Diseño del sistema de control.

Plankton are small plants and animals that move little in relation to the water mass in which they live. Phytoplankton are microscopic, freely suspended, normally single-celled algae. They exist mainly in the upper layers of the sea, where light supports photosynthesis. In the winter only small amounts of phytoplankton occur in the water, but in spring a bloom occurs with the increasing sunlight, higher nutrient values in the water, and formation of a stable surface layer. The spring bloom starts in March-April and lasts until the end of June. During the summer the amounts of vegetable plankton decrease as grazing by zooplankton accelerates and nutrient salts become in short supply. Between August-September-October a new bloom occurs, but the autumn bloom is usually smaller than its spring counterpart (Colebrook & Robinson 1965). Joint & Pomeroy (1992) estimate primary production in the central North Sea Area to be 100–150 gr. C/m2_/year.

Zooplankton belong to different fauna groups, including single-cell animals, crustaceans (copepods), jelly fish, and fish. They consist of two groups, those organisms that live as plankton their entire life (holozooplankton), and those that pass through a plankton stage during parts of their life cycle (merozooplankton). Small crustaceans dominate the first category, and can make up 70–80 per cent of the

Fixed installation Sampling point THC 1996 0 0.5 1 2/4-A 2/4-T 2/4 T Figure 14 Total hydrocarbon content (oil) around Ekofisk Complex. Red indicates values above 21 mg/kg, orange 16–21 mg/kg, yellow 11–16 mg/kg, and green 6–11 mg/kg (from RF and

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biomass of animal plankton (NSTF 1993A). Spawn and larvae of fish constitute an important part of the other category. Our knowledge of the breakdown of animal plankton in the North Sea is very limited. In general it can be said that the volume of animal plankton follows the bloom of vegetable plankton, with a lag of a couple of weeks. In the North Sea, copepods predominate (Fransz et al. 1991). Particularly numerous are the copepod, Calanus finmarchicus, which are carried with the current from the North Atlantic and drift with the upper water layers from March until August (Colebrook & Robinson 1965). This creature spends its winters in deep water, rising to the surface in February-March. The largest volumes occur in the upper layers from March to August. Generally large numbers of copepods occur for much of the year in the middle North Sea region, except during the winter months December, January, February and part of March (Colebrook & Robinson 1965). Simultaneously with the culmination of the biomass of the copepods, there are also large volumes of pelagic larvae and fry from many species of fish in the water. The copepods are a key food for them all. The copepod is also the key food source for adult herring. Krause & Martens (1990) estimated the production of zooplankton biomass to be greater than 10 grams dry weight per square metre of water in the period 2 May–13 June 1986.

5.3.2 Fish

The North Sea is one of the most richly stocked fisheries in the world. About 5 per cent of the total fish harvest in the world comes from the North Sea. The main pelagic species are herring, mackerel and sprat (Anon 1997-b). Of demersal species the significant fish are cod, saithe, haddock, whiting, plaice and sole, sandeel and Norway pout (Anon 1997-b).

Most populations of bottom-dwellers in the North Sea have now been reduced to very low levels due to over-fishing. The spawning population of herring has been very much reduced for many years, although its prospects are reported as improving (Toresen, 1998). For cod the spawning population remains very low and recruitment is generally poor. The mackerel stock has gone through a historical low point, but the Institute of Marine Research now reports that the decline has ceased. Also the stocks of haddock, whiting and saithe (coal fish) are generally low compared with previously, but at least an increase is predicted for haddock and whiting (Toresen, 1998).

In this section of the North Sea occur eggs and larvae of many fish species. Mackerel, Norway pout and sandeel all have important spawning grounds here, and large volumes of spawn from the North Sea herring drift into the area. There are also larval stages of plaice, cod and sprat.

Cod

Cod spawns all over the North Sea. The highest egg concentrations are normally in the English Channel, Dogger Bank, and along the Scottish coast. Important growth areas are the German Bight and

south-eastern part of the North Sea.

Until the sixties the annual harvest of cod in the North Sea was 50,000–100,000 tonnes. From then on the harvest increased to a peak of 350,000 tonnes in 1972. This volume was far more than the population can withstand, and

in recent years catches have been significantly less (approx 100,000 tonnes). The Norwegian catch is very modest indeed: in 1997 Norway took some 6,500 tonnes of a 9,850 tonne quota.

The status of cod in the North Sea is described as «worrying» (Anon. 1997-b).

