The Outer Hebrides comprise approximately 1·3% of the land area but 15·8% of the area of standing waters of Great Britain. Streams there are generally small and only three are as large as fifth order. The majority of the lochs occur below 200 m altitude and are less than 25 ha in area. A total of 1375 stream systems enter the sea, and drain the land mass through 9240 stream segments. There are just over 6000 lochs, of which less than half are connected to these networks. Most of the water systems are on geologically base-poor rock or soil, and less than 3%, situated on machair, are base-rich and eutrophic. The islands, due to their situation, receive precipitation in the form of very dilute sea water (with some non-marine sulphate probably derived from fossil fuel combustion) and this leads to unusual concentrations of chloride in the inland waters. Metamorphic rocks are resistant to weathering, so there is usually little variation in the composition of fresh waters except in the machair areas, where calcareous sands affect calcium concentrations and alkalinities.

The flora and fauna reflect the aqueous conditions and there are some unique successions of species from high to low salinities even in the same water body. The lochs provide a wide range of habitats for aquatic vegetation, ranging from the rich machair lochs to the oligotrophic peaty lochs (which show marked similarities with their counterparts on the mainland). Three broad categories can be delineated on the basis of their water quality and vegetation: brackish lochs with varying degrees of marine influence, calcareous machair lochs which may also be affected by sea connections, and the peaty lochans of low alkalinity. A few brackish water animals are sufficiently euryhaline to exist in the machair lochs, which are otherwise dominated by freshwater species. The occurrence of chloride in inland waters enables some crustaceans, e.g. Neomysis integer, Gammarus duebeni and Diaptomus wierzejskii to extend into what are poor freshwater habitats. The main constituents of the freshwater fauna are molluscs and arthropods, and a few species of euryhaline fishes, i.e. salmon, sea and brown trout, three-spined and ten-spined sticklebacks and eels. There are some populations of non-migratory charr. No amphibians are present.

Three main types of Hebridean freshwater lochs occur in the Loch Druidibeg National Nature Reserve; eutrophic calcareous machair lochs near the Atlantic coast, mesotrophic lochs on the edge of the moor and oligotrophic lochs on the peatlands. Chloride/calcium concentrations, highest in lochs near the sea, fall away rapidly inland and are lowest in the moor lochs. The fauna of eutrophic and mesotrophic lochs includes a few euryhaline brackish-water species Potamopyrgus jenkinsi, Diaptomus wierzejskii, Neomysis integer and Gammarus duebeni. G. duebeni alone penetrates the oligotrophic lochs. Freshwater sponges are widespread but are absent from the machair lochs and there are no triclad planarians. The fauna of fast and slow stream segments is described. Loch Bee, a man-made lagoon, unlike the open brackish lochs of North Uist, is an unstable habitat and its fauna has changed considerably since it was investigated in 1933. The fauna consists mainly of brackish-water species and a few stenohaline marine species which are confined to its seaward ends. The sequence of faunistic change in the Howmore estuary, Lochs Roag and Fada, a Site of Special Scientific Interest (SSSI), is described; the brackish-water fauna is reduced by the volume of fresh water passing through the system. A brief account is given of the unusual dominance of Hyalinella punctata and Potamopyrgus jenkinsi in Loch Ardvule (SSSI), South Uist.

Only euryhaline fishes were able to recolonize Outer Hebridean fresh waters after the last glaciation so that inland waters now contain only six indigenous, truly freshwater species; salmon, trout, charr, three- and ten-spined stickleback and the freshwater eel. Parts of some lochs are invaded by the sea at stages in the tidal cycle and some 20 species of marine fish may be found at times in inland waters.

Both salmon and sea trout occur in all the larger freshwater systems and sea trout are also found in many of the minor ones.

The original sea trout gave rise to sedentary brown trout populations similar to those of the mainland.

Fluctuations in the Hebridean salmon catch suggest that the stock is affected by the same factors as those affecting Scottish west coast stocks generally: the vast-majority return to fresh water as grilse, while sea trout stocks have been declining over the last 25 years.

Brown trout and eels are almost ubiquitous. Charr occur in Lewis and parts of North Uist but have not been reported from Harris or South Uist and are now only present as sedentary forms. The three-spined stickleback is more widely distributed than the ten-spined.

Outer Hebridean lochs in general have special scientific value because, unlike many Scottish mainland lochs, they have not been affected by the introduction of alien species and some are still in an unexploited state.

During June and July 1976 the Institute of Terrestrial Ecology carried out a limited invertebrate survey on 18 sites in the Outer Hebrides.

