Natural Heritage Trends: Scotland 2001

The Sea

The natural heritage of Scotland’s coastal waters, to the 12-nautical-mile territorial limit and beyond that to the continental shelf, are equally distinctive. Lying in a transition zone between sub-tropical waters to the south and sub-polar waters to the north, many marine species occur at their northern or southern limits within Scottish waters. The prevailing northerly currents and relatively warm waters of the west coast allow species associated with more southerly latitudes to establish. In contrast, the colder North Sea favours species of temperate-arctic waters. Between the two, and biogeographically isolated by the fluctuation of temperature seasonally, are the northern seas around the Fair Isle and Shetland Islands. Consequently, Scotland’s marine flora and fauna are rich and diverse, with major commercial fish and shellfish stocks, as well as internationally important populations of seabirds and grey and harbour seals.

Chapter 14 summarises trends in the marine environment. It is only comparatively recently, from the 1970s onwards, that marine surveys have started to reveal the habitats and wildlife in the seas around Scotland. Some 25 species of whales, dolphins and porpoises have been recorded in British and Irish waters within the last 100 years, and 23 within the last 20 years. Improved water quality in the Clyde and Forth estuaries in recent decades has allowed invertebrates and fish to recolonise formerly polluted areas. The clean and sheltered waters of the west coast have attracted fish farming, with rapid expansions in salmon and shellfish production from the 1980s onwards. Sea levels are rising relative to land in places, with increased storminess. Short-term increases in sea temperature provide no clear long-term trend. Harbour seal numbers remained largely stable over the last decade but grey seal numbers increased by around 6% per year. Eleven out of 18 seabird species showed a marked increase in their breeding population and four showed a marked decline, between around 1970 and 1987.

Chapter 15 provides an overview of Scottish marine fishery trends. In view of their ecological importance, commercially exploited fish species have been identified for biodiversity conservation action through Species Action Plans. Of the 1000-or-so species of fish that inhabit the seas around Scotland, some 2% account for 95% of total fish biomass. Scottish data are available for 12 commercially exploited species, nine of which are considered to be outside safe biological limits. At current levels of exploitation, the stock of some, such as cod, are at risk of collapse. The status of 12-or-so commercially exploited deep water species is mostly unknown or outside safe biological limits.