Baltic cod

Baltic cod (Gadus morhua) is a demersal, marine species, which mainly feed on molluscs, crabs, starfish, worms and small fish (i.e. herring and sprat, but also cod juveniles and eggs).

Distribution

The abundance and distribution of Baltic cod has varied considerably over time owing to natural as well as anthropogenic causes.

Two populations are present in the area: The western Baltic cod stock (Gadus morhua morhua) and the eastern Baltic cod stock (Gadus morhua calarias). These stocks have different morphological characteristics and population genetics. The eastern cod stock occurs in the central, eastern and northern Baltic Sea, while the western cod stock inhabits the areas west of Bornholm, including the Danish straits. The two stocks overlap in the area around Bornholm.

The eastern population is the largest, accounting for approximately 90% of the cod stocks in the Baltic Sea; however, the sub-population of the Gdansk and Gotland Deep is considered seriously reduced and has not spawned since the 1980s /172/. The eastern Baltic cod stock declined from its historically highest level in 1982-1983 to the lowest level on record in 2004-2005 /172/. The decline is attributed to reduced reproductive success in combination with increasing fishing pressure. ICES reports that the eastern Baltic cod stock is still at historically low levels, even though the stock has increased continuously since 2005 /172//175//176/.

The western Baltic cod stock has been decreasing over the last three generations, but the decrease has levelled off since the cod management plan was put into action in September 2007. ICES clas-sifies the stock as being at risk of reduced reproductive capacity, suffering from too high fishing pressure /175/. While the abundance of smaller individuals has increased in recent years, the num-ber of larger individuals has decreased. The reason for the reduced numnum-ber of larger cod is unclear, but may be associated with either increased mortality of older cod and/or low individual growth.

Studies indicate that size-specific trawl fishing, where small cod are released from the trawl, only catching large cod, could further hamper growth, since the result is a large biomass of small cod with high intraspecific feeding competition and hence a lower growth rate

The availability of suitable habitats for cod varies between areas and years depending on the pre-vailing environmental conditions. The fish may be periodically or permanently absent in some areas, e.g. in the bottom layers of deep basins due to low or no oxygen content.

Spawning

The female cod spawn in batches and as the time between releases of batches varies, there are no fixed periods for various spawning phases. Egg fertilisation, development and larval periods are completed in parallel to one another all through the spawning period. The time from fertilization until hatching varies between two and four weeks, depending on temperature. A few days after hatching, larvae avoid critical oxygen levels by migrating vertically into the upper water layers with sufficient light conditions and prey concentrations for feeding /177/.

The spawning grounds and nursery areas for Baltic cod are shown in Figure 7-35 /175/. Cod under-take migrations between spawning and feeding areas and have a strong homing behaviour. Spawn-ing and nursery areas in the Baltic Sea include the deeper regions of the Bornholm Basin, Arkona Basin, Kiel Bay, Fehmarn Belt and the Mecklenburg Bay. The Arkona Basin is used for spawning by both western and eastern stocks, whilst the Bornholm Basin is used by only eastern Baltic cod /178/.

In order to enable undisturbed spawning, the cod fishery is regulated by a seasonal closure from 1 May to 31 August in areas shown in Figure 7-35. Closure for all fisheries in a specific part of the main spawning area in the Bornholm Deep has been implemented during the main spawning sea-sons since the mid-1990s /172//175/. The NSP2 route crosses this area in both Danish and Swedish waters, for a total stretch of 65 km. Of these, 15 km are within the Danish EEZ.

Figure 7-35 Traditional spawning grounds and nursery areas for Baltic cod. During the last decades, cod spawning has taken place only in the southern parts of Bornholm Deep and in Slupsk Furrow (the small area east of the Bornholm Deep) /182/. Since the late 1980s, spawning in the Gdansk Deep and the Gotland Deep was almost eliminated /175/. The figure also shows areas which are closed to fishery due to spawn-ing.

Successful egg development requires a minimum oxygen level of approximately 3 mg/l seawater and salinity higher than 11 psu in the reproductive volume, at which the buoyancy of cod eggs is neutral /179//180/. The volume of water where this is fulfilled is called the “reproductive volume”, and the location of this differs due to external factors affecting salinity and oxygen levels (e.g.

storms and inflow of marine water). In periods without major inflows, oxygen depletion of the saline water affects the survival of the eggs. The spawning of the eastern Baltic cod is generally confined to areas 40-70 m deep, e.g. in the waters of the Bornholm Deep and previously in the Gdansk Deep and Gotland Deep. As the Gdansk Deep and Gotland Deep are considerably farther from the saline water inflow from the North Sea, the salinity, oxygen and halocline depth conditions in these areas are more variable than in the Bornholm Deep, which directly affects reproductive success