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Marine communities in the Mulroy Bay and Lough Swilly area


Limaria hians
The bright red tentacles of the file shell Limaria hiams. 'Nests' of this bivalve mollusc consolidate the gravel seabed and form areas of biogenic hard substrata.

There is little published work on the fauna, flora and biotopes of Mulroy Bay and Lough Swilly. A study of the intertidal algae exists (Parkes, 1958a, 1958b), otherwise there are only a few papers dealing with individual species of plants and animals of the area (Praeger, 1894; Morton, 1978; Minchin, 1981, 1987, 1988; Minchin, et al., 1987; Somerfield, 1985).

There are a number of areas of scientific interest adjoining the sea within the area described here but none of these has been designated on the basis of marine life. Most are of ornithological or botanical interest. This abstract presents the results of a survey undertaken between 9th and 23rd July 1993. Eight littoral sites and 32 sublittoral sites were surveyed.

Physical conditions

Mulroy Bay is the most convoluted of the marine inlets in north-west Ireland. It is approximately 12m long in a north-south direction. The entrance to the bay is a narrow embayment leading to a winding entrance channel 10m long. This channel varies in width and depth, with three significant narrows only 100-150m across, where the current reaches maxima of 3-5 knots. It opens into the Broad Water, an open shallow sea lough 8m from north to south and 2.5m from east to west, generally less than 20m in depth and with many small rocky islands and islets. Leading back towards the north is another narrow channel which opens into a deep, extremely sheltered body of water known as the North Water. The North Water measures 3m north to south and 1.5m east to west, with a maximum depth of 51m, a considerable area deeper than 30m and a narrow rocky intertidal zone. The North Water is surrounded by hills and extremely sheltered with only a relatively small catchment area. It retains a high salinity and has a pronounced halocline in wet periods of weather. Bay is a strange term to use for this complex body of water, which would certainly be described as a sea loch in Scotland. Mulroy Bay is in fact a fiard, formed by glaciation. Rocks on both sides of the bay are ice-polished with striations indicating a south to north direction for the movement of ice (Hull et al., 1895). The Moross peninsula, separating the North Water from the Broad Water, is a large glacial drumlin as is the area of boulders known as Scalpmore, to the south of where the main channel enters Broad Water (Charlesworth, 1924, 1953). The tidal regime within Mulroy Bay is affected by the narrows, which both delay the times of high and low water and reduce the tidal amplitude. Parkes (1958a) quotes a delay of 2 hours 33 minutes between Mulroy Bar (at the entrance) and the village of Cranford on Broad Water. She also reports a reduction of height of spring tides from 4.5m to 1.6m between the same places.

Lough Swilly is the longest marine inlet in north-west Ireland and is very different in character to Mulroy Bay, from which it is separated by the narrow Fanad peninsula. It is 7km wide at the entrance and approximately 38km in length, tapering gradually to the south-south-east and then turning south-south-west. There are extensive intertidal mud flats along the shores of the inner lough, and large banks of coarse sand and gravel in 10m or less in the middle portion, opposite the town of Buncrana. A channel in the centre of the inner arm of the lough reaches 19m depth. The river Swilly flows into the lough at its head and the river Leanan enters on the western side about one third of the way from the head. The lough has more of the qualities of an estuary than a sea lough.

Human impacts

The majority of the area is not densely populated. The largest centres are the towns of Buncrana (population 2800) on the east coast of Lough Swilly and Letterkenny (population 3500) at the head of Lough Swilly (1971 census, Royal Irish Academy 1979).

Mulroy Bay is used for aquaculture, particularly salmon farming and is important for scallop spat collection. One of the oldest salmon farms in Ireland, Fanad Fisheries, has its headquarters located on the northern shore of the North Water. Salmon cages are no longer located in the extremely sheltered North Water (apart from holding cages), but are now located in the Moross Channel and in Mill Bay in the main entrance channel as well as in Lough Swilly. Locations of aquaculture sites were taken from observations in the field.

The North Water of Mulroy Bay is a designated area for collection of Pecten maximus spat. Dredging for Pecten maximus is prohibited by a fisheries bye-law and to date the Department of the Marine has not permitted mussel culture in this area. A scallop fishery has existed within the Broad Water of Mulroy Bay for many years but is undertaken mostly from small local boats which have depleted the stocks less than in many other areas. Tourism in the area is of minor importance, with no large tourist centres and accomodation in scattered hotels, bed and breakfast establishments and caravan sites. Holiday homes owned by residents of Northern Ireland account for much of the tourism. Popular beaches are situated on Lough Swilly but these are generally uncrowded.

Map showing survey site locations


Charlesworth, J. K., 1924. The glacial geology of the north-west of Ireland. Proc. Roy. Ir. Acad. 36B 174-314.

Charlesworth, J. K., 1953. The geology of Ireland. Oliver & Boyd, 276pp.

Hull, E., Kinahan, G., Nolan, J., Cruise, R., Egan, F., Kilroe, J., Mitchell, W. & McHenry, A., 1895 Explanation of sheets 3, 4, 5, 9, 10, 11, 15 & 16 of the geological survey of Ireland. Mem. Geol. Surv. Ireland.

Minchin, D. 1981. The escallop Pecten maximus in Mulroy Bay. Fisheries Bulletin (Dublin), 1: 21pp.

Minchin, D. 1987. Sea-water temperature and spawning behaviour in the sea star Marthasterias glacialis. Marine Biology, 95: 139-143.

Minchin, D. 1988. Couch’s goby, Gobius couchi (Teleostei: Gobiidae), from Irish Waters. Journal of Fish Biology, 33: 821-822.

Minchin, D., Duggan, C.B. & King, W. 1987. Possible effects of organotins on scallop recruitment. Marine Pollution Bulletin 18(11): 604-608.

Morton, O. 1978. Some interesting records of algae from Ireland. Irish Naturalists' Journal, 19 (7): 240-242.

Parkes, H.M. 1958a. A general survey of the marine algae of Mulroy Bay, Co. Donegal. (to be continued). Irish Naturalists' Journal, 12 (11): 277-283.

Parkes, H.M. 1958b. A general survey of the marine algae of Mulroy Bay, Co. Donegal II. Irish Naturalists' Journal, 12 (12): 324-330.

Praeger, R.L. 1894. Fauna of Mulroy Bay, Donegal. Irish Naturalist. 3: 113-114.

Royal Irish Academy, 1979 Atlas of Ireland, Dublin.

Somerfield, P. 1985. A study of Haminea navicula (Da Costa) and its environment, the Wee Sea, County Donegal. Mod. Thesis, Zoology, Trinity College, Dublin.

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