., Taylor S. 2001. Cetaceans of the Atlantic Frontier, north and west of Scotland. Continental Shelf Research, 21: 1047-1071

Wilson, L.J. and Hammond, P.S. 2019. The diet of harbour and grey seals around Britain: Examining the role of prey as a potential cause of harbour seal declines. Aquatic Conservation: Marine Freshwater Ecosystems. 2019; 29(S1): 71– 85.


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