The distribution of seaweeds in the Firth of Clyde is described. The patterns of algal distribution are discussed in relation to propagule dispersal and recruitment, and the physiological ecology of the plants. Particular attention is given to the influence of biotic factors: competition, grazing by invertebrates and the effects of man.

Present knowledge of animal communities associated with the algae of the Firth of Clyde is briefly reviewed. New work is presented on Hyale nilssoni (Rathke), the commonest amphipod inhabiting high littoral seaweeds, which describes the life-history characteristics over three years of a population associated with Pelvetia. Seasonal changes in growth rate, sex ratio and breeding status are described. Fecundity is investigated and data on brood and egg sizes compared with populations from other latitudes. The dynamic equilibrium between the grazer (Hyale) and the grazed (Pelvetia) is described.

Present knowledge of long-term changes in benthic species in the Firth of Clyde is reviewed.

Recent work on the annual variations in seasonal cycles of the sand-dwelling bivalve Tellina tenuis da Costa shows some correlations with natural and man-made variations in seawater temperature. Between 1973 and 1984 inclusive good settlements of young occurred on the two beaches examined in the autumns of years when higher mean seawater temperatures occurred in June and July. The implications are discussed in relation to cycles of climatic change. In general, settlements at Hunterston, a thermally enriched area, were considerably greater than at Kames Bay, a site affected only by natural temperature changes. However, T. tenuis at Kames Bay grew to much larger maximum sizes than at Hunterston, except in 1979, when Hunterston animals were larger. The possible interactions of food availability and the modifying effects of heated discharges are discussed.

Earlier results by Dr A. C. Stephen between 1926 and 1951 are reassessed in the light of present knowledge.

An account is given of the benthic harpacticoid copepods of a medium grade subtidal sand at a depth of 8–5 m (below chart datum) off the Ayrshire coast at Hunterston. It is the first account of seasonal changes in subtidal harpacticoids in the Firth of Clyde.

Most of the population is restricted to the top 1 cm layer of sand and this is considered to be related to food availability. There were considerable variations in total densities with largest numbers occurring in late June, as seawater temperatures increased rapidly towards the summer maxima.

Two species examined in detail, Asellopsis hispida and Harpacticus flexus each had one distinct period of reproduction during the year. Gravid females appeared first during the coldest months (January and February) followed by large numbers of copepodites from late April to June, culminating in large numbers of adults in June. Adult males clasped female stages only in May and June, after the main period of egg production. It is thought that viable sperm is carried by the adult females for about six to seven months, supposedly until the next period of egg production the following year. Adults of Harpacticus flexus are thought to be an important source of food for juvenile and small fish, particularly during July.

The coastal areas in the Firth of Clyde, of which Irvine Bay is a representative example, receive important amounts of industrial and urban wastes which are a cause of economic and environmental concern.

Surveys of the benthos of Irvine Bay revealed a varied fauna dominated by large densities of opportunistic polychaetes such as Chaetozone and Spio and nematodes whose distribution correlated very strongly with the high organic carbon levels in the bay. The high densities and large biomass recorded in Irvine Bay were found to be 2–3 times higher than values from Loch Ewe, a clean area with normal carbon values.

All evidence from this survey points towards a situation in which the fauna is enhanced, yet does not suffer a species reduction.

A general survey of the biomass of the benthic infauna of the Clyde Sea Area and the distribution and abundance of the dominant species throughout the area, based on data from surveys undertaken in 1972 and 1974, is described. Recent surveys of the distribution of species along a well-defined gradient of organic enrichment on the Garroch Head sewage sludge disposal grounds in the central Firth of Clyde are used comparatively to suggest that benthic populations in the inner sea lochs, Kilbrannan Sound and in areas along the Ayrshire coast are markedly enriched. It is suggested that this enrichment may be caused by a general eutrophication of the Clyde Sea Area enhancing the effects of localised carbon inputs from urban areas.

Underwater television, remote photography and SCUBA diving have revealed that Clyde Sea Area sublittoral muds are extensively bioturbated by megafaunal species that construct conspicuous burrows. These species arc mainly decapod crustaceans and fish, each with a characteristic burrow configuration and optimum horizon for excavation. Burrow structures have been revealed by resin casting and densities and distributions have been studied using camera and SCUBA techniques. Burrowing behaviour has been investigated by field and laboratory studies, while physiological studies of some species have shown respiratory adaptations to the burrowing mode of life. Megafaunal burrowing appears to be an important determinant of benthic community structure.

Marked interspecific differences in recent population trends have been recorded for seabirds breeding in the Firth of Clyde. Auk populations have remained comparatively stable, while considerable reductions have taken place in tern and kittiwake numbers. Shags, cormorants, gannets and great black-backed gulls have all increased, as have fulmars and herring gulls. The striking increases in the latter two species are considered in detail. To some extent, these population changes can be related to differences in the feeding ecology of the various seabirds; in particular the variation in the number of breeding terns is related to variation in food availability.

The Clyde Estuary receives a relatively high pollutant load derived from the domestic and industrial wastes of half of Scotland's population. In order to monitor contaminant concentrations throughout the area, the Clyde River Purification Board (CRPB) initiated in 1980 an extensive ‘mussel-watch’ programme, having selected the common mussel as the most suitable indicator organism.

