Deep Water From WGRED 2008 From WGRED 2008

northwards to the Norwegian Sea has undergone changes in the last few decades with an earlier migration occurring in recent years (Reid et al., 2006).

Eggs and larvae of Blue Whiting may be influenced by hydrographic conditions during the spawning season which affect the relative amounts of eggs and larvae drifting to northern and southern nursery areas; a certain spawning area may seed northern areas in one year, southern areas in another (Skogen et al., 1999). There is a positive effect of the large inflow of warm Atlantic water to the Barents Sea (as indicated by a positive salinity anomaly on the

Fugløya-Bear Island section) on abundance of blue whiting in the Barents Sea one year later (Heino et al., 2003).

The strength of year classes as 0-group in the North Sea is only weakly coupled to the strength of year classes in the main Atlantic stock. This suggests either local recruitment or variation in transportation of larvae into the North Sea. Increased inflow of Atlantic water into the Norwegian Sea through Faroe-Shetland Channel (as indicated by a positive temperature anomaly, e.g. Hátún et al., 2005) coincides with increased recruitment, although earlier warm periods have not witnessed a similar increase in recruitment.

For Norwegian Spring Spawning Herring the inflow of Atlantic water into the Norwegian Sea and Barents Sea (NAO-index) seems to influence the condition and hence fecundity of adult fish as well as the survival of larvae (Toresen and Østvedt , 2000, Fiksen and Slotte, 2002, Sætre et al., 2002). There is very good correlation between environmental changes locally at spawning grounds and nursery areas and the large-scale variations in Atlantic water inflow. The survival of larva is also influenced by changes in currents; some years retention areas may be stronger. It has been demonstrated that the tendency of retention may increase larval survival, i.e., the larvae stay for a longer period in warmer water, drifting slower towards the north (Sætre et al., 2002). The environmental conditions also affect the condition of the fish, which again may cause reduced fecundity (Oskarson et al., 2002). The strong year classes have occurred in periods of good condition and high temperatures.

1.2.3.4 Deep Water

There is little commercial exploitation of large invertebrates in this region. Deep-water trawling is known to have a small bycatch of cephalopods, the landings are often reported as

miscellaneous cephalopods. The crab Chaceon affinis occurs at slope depths over the advisory region and is a bycatch of deep-water trawling and netting and a target of pot and net fisheries.

Biogenic habitat occur along the slope, the most well-known of these being formed by the scleractinian Lophelia pertusa a colonial coral, which locally forms large bioherms or reefs, along the slope, on the offshore banks (Rockall and Hatton), on the mid-Atlantic Ridge and on

seamounts

Figure 1.2.63) (Freiwald, 1998; Rogers, 1999). Many areas remain to be surveyed for Lophelia pertusa. Some of these reefs are large, for instance, to the south and west of Ireland several reefs have built mounds of 150 to 200 m height and about 1 km wide are known. The bases of these mounds are comprised of dead coral rubble with some infill; live corals grow on top of the mounds.

Figure 1.2.63: Distribution of deepwater Lophelia reefs in the North East Atlantic and wider (Freiwald, 1998).

A dense and diverse fauna is associated with Lophelia reefs. This includes fixed (e.g.

anthipatarians, gorgonians) and mobile invertebrates (e.g. echinoderms, crustaceans). The species richness of fauna associated with coral reefs is up to three time higher than on surrounding sedimentary seabed (Mortensen et al., 1995). Several species of deepwater fish occur on corals, some are more abundant around corals but possible functional links between fish and coral have proved difficult to demonstrate (Husebo et al., 2002).

1.2.3.4.2 Fish community

Large pelagic fish (tunas, swordfish, some sharks) are not considered in this section.

In the advisory region the two major small pelagic species are blue whiting Micromesistius poutassou and greater argentine Argentina silus. Both occur mainly over the slope and at the shelf edge. Blue whiting is a major prey of some deepwater (e.g. black scabbard fish Aphanopus carbo) and shelf (e.g. hake Merluccius merluccius) fish.

The mesopelagic zone (200–1000 m) has a high diversity of small fish species with striking morphological characters and adaptations such as large mouths, light organs and specialised eyes. The most abundant families are Myctophidae and Gonostomatidae (with Cyclothone, the most common vertebrate genus on earth), these may form up to 50% of a sample catch. The most diverse (number of genus and species) families are Myctophidae and Stomiidae. Many, if not all, mesopelagic fish migrate to feed on pelagic prey in upper water layers during the night.

They return to the depths during daytime probably in order to avoid epipelagic predators. This is another mechanism by which nutrients reach deeper water layers.

A similar, but less abundant, fauna is found in the bathypelagic zone (1000-3000m).

Bathylagidae is the most common family; other common families are Platytroctidae and Searsidae.

