2.1 Richest biodiversity on earth?

The oceans cover two-thirds of the surface of the Earth, to an average depth of almost 4,000 metres. These depths provide more than 150 times as much living space as all terrestrial and freshwater environments combined (Rice 1996).

Our knowledge of the life in the deep ocean is of fairly recent origin and we are a long way from a complete understanding of deepwater ecosystems (Gordon 1996a). The view little more than two decades ago was that the deep-sea was impoverished in species owing to its demanding conditions for life. Edward Forbes suggested in 1844 that, on theoretical grounds, no animal life could exist at depths below about 600 metres (Gordon 1996a; Carter 1961), however this theory was finally disproved in the late 1860s (Gordon 1996a). It is now known that almost all of the entire volume of the oceans support life, from the surface to the greatest depths (Gordon 1996a). In 1951 scientists even found life at 33,400 feet deep in the Philippine Trench. It is notable that Sir Edmund Hillary and Tenzing Norgay observed no sign of life at 29,028 feet, on the summit of Mount Everest (Earle 1996).

Modern research has recently demonstrated an astonishingly high local species diversity in the deep-sea. It was only in 1992 that scientists (Grassle and Maciolek 1992) estimated that the deep-sea could harbour 10 million species that have not yet been described and named (Gage and Gordon 1995a). The deep-sea benthic fauna has been found to be extremely species-rich, possibly rivalling that of any environment on Earth (Rice 1995). However, these species are often thinly distributed or poorly represented, or rare.

2.2 Living fossils

The discovery of richness in biodiversity now prompts a view of the deep-sea as a site where there has been wide-spread evolution of new species and perhaps the place of origin of wider groups or families of marine species (Hessler and Wilson 1983; Rice 1996).

The deep oceans are an environment where evolution may have proceeded much more slowly than in shallow waters (Carter 1961). This view had kept alive the old idea of the deep-sea as a refuge for species long extinct elsewhere (Zenkevitch and Birstein 1960). This is particularly the case in the Atlantic Ocean where the deepwater fish fauna is largely made up of species which have adapted to life at great depth over a long time-period. These fish are referred to by scientists as the ‘ancient deepwater species’ (Gordon 1996a), making them like living fossils from a by gone era. By comparison, the deepwater fish fauna of the Norwegian Sea has close affinities with the continental shelf species and are generally known as the secondary deepwater forms. They are characterised by species which have invaded the deepwater in more recent times (Gordon 1996a), unlike those in the Atlantic Ocean.

On the upper continental slope of the Atlantic Basin there are 300 to 400 species of fish known to science, variably distributed throughout the whole region (Marine Panel 1996). In the deepwaters off the west of the British Isles there are over 200 species of fish known to science (Gordon and Hunter 1994a). Between the depths of about 250 and 3,000 metres there are over 100 different types of fish, of which many are quite rare and others reach only a small adult size (Gordon and Hunter 1993). They are quite distinct from the fish which fisherman are used to on the continental shelf around Britain. Some are long lost relatives of the cod or salmon families, however, these creatures bear no resemblance to, or have few similar characteristics of, their ancestors.

2.3 Beyond the reach of light

A frigid realm beyond the reach of sunlight, the pressures at depths of about 1,000 metres are around 100 times greater than that at sea level. Energy is sparse in the deep-sea, making reproduction expensive. To feed, deep-sea species depend on organic material from the upper sea or on hunting. But hunting is also expensive and the predator may have to travel long distances to find its prey and even risks not running into it (Earle 1996).

Deep-sea fish are not slimy like the fish found on the continental shelf, owing to the lack of a mucus coating. Many of the deep-sea fish have different body shapes, often with large heads and large eyes (Gordon and Hunter 1994a).