Herring

Herring in the North Sea belong to several stocks, the autumn spawning stock being the largest in size (Anon 1997-b). Spawning takes place on the fishing banks east of the UK and Shetland/Orkneys in August (Table 8). The herring larvae drift with the currents into the North Sea and Skagerrak, staying normally within the top 30–40 metres of water, and making diurnal vertical migrations through the water column. Normally larval stages of autumn herring will drift across the whole North

Sea to Skagerrak and German Bight. They will therefore encounter the Ekofisk area in the autumn and winter. Spring-spawning herring spawns, among other places, in the western parts of the North Sea and in the Skagerrak and Kattegat (Pethon 1989, Anon 1997-B).

The spawning population of North Sea herring has shrunk from an estimated 5 million tonnes after the second world war to less than 500.000 tonnes estimated in 1997. Extensive over-fishing is the culprit causing this collapse, and today the North Sea herring stock is considered fairly vulnerable (Toresen 1997).

Mackerel

The North Sea stock of mackerel has its primary spawning grounds in the central areas of the North Sea (Bakke et al. 1987, Anon. 1978), but also spawns northward along the Norwegian coast. Mackerel spawns between the middle of May and end of July (Table 8), with a maximum occurring in mid-June. Eggs and larvae disperse with the current systems in the area (Bjørke et al. 1990). The eggs generally occupy the top 10

metres, and hatch after 3–7 days. Newly hatched larvae drift with the surfa- ce water, most being found between 10 and 20 metres down.

Sandeel

This industrial fish is found over much of the North Sea. It occurs mainly in shallower parts where the bottom is sand or gravel. The primary areas are from the Viking Bank across to the Danish coast along the Egga Edge, across Dogger Bank, and in the coastal areas of Shetland, England, and Denmark (Lahn-Johannesen 1987). Sandeel likes to spawn on a sandy bottom, and spawning and hatching occur during much of the year depending on variety and population. Spawning occupies a large area in the central and southern parts of the North Sea. The larvae live freely in the water column before ultimately diving for the bottom. The spawning

grounds for sandeel in the Norwegian sector are not well known, despite the industrial significan- ce of this fish (Anon 1997-B). Spawning has been recorded in the Ekofisk Area (Figue 14).

5.3.3 Seabirds

The Ekofisk Area lies outside the places in the North Sea where the greatest concentrations of seabirds are observed (NSTF 1993-a, Carter et al. 1993). The variations in occurrence of seabirds in the open sea are closely connected with their activities, which are controlled in turn by the seasons (Table 9).

As June turns into July an immense number of seabirds migrate from their nesting sites and into the central parts of the North Sea. To begin with the young of the auks are unable to fly, and there- fore they migrate on the surface. In company with moul- ting adults they therefore swim across the North Sea to their winter quarters in Skagerrak and Kattegat. From July to September a large number of birds will be found in the central parts of the North Sea. The auks (guillemots, auks, little auks, puffins) are the most numerous.

Other species too, like Fulmars, Skuas, Gannets and Kittiwakes occur in large numbers. As autumn draws on into winter (September-February) these types remain in the central North Sea, though large numbers have also moved further east to Skagerrak and Kattegat or the southern parts of the North Sea.

Seafowl are not expected to suffer in any way from the offshore removal processes. If underwater blasting is carried out, individual birds may be hit. Table 8

Spawning period (colour) for selected fish populations in the North Sea (after Pethon 1989, for herring also Anon 1997-B).

Species Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mackerel Sandeel Herring Plaice Cod Norway pout Sprat Saithe Haddock Whiting Sole Activity J F M A M J J A S O N D Nesting

Swimming migration (aucks) Moulting

Migration (flying) Wintering

Table 9

Key periods in life cycle of seabirds (after NSTF 1993B) (colour)

Figure 15

Spawning areas for selected fish species in the North Sea. Ekofisk is marked with a dot (•). Data taken from MRDB®.

Saithe Cod Sandeel Whiting Haddock Mackerel Herring Norway pout Plaice

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5.3.4 Marine mammals

Several types of whale are seen regularly in the North Sea, but most observations are sporadic and the migration patterns and sizes of the stocks are not well known (Reijnders & Lankester 1990). The predominant species in the North Sea are the Minke whale and Pilot whale. The former is mainly observed north-west of Ekofisk. Observations of Minke have also been made within the Ekofisk Area (Øien 1990). The whales that occur in the North Sea are believed to make up a tiny portion of the total stock, which largely comes from southern parts of the North Atlantic and follows the North Sea up to the Barents Sea. The smaller whales (like the porpoise) largely stay close to shore, although they also occur in the North Sea and can therefore be expected to visit Ekofisk. Seals are also occasional guests, occurring individually in the area. Marine mammals are not expected to suffer from any of the operations associated with cessation.