Benthic fauna along the Danish NSP2 route

In 2017, seabed samples were collected at regular intervals along the proposed NSP2 route for consequent analysis of the presence of benthic fauna. In addition to sediment sampling, depth, temperature, salinity and oxygen concentrations in the water column were measured at all stations (see section 7.1).

Overall, the 2017 seabed survey in the Danish EEZ showed large variations in both environmental and physical parameters, as well as in the number, abundance and biomass of infauna species. The infauna were, in general, sparsely distributed.

The number of species found per infauna station along the proposed NSP2 route ranged from 0 to 22 species per station. In general, the highest numbers of species per station were found in the shallow Rønne Banke area southwest of Bornholm, while the lowest numbers of species per station were generally found at the stations in the deeper, northern part of the route.

The dominating species present (based on abundance) were blue mussel (Mytilus edulis), the pol-ychaete Pygospio elegans, the polpol-ychaete Scoloplos armiger, annelids (Oligochaeta), the mudsnail (Peringia ulvae), northern astarte (Astarte borealis), the polychaete Marenzelleria, Baltic macoma (Limecola balthica), the oligochaete Tubificoides benedii and the polychaete Hediste diversicolor.

The highest accumulated biomass was found for blue mussel, which was present in 73% of all replicates. The highest accumulated biomass was generally found for bivalves, including blue mus-sel, northern astarte (Astarte borealis) and Baltic macoma (Limecola balthica).

The stations with the highest species richness and diversity were generally situated in areas with high variation in sediment composition. Statistical correlation analyses between species numbers and physical characteristics of the station sediment and water revealed that oxygen availability, salinity, silt/clay fractions and depth were important intercorrelated variables affecting the infauna communities along the sampling transect. The best distance-based linear model, including seven variables, explained 67% of the overall variation in infauna composition. Salinity and the silt-clay fraction had the highest contribution to the overall variation of the infauna composition.

Additional results can be found in the benthic fauna survey report /159/.

7.8.2.4 Biodiversity

In 2017, HELCOM assessed the integrated biodiversity status for benthic habitats using the core indicator “State of the soft-bottom macrofauna community”, but this assessment did not cover the Arkona Basin or Bornholm Basin /104/.

Of relevance to benthic fauna is the eutrophication indicator “Oxygen debt”, which is assessed as

“not good” in the Bornholm Basin /104/.

The percentage of disturbed benthic habitat has been assessed by HELCOM. The disturbed area is estimated based on spatial information of the distribution of human activities connected to the pressures. In the Arkona Basin and Bornholm Basin, the disturbed area is 80-100% /104/.

7.8.2.5 Conservation status

The Danish Red List /150/ does not include benthic marine fauna.

The HELCOM Red List Assessment for the Baltic Sea provides information on the status of benthic species. The Red List includes 19 species of macrofauna categorised as Threatened. One species, the amphipod Haploops tenuis, was categorised as Endangered (EN) and 18 species were catego-rised as Vulnerable (VU). The majority of these occur in the Kattegat or the western-most part of the Baltic Sea, some of them at the border of their distribution area with respect to salinity /151/.

None of the benthic species identified along the proposed NSP2 route are listed as Near Threatened, Endangered or Vulnerable on the HELCOM Red List. HELCOM Red List species observed during the survey included the two amphipods Monoporeia affinis (a Baltic glacial relict) and Pontoporeia fem-orata, both of which are listed as Least Concern.

A HELCOM threat assessment for the Baltic Sea has also been prepared for the characteristic living environments for species, so-called biotopes and biotopes complexes. The Red List includes 17 biotopes evaluated as threatened and one evaluated as critically endangered /160/. None of the benthic habitats identified along the proposed NSP2 route are listed as Near Threatened, Endan-gered or Vulnerable on the HELCOM Red List.

Ten biotope complexes recognised in HELCOM HUB /160/ are also listed in the EU Habitats Directive.

A description of the habitat types as part of Natura 2000 is presented in section 10.