Some deep-sea organisms have lost their eyes or sight completely. Many species of fish, squid and shrimp inhabiting the ocean’s upper 800 metres have developed an assortment of lights on their underbellies. Some, such as the hatchetfish, the appropriately-named lanternfish, and certain squids have an ability to fine-tune the level of blueness to that of the changing levels of illumination from above. The hatchetfish have large tubular eyes which are directed upwards, while an impressive array of gleaming light organs shine blue light downwards. A hatchetfish can illuminate its body with a glow to match light from above, and therefore reduce the risk of its silhouette being seen (Earle 1996).

The bathypelagic fish occur from 1,000 metres down to abyssal depths and are generally highly adapted, often in bizarre ways, to life in a dark, food-poor environment. Some examples include the deepwater angler fish where the males only exist as a parasite or the gulper eel, which has an enormous mouth (Gordon 1994a).

About 80 per-cent of the animals in the dimly illuminated depths at 200 to 1,200 metres have the ability to produce light, which can have different functions for different animals. Some use it to locate food or use as a lure, sometimes to signal a mate, and perhaps some to frighten, distract, or decoy a determined predator (Earle 1996).

2.4 Call for protection

The deep-sea fauna in the northern Rockall Trough off the British Isles has been described by scientists as being remarkable in its diversity. Many species that occur there are known only from depths elsewhere (Gage et al. 1983, 1985; Scottish Association Marine Biological Association - unpublished - in Gage 1992). Because of this, and because of its historical importance in deep-sea research, John Gage concluded in a report for Mobil Oil that this “site is one that should be protected from gross disturbance” (Gage 1992). Protection should include an end to demersal trawling which currently takes place in this area, which is known to a have a heavy impact on the deepwater coral, Lophelia pertusa.

Studies of the Rockall Trough and the Porcupine Sea Bight indicate that the upper and middle continental slope supports a high biomass of fish with a peak at about 1200 metres but a marked reduction in biomass at depths greater than about 1500 metres (Gage and Gordon 1995b). The depth ranges of well over 100 benthopelagic species have been recorded for Rockall Trough (Gordon and Duncan 1985) and for the Porcupine Sea Bight (Merrett et al. 1991a,b). Each individual species has its own depth range, ranging from a few hundred to several thousand metres (Gordon 1996a). This suggests that there is no zonation of the fish fauna, but rather a continual change, with species arriving and departing at different depths (Gage and Gordon 1995b).

The “cut-throat” eel, Synaphobranchus kaupi, so-called because of its large mouth which reaches to its throat and sharp needle-like teeth, is the most abundant of the bottom-living species in the Rockall Trough and Porcupine Sea Bight. It has a depth range from about 400 to 2,500 metres. In common with many other species these cut-throat eels display a ‘bigger-deeper’ life style, such that the juveniles live at the shallow end of the depth range and as they grow they migrate down the slope (Gordon 1996a; UNESCO 1989). Despite their small size they often occur in commercial catches (Gage and Gordon 1995b; Gordon et al. 1995).

The two species of monkfish (Lophius piscatorius and Lophius budegassa) traditionally caught in the shallower waters of the continental shelf, are now being caught down to depths of 800 metres on the continental slope (Copius 1997; Fishing News 1997a). The distribution of Lophius piscatorius is known to go down to 1,200 metres to the west of the British Isles (Gordon and Hunter 1994a). Other species such as the tripod fish found in the Rockall Trough has a very limited range at about 1500 metres (Gage and Gordon 1995b), and is therefore unable to take refuge at greater or shallower depths in the event of changes to its population or habitat from fishing activities.

The blue hake (Antimora rostrata) which makes up a significant part of the biomass of the lower slope of the Rockall Trough (Gordon and Bergtad 1992), was considered by Pohle et al. (1992) as a possible resource off Canada (Gage and Gordon 1995b). The Baird’s smoothhead, Alepocephalus bairdii, also makes up a significant part of the biomass on the continental slope to the west of Scotland. Occurring at depths from about 700 to 1800 metres, but particularly at about 1,200 metres or more it can comprise over 50 per cent of the trawl catch. As it has no commercial value, it is considered a nuisance to fishermen (Gordon and Hunter 1994a; Fishing Monthly 1997).