Portable battery-operated UV light traps were installed at each site and left to operate for eight nights. Bad weather in June greatly reduced the numbers of Lepidoptera caught. During July moths were trapped at 16 of the sites. Details of the combined catches are given. Pitfall traps were also installed at all sites and all beetles, spiders, harvestmen, snails, millipedes, centipedes and woodlice have been identified. In addition a list of species of littoral Coleoptera recorded from 24 Hebridean localities is given.

This survey was part of a much wider project around the coasts of Scotland, commissioned by the Nature Conservancy Council, to whom a full report will be submitted when identification and analysis of the results has been completed.

A description is given of the birds of the major terrestrial habitats of the Outer Hebrides: woodland, moorland, croftland and farmland. Changes in these habitats have affected the status and distribution of species. Most important of these changes is the growth of plantations, particularly those in Stornoway Castle grounds, Lewis, which have caused the colonization of the comparatively treeless islands with many woodland species previously absent from the Outer Hebrides. Landuse, particularly sheep husbandry and muirburn, has had a great effect on birds of the moorland including such predatory species as the golden eagle, peregrine, kestrel, merlin, hen harrier and short-eared owl.

An account is given of the breeding land birds of the outlying islands (except St Kilda) and a check list is provided for all species of birds in the Outer Hebrides.

The wading bird communities of four major habitat zones are described. Shore habitats support few breeding birds but hold significant numbers of migrant and wintering waders. Oystercatcher, ringed plover, bar-tailed godwit and dunlin are the most numerous species on the sand flats; the Atlantic beaches are particularly important to turnstones, purple sandpipers and sanderling. The wintering sanderling population is of national significance. Densities of breeding waders on the machair are greater than elsewhere in Britain. The dominant species are oystercatcher, ringed plover, lapwing and dunlin. The machair is nationally outstanding for breeding ringed plovers and the Outer Hebrides supports 14% of the British and Irish population. The machair is also unique in Britain for its concentration of coastal breeding dunlin. At other times of the year flocks of lapwings, golden plovers and curlew feed on the machair. Rough grazing (blackland) inland from the machair supports high densities of breeding snipe and redshank. Large numbers of grassland plovers and snipe feed here in winter. Seven species breed on the uplands and peatlands: golden plover, snipe, curlew, whimbrel, common sandpiper, greenshank and dunlin. Most of these species are more abundant north of the Sound of Harris and the north Lewis moors hold relatively high densities of golden plovers and dunlin.

Waterfowl habitats are briefly described. These consist of nutrient-poor moorland with peaty lochs and bogs; nutrient-rich machair wetland with calcareous lochs and marshes; brackish water and marine habitats of estuary, shore and off-shore islands. The breeding waterfowl populations for moorlands, machair wetlands (including the Balranald Reserve) and the off-shore islands are described by species. Migrating and wintering assemblies of waterfowl are described by species: divers and grebes, sea-ducks, geese and swans, ducks on machair wetlands. The paper concludes with a review of the factors of land use most affecting waterfowl: traditional crofting practice, reseeding and fertilizing of moorland and grassland, drainage, wildfowling, sheep husbandry, dumping of rubbish and human disturbance relative to predators such as the hooded crow and the herring gull. Such factors constitute a serious threat to the wellbeing of stocks of waterfowl in the Outer Hebrides which is not overcome by present conservation measures.

Coastal water moving north up the coast of Europe mixes with oceanic water from the Atlantic off the north of Scotland, which leads to a high marine productivity. In summer the mixing tends to be concentrated along fronts which form between the stratified water out at sea and that mixed by coastal currents, and in the sounds between the land masses. Small fish, and the birds which feed on them, frequent these areas, and the birds breed nearby, so that the area is a major nursery for temperate North Atlantic seabirds. Most of the large breeding colonies of pelagic species are not found along the Outer Hebridean island chain, but along the mainland coast and on the outlying islands. There is also a substantial population of birds which feed along the coast scattered throughout the area. When the stratification of the water out at sea breaks down at the end of the summer, many moulting guillemots, razorbills and kittiwakes move inshore with their young for a time. Some birds also tend to move north at this time, presumably following some change in the distribution of the available food. As the weather deteriorates in the winter, many of the seabirds disperse. Some auks move into the more sheltered North and Irish Seas, the shearwaters, small petrels, skuas and terns migrate to the southern hemisphere, and the puffins, kittiwakes and young fulmars scatter throughout the temperate North Atlantic. Variable proportions of many species, including cormorants, gulls, auks and the adult fulmars remain in the area, retiring to more sheltered waters during bad weather. There may have been some long-term changes in populations; thus the southern Manx shearwater declined and the northern puffin increased in the last century, and the process was reversed in this one, possibly as a result of changes in climate. Some species, notably the gulls and fulmar, are now increasing following their exploitation of human wastes for food, and all species are benefiting from the cessation of human exploitation and its replacement by protection over the last century.