The demersal deep water fish community includes several larger species. Species composition

In this deep water region, dominant commercial species at 200–2000m include species such as ling, tusk, roundnose grenadier, orange roughy and deep-water sharks and chimaeriforms (Table 1.2.9) and other species such as redfish, monkfish and Greenland halibut that are dealt with elsewhere. Amongst sharks, Centroscymnus coelolepis and Centrophorus squamosus, the two main commercial species (1 to 1.5 m long) are seriously depleted. The status of a number of smaller or less common species (Centroscymnus crepidater, Deania calcea, Dalatias licha, Scymnodon ringens, Etmopterus spp. Galeus spp. Apristurus spp.) is less clear. Chimaeriforms occur at least down to 3000 m but are more abundant on the upper slope, 400–800m (Lorance et al., 2000). All deep-water shark species and most larger deepwater demersal fish are assumed highly vulnerable to over-exploitation, having a low reproductive capacity. For example, the maximum sustainable exploitation rate of orange roughy is estimated between 1 and 2% of the unexploited biomass (Koslow et al., 2000). Most stocks of the larger species are overexploited. Orange roughy, which forms dense aggregations (Koslow et al., 2000; McClatchie et al., 2000; Lorance et al., 2002) was depleted in the early 1990s in some ICES areas, in

particular off west Scotland and Ireland (Lorance and Dupouy, 2001; ICES, 2004). The blue ling, exploited on the upper slope, was depleted by the 1980s. The status of chimaeriform

populations is unknown. Most of these species are discarded but there is some directed fishing for Chimaera monstrosa on the upper slope.

Table 1.2.9: Broad distributional description of some important deep water fish in the North Atlantic.

SPECIES LATITUDINAL

DISTRIBUTION

DEPTH DISTRIBUTION

(M)

OTHER INFORMATION

Blue ling Molva dypterygi 79°N–48°N 150–1000m Found mostly from

350–500 m depth on muddy bottoms

Ling Molva molva 75°N–35°N 100–1000m Occurs mainly on

rocky bottoms in fairly deep-water, usually 100–400 m

Tusk Brosme brosme 83°N–37°N 18–1000m Far from the shore,

near the bottom, mostly 150–450 m Roundnose grenadier Coryphaenoides rupestris 67°N–20°N 400–2200 m Bentho-to

bathypelagic in about 400–1200 m depth. Large schools at 800–1000 m Orange roughy Hoplostethus atlanticus 65°N–56°S 180–1809 m Inhabits deep, cold

waters over steep slopes, ocean ridges and sea-mounts.

Black scabbardfish Aphanopus carbo 69°N–27°N 200–1800 m Occurs on slopes from 200m off the British Isles to 1800m off Madeira Black dogfish Centroscyllium fabricii 68°N–51°S 180–1600 m Found on the

outermost continent.

shelves and upper slopes, mostly below 275 m

Portuguese dogfish Centroscymnus coelolepis 64°N–48°S 150–3700 m Commonly found on continental slopes and abyssal plains.

Leaf-scale gulper shark Centrophorus squamosus 69°N–54°S 145–2400 m Found on or near the bottom of continental slopes.

Many demersal slope species are not commercial because they do not reach sufficient size while the alepocephalid are large but have a low palatability due to the high proportion of water in their flesh. At 1000 m–1500 m Alepocephalus bairdii is the dominant species by biomass to the west of the British Isles (Gordon, 1986; Gordon and Bergstad, 1992) so that it makes the bulk of fisheries discards (Allain et al., 2003).

1.2.3.4.3 Seabirds

The only breeding birds in the advisory region are on the Azores where, the main species are Cory’s shearwater, Calonectris diomedea (189 000 pairs), common tern, Sterna hirundo (4000), yellow-legged gull, Larus cachinnans (3000), little shearwater, Puffinus assimilis (1200),

Madeiran storm-petrel, Oceanodroma castro (1000), roseate tern, Sterna dougalii (700) and Manx shearwater, Puffinus puffinus (180). These deep offshore waters are visited by migrant birds breeding elsewhere outside their breeding seasons; most are Procellariformes and include northern fulmar Fulmarus glacialis, from colonies around the North Atlantic and sooty Puffinus griseus and great P. gravis shearwaters from the South Atlantic.

long-finned pilot whale Globicephala melas, Risso’s dolphin Grampus griseus, fin whale Balaenoptera physalus and sperm whale Physeter macrocephalus. Those abundance estimates that exist for these species have wide confidence intervals.

See section 1.2.3.5.4 Marine mammals and reptiles

1.2.3.4.5 The major environmental impacts on ecosystem dynamics The deep sea environment is considered to be less variable than surface systems. Moreover, due to the long life span of exploited species, variations in annual recruitment have a relatively minor effect on the standing biomass so short-term variability in the environment is unlikely to have great effects on stocks. The North Atlantic Oscillation may influence the composition of the deep sea fauna over time. It has been suggested that an outburst of sea cucumbers and brittle stars on the abyssal plain of the North Atlantic might be linked to the extremes of NAO seen in these years. It is not known how global warming might change the deep seas in the longer term.