7.9 Fish

Fish are an important component of the marine food chain and ecosystem in the Baltic Sea; they are also a valuable component of the Danish economy (commercial fishery and the value of fish are described in section 7.15). Given this, in combination with the fact that a number of fish species present along the proposed NSP2 route have protected status under national/international

legisla-Fish in the Baltic Sea

Fish are a human food source, but are also prey for marine mammals and seabirds. Fish themselves feed on benthic species, zooplankton, and smaller fish, and are thereby a link between different parts of the food web. When migrating, they also have an ecological role in connecting different sea areas /104//162/.

Fish include both bony fish, a diverse taxonomic group of fish that have skeletons primarily com-posed of bone tissue, and cartilaginous fish, which have a skeleton made of cartilage (e.g. sharks).

The distribution of fish species is mainly determined by salinity levels, and marine fish species dominate the Baltic Proper, while freshwater species occur in the coastal areas and in the innermost parts of the Baltic Sea. Furthermore, the composition of fish communities varies between different regions of the sea in relation to different habitat characteristics as well as differences in salinity, water temperature, oxygen content and nutrient availability. The Baltic Sea fish populations are also affected by fishing, eutrophication, oxygen depletion, high levels of hazardous substances, as well as natural factors such as cold winters and varying salinity levels /104//162//163/.

Fish communities, especially in the coastal areas of the Baltic Sea, underwent dramatic changes during the late twentieth century as a result of both human activities and natural factors /104//161//162//163/. Fish are subject to a number of anthropogenic impacts, as described above, such as enhanced nutrient loads (eutrophication); contamination by heavy metals, organic contam-inants and hormone-like substances; destruction of recruitment habitats; introduction of non-indig-enous species and increased fishing pressure. Climate-driven changes in the salinity, temperature and oxygen content of the water can also affect recruitment and growth. Hydrophysical-climatic variability (i.e. low frequency of inflows of saline oxygenated water from the North Sea and increas-ing temperatures) in combination with heavy fishincreas-ing over the last 10-15 years has led to a shift in the fish community from cod to clupeids (i.e. herring, sprat) /161/. This shift is explained by a weakened cod recruitment and subsequent favourable recruitment conditions for sprat /161/.

Approximately 230 fish species are known from the Baltic Sea, of which 70 are marine species (including lampreys) /104//162/. The number of marine species is low compared with more saline waters.

Marine species are well-adapted to the Baltic Sea conditions and occur in high population densities.

Cod (Gadus morhua), Baltic herring (Clupea harengus) and sprat (Sprattus sprattus) comprise the large majority of the Baltic Sea fish communities in terms of biomass and numbers. These three species are also the most important commercially exploited species, and comprise the majority of the commercial catches in the Baltic Sea. Marine fish species such as cod, sprat, flounder (Platich-thys flesus), plaice (Pleuronectes platessa), dab (Limanda limanda), turbot (Psetta maxima) and brill (Scophthalmus rhombus) prefer more saline areas and are therefore more abundant in the southern Baltic Sea and/or the Baltic Proper. Other marine species migrate from time to time from the North Sea into the Baltic Sea. Such species include whiting (Merlangus merlangus), European anchovy (Engraulis encrasicolus), mackerel (Scomber scombrus) and grey mullet (Liza ramada).

Due to unfavourably low salinity conditions, these marine species are unable to form self-sustaining populations in the Baltic Sea.

Demersal marine fish species, such as flounder, plaice and turbot, live in the central and south-western parts of the Baltic Sea. In the deeper waters, demersal fish are not common due to the low oxygen content and limited presence of benthic fauna. Conversely, shallower waters with high levels of oxygen encourage a more diverse and abundant community of benthic invertebrates as well as small- and medium-sized bottom-dwelling fish species (i.e. gobies, juvenile cod and flatfish). Top predators such as cod and salmon strongly depend on this food chain.