The demersal fishery in Hebridean seas is based mainly on haddock, whiting and cod, mostly from local spawning grounds, with some recruitment from the North Sea. An important seasonal fishery for dogfish is based on a highly migratory stock. All these stocks are probably fully exploited.

Herring spawning grounds occur west and north of Lewis, and larvae drift eastwards to nursery areas, mainly in the North Sea. Enhanced recruitment in the late 1960s and an increase in fishing effort produced a large increase in landings but these have subsequently declined. The west coast herring fishery is subject to quota regulations for conservation. There are also important exploitable stocks of mackerel and sprats. Since 1971 an industrial fishery for Norway pout has taken a few thousand tonnes per annum.

Shellfishing by small boats goes with crofting. Since 1962 the number of larger boats has increased with full-time fishing, particularly for Norway lobsters and lobsters. Exploitation has been helped by processing and lobster-holding facilities. Scallops and periwinkles are also important commercially and there are good prospects for cultivating oysters and mussels.

By far the largest underexploited fish stock is blue whiting, which migrates south to spawn in March-April on the continental slope in depths of over 400 m to the west of Scotland. The spawning stock in this area is estimated to be 5-15 million tonnes. Sandeels may also occur in commercial quantities in some areas.

The growth of the Hebridean Grey seal population is described and its probable causes are discussed. The status of the Hebridean assemblies, in relation to those at North Rona and Orkney is reassessed on the basis of recent evidence supporting the hypothesis that there is considerable mixing of breeding individuals throughout Scotland. Consideration of the consequences of the population growth with regard to fisheries, conservation and commercial sealing prompted an investigation of the efficacy of different management strategies which might be used to adjust the population to a level at which it can be maintained.

Land use and land tenure in the Uists has evolved over the past two centuries fashioned by the possibilities offered by the varying qualities of soils and terrain and by economic circumstances, estate and government policies. At the end of the 18th century, estate surveys and plans provide a detailed account of the patterns of tenure and land use. The crofting system, created between 1814 and 1818, with the aim of improving agricultural standards and as a means of maintaining a labour force to manufacture kelp, led to an expansion of the cultivated area. In the middle of the 19th century, large farms displaced much of the population and resulted in congestion in the remaining crofting townships and very intensive use of the available land. Government intervention led to the settlement of crofters on the majority of the farms after 1890. Since the ‘agricultural revolution’ of the 1950s, if the number of active crofters and croft units and the area of cropped land has declined, the size of the effective croft units and their cattle stocks has increased, particularly in the more recently formed townships. Cereal cropping is now virtually confined to the machairs and most of the crofts are under permanent grass.

Cattle and sheep husbandry in the Outer Hebrides depends on hill grazing supplemented by small areas of improved or rotation grass and for winter fodder on cereals and on grass conserved as hay. The acreage of crop has decreased because of economic and social factors such as an ageing population, part-time crofting, difficulties in livestock marketing and in procurement of suitable machinery and seeds. These problems are likely to increase.

In Lewis and Harris and in the acid soils of the Uists cropping may continue as at present. The potential for development and particularly for any improvement in sheep husbandry, on which crofting depends, must be in hill pasture improvement. In the Uists machair is more important. There is need for more investigation of machair and machair cultivation. There must be greater use of grass for grazing, for conservation and as a means of raising soil fertility. Cereals may still be grown but both cereals and grass may have to be conserved as silage.

Preliminary stratification of the Outer Hebrides provides a basis for new surveys, and can be developed from the information held in the Institute of Terrestrial Ecology's Terrestrial Environment Information System. Characterizations of the area by environmental variables at various scales indicate several distinctive stratifications with predictive capabilities and suggest possible topics for future survey and research.

This, the concluding paper of the Symposium, reviews the main issues in the Outer Hebrides concerning the interaction between the constituent parts of the environment and man's use of them, seeking to identify the existing impacts on the natural environment and speculating upon the character of possible future impacts. It aims to demonstrate how the findings of the environmental sciences, as exemplified by the information in the symposium papers, are relevant to the understanding and resolution of these issues and to identify important gaps in knowledge.

The relationship between land use planning and development control is discussed, particularly as it is reflected in the present system of Regional Reports, Structure Plans, Local Plans and the recently published National Planning Guidelines, and as it affects options for influencing future use of environmental resources in the Outer Hebrides.

Developments need management and, in some cases, monitoring. Both these activities require a contribution from the environmental sciences through an effective institutional framework.

The closing discussion examines some of the more general social issues, relating to the potential conflict between utilization of the environmental resource and conservation of the natural environment, which characterize isolated rural areas.