This 1980 survey served not only to identify shores with elevated trace metal concentrations and those which were relatively unpolluted, but also to assess changes in overall contamination of the area in comparison with previous surveys.

Following the success of the 1980 survey it has been gradually extended over the years, both in the number and geographical spread of sites visited, and in the substrates and determinands measured, as the CRPB's monitoring requirements have changed.

The 1984 CRPB ‘mussel-watch’ survey involved the collection of over 1200 mussels at twenty-four sites throughout the Firth of Clyde (Fig. 1), with subsequent determination of concentrations of the trace metals Cd, Cr, Cu, Hg, Mn, Ni, Pb and Zn, and of other toxic, persistent substances such as organochlorine pesticides and PCB residues.

Attention is focussed here on trace metal body burdens for Clyde mussels collected during the 1984survey.

The effluents from about half of Scotland's population and industry drain to the Clyde Estuary and Firth, where they cause a number of problems of contamination and pollution. This paper outlines historical aspects of urban growth, effluent production and disposal, pollution and pollution control. The sources, nature, quantities and main effects of effluents discharged are described, as is recent national and international legislation intended to prevent or control pollution. Reference is also made to the determination of licence conditions for effluent discharges by means of the Environmental Quality Objective/Environmental Quality Standard approach.

The herring population inhabiting the Firth of Clyde has supported an important fishery for hundreds of years. The basis for this fishery was until recently a stock of spring-spawning herring which spawn on Ballantrae Bank in the period February-April and which was investigated intensively by means of larval surveys from 1958 to 1979, when it had decreased to a very low level. In the late 1960s there was evidence of a change in the composition of the landings and during the 1970s and in the present decade the majority of fish caught have been of autumn-spawned origin. There is no evidence of significant autumn-spawning in the Firth of Clyde and autumn-spawning herring in advanced stages of maturation appear to leave the Clyde, returning by the time the fishery opens the following spring.

The spawning origin of autumn-spawned herring in the Clyde has been investigated by tagging experiments, and recaptures have been made in the Irish Sea, off the northwest Irish coast and further north along the west coast of Scotland. However the spawning area of the majority of the fish is unknown.

The herring fisheries in the Clyde, originally carried out by ring-net and anchored drift-net but now mainly by pair-trawl, have been managed by closed seasons and in the past few years by total allowable catch regulations under the control of the European Community.

There is also a population of sprats in the Firth of Clyde which supports a small fishery and spawning of this species takes place earlier in the Clyde than elsewhere around Scotland. Mackerel also make annual incursions into the Clyde and support a small summer fishery. Factors affecting the abundance of pelagic fish species in the Clyde are discussed.

Data from the Clyde demersal fishery during the period 1960 to 1984 are presented. Annual landings of demersal species at Chde ports in recent years have ranged from 4000 to 10,000 tonnes, with a first sale value of £2–5 million. In the 1960s the fishery was undertaken mainly by seine-net vessels, but the present fleet consists largely of dual-purpose light/Nephrops trawlers. The demersal fishery is seasonal and during the winter and spring there is an influx of ‘stranger’ vessels. Five species (cod, whiting, saithe, hake and haddock) make up more than 80% of the demersal landings from the Clyde. The landings of saithe have fluctuated very considerably during the period under review. Other relatively important species are dogfish, plaice, skate and ray and angler fish. In general the Clyde fishery represents only a small proportion of the total Scottish landings but in recent years landings of hake from the Clyde grounds have averaged approximately 40% of the Scottish hake total. The Clyde demersal fishery depends to a large extent on young fish and both the total annual landings and the relative importance of the principal species fluctuate from year to year in response to variations in the strength of the recruiting year classes. The catch rates of the principal species vary greatly throughout the year. Although there is no conclusive evidence that the Clyde populations are self-contained, all the major species are known to spawn within or close to the area and tagging experiments have suggested that there may not be much mixing between Clyde fish and those from surrounding areas. This may in part, however, reflect the fact that fishing intensity within the Clyde is high and fish are recaptured before they have had time to leave the area. For management purposes, the Clyde demersal stocks are included in ICES Sub-area VI.

The Clyde Sea Area has long been associated with shellfish fishing and has shared in the large expansion which has occurred during the past thirty-five years in the Scottish industry. Here, as elsewhere, the expansion has been helped by the establishment of processing facilities and in 1983 the landings of shellfish from the Clyde reached £4–5 million in value.

The sea bed and shores in the Clyde provide a wide range of habitats which is reflected in the commercially important species which are taken. They range from the fine mud in which Norway lobsters burrow, through sand in which cockles occur, and sandy gravel which is suitable for scallops and queens, to the hard ground inhabited by lobsters and the rocky shores on which periwinkles abound.