Freshwater species inhabiting the Baltic Sea include e.g. perch (Perca fluviatilis), pike (Esox lucius), pikeperch (Sander lucioperca), bream (Abramis brama), roach (Rutilus rutilus) and burbot (Lota lota). These freshwater species naturally prefer less saline areas and so they colonize mostly the coastal areas, especially in the northern Baltic Sea, where the salinity level is lower.

Migratory fish are species that undergo periodic migrations. Fish that migrate to spawn can be divided into anadromous and catadromous species. Anadromous species live and feed mostly in the sea and migrate to freshwater to breed, whereas catadromous species live mostly in lakes or rivers and migrate to the sea to breed. In the Baltic Sea, anadromous fish include Atlantic salmon (Salmo salar), sea trout (Salmo trutta), whitefish (Coregonus species), vimba bream (Vimba vimba), river lamprey (Lampetra fluviatilis), grayling (Thymallus thymallus) and smelt (Osmerus eperlanus). The catadromous European eel (Anguilla anguilla) migrates a long way from the Sargasso Sea in the north-west Atlantic Ocean into the Baltic Sea area rivers and lakes as a juvenile and back as an adult.

Lastly, species of sharks, rays and chimaeras (cartilaginous fish) have been recorded in the Baltic Sea and Kattegat. Some of the most common include: spiny dogfish (Squalus acanthias), thorny skate (Amblyraja radiata) and small-spotted catshark (Scyliorhinus canicula).

Fish in the Danish section

In the southern part of the Baltic Sea (i.e. covering the Danish sector), the most important com-mercially exploited species are cod, sprat and herring, which comprise 90% of the commercial catches in the Baltic Sea. Other commercially important species, especially in the southern part of the Baltic Sea, include flounder, plaice, turbot, eel and salmon /162//164/.

The most important commercially exploited pelagic and benthic fish species in the southern part of the Baltic Sea also happen to be the most common in the Danish section. These species and their spawning periods are listed in Table 7-17 and described below in more detail.

Table 7-17 Spawning periods for commercially important fish stocks in the Baltic Sea. The acronyms N, S, E and W refer to the spawning population – see text below.

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

1: Spawning periods for spring spawning stocks of different herring populations in the Baltic Sea:

• Western Baltic Sea: March-May;

• Central Baltic Sea: April-May (ICES 25), March-May (ICES 26, Polish coastal waters), April-June (ICES 28), May-June (ICES 29);

• Gulf of Finland (ICES 32): May-June.

Demersal eggs with an adhesive layer that attaches them to the substratum/vegetation in shallow waters /165/.

2: The spawning period for salmon depends on latitude and the geographical locations of the breeding rivers.

Demersal eggs are buried in river-gravel bottoms /166/.

3: There are two different types of flounder in the Baltic Sea: a northern type (N) with demersal eggs, and a southern type (S) with pelagic eggs. The former may reproduce successfully in the northern Baltic Proper, the Bothnian Sea and the Gulf of Finland. The spawning period for the southern stock with pelagic eggs is March-June. The main spawning period for the northern stock is May-July /167//168/.

4: Turbot eggs are demersal at the low salinities occurring in the Baltic Sea /169/.

5: Winter spawning (Nov-Jan) of sprat (win) occurs following summers with exceptionally warm surface water in the Baltic Sea. However, the contribution of winter spawning compared with annual egg and larval production is negligible /170//171/.

6: Spawning in Dec-May /167/.

7: Two populations in Baltic Sea: eastern (E) and western (W) Baltic cod. There are significant inter-annual vari-ations in the spawning time of the eastern Baltic cod, and in the 1990s, a remarkable shift in the timing of spawning from April-June to June-August was observed. The spawning period for the western Baltic cod (also known as the Belt Sea cod) is Jan-April /161//172//173//174/.

In the following sections, a description is provided for each fish species that is considered im-portant in the Danish section. Importance is given based on commercial value and conservation status (see section 7.9.4). The descriptions are based on peer-reviewed literature as well as sci-entific knowledge from e.g. HELCOM and International Council for the Exploration of the Sea (ICES).