The most important shellfish in the Clyde, with annual landings valued at £3–75 million, is the Norway lobster, which is dealt with by Bailey et al. (1986). This contribution considers all the other species. The most valuable are scallops and queens, which now together exceed £400,000 p.a. in value. Periwinkles are gathered extensively on rocky shores. Squid are taken sporadically, by light trawlers and seine-netters, and occasionally, when they are especially abundant, form the basis of a directed fishery. Creel fishing for lobsters, edible crabs and velvet swimming crabs is less widespread than elsewhere round Scotland, but provides a useful income for a few boats. Mussels and cockles are gathered on a small scale, and cockle stocks in the Clyde proved especially valuable after English and Welsh stocks had been depleted by the severe winter of 1962–63. The only surviving fishery for the native oyster in Scotland is in the Clyde, in Loch Ryan, and attempts are being made to improve its production. Sea lochs in the Clyde offer conditions, particularly shelter, suitable for cultivating filterfeeding bivalve molluscs.

Three comprehensive surveys of Clyde vertebrates have been carried out during the past twenty-five years, and this exceptional degree of investigation permits some very valid comparisons to be made. Distinct changes in status have taken place throughout all the vertebrate classes. The work of man has had the most obvious impact. This is sometimes deliberate, as in direct attempts at species protection or destruction. At other times the impact of man is indirect, with effects on food supply, on habitat, or on health. Occasionally the effects are accidental. Many status changes are easily explained under the above headings, but others are not, and some show apparently opposite changes in closely-related species. Some changes in status are very real, and indeed are sometimes quite dramatic and obvious for all to see. Others are more apparent than real, presumably arising from increased observation. A representative selection of species is discussed.

An indication is given of the importance of the Clyde Estuary for economic, scientific and aesthetic purposes. The main thrust of the World Conservation Strategy and the Conservation and Development Programme for the U.K. is explained. In the light of this the attitudes and actions of a range of agencies operating in the estuary are examined.

The paper describes the salmon and sea trout stocks of the rivers entering the sea via the Estuary and Firth of the Clyde including the Rivers Clyde, Leven, the Carts, the rivers of the Ayrshire coast and those entering via the sea lochs and from the Mull of Kintyre. Particular attention is paid to the recent return of salmonids to the Rivers Clyde and Cart, which have been denied to migratory species for over a century because of pollution in the upper Clyde Estuary. The changing environmental conditions, which have made the reappearance of migratory salmonids possible, are described and the future importance of the fishery assessed.

Information concerning the abundance and distribution of plaice Pleuronectet, platessa L. eggs and larvae has been gathered from the examination of old records and from cruises carried out for this precise purpose. Additional information on the spawning of plaice is presented from the capture of ripe specimens.

Plankton samples have been taken in the Clyde Sea Area over many years by the Department of Agriculture and Fisheries for Scotland for a variety of purposes. All samples taken since 1950 have been examined and data on plaice eggs and larvae have been extracted and collated. The data have been standardised by expressing all the results in terms of number/100 m3.

The data show that plaice spawn in the Clyde between February and April, with maximum concentrations of eggs and larvae occurring in early March and April, respectively. Although the eggs are widely distributed, they are commonest to the east of Arran, the greatest densities being found off the Heads of Ayr, in Ayr Bay, close to Ailsa Craig and in Irvine Bay. The distribution of eggs and larvae in relation to the position of the nursery grounds for the young fish is discussed.

The fishery for Norway lobsters Nephrops norvegicus began in the 1950s and has since become one of the most valuable assets of the Clyde Sea Area. Landings of Nephrops reached a peak of 26001 in 1972. Following a decline in landings up to 1975 there has been a further increase to over 45001 in 1983, valued at £4–5 million. Most of the landings are made by inshore trawlers working from Ayr, Troon, Girvan, Largs, Campbeltown and Tarbert. Recently a small creel fishery has developed in some sea lochs. The abundance of the stock appears to have been maintained; landings per unit of fishing effort have fluctuated without any obvious trend.

Since September, 1979. several aspects of the biology of Nephrops have been studied in an area south of the Little Cumbrae at the north end of the east Arran Basin. Data are presented on the density, size composition, growth rate, reproductive biology and feeding of Nephrops in this area. The results are compared with those obtained for other Nephrops populations, revealing some unusual features of Clyde Nephrops. In the Clyde, densities were lower than in a number of other areas, while growth rate was much higher and animals grew to larger sizes. Sexual maturity was attained at a larger size in Clyde Nephrops and studies of ovary development revealed that considerable numbers of large females did not reproduce annually, in contrast to some other areas. Recent observations suggest that the biological characteristics of Nephrops populations in an area may be influenced by features of the local physical environment such as the particle size distribution and organic carbon content of the sediment. Differences between Clyde Nephrops and those from other areas are discussed in relation to this.

The saltmarshes of the Firth of Clyde are restricted both in area and number. Despite this they exhibit considerable variation in terms of their physical environment, floristic composition and range of plant communities. The absence of grazing at most of the sites has resulted in their exhibiting plant communities atypical of most of the other saltmarshes of western Scotland. Like many other saltmarsh areas they have suffered from reclamation of the older, upper marsh. However, despite this, there are still important transitional communities to non-tidal vegetation. The prevention of further incursions into the sites for roads, industrial development, caravans and the like, is important if these sites are to remain as examples of natural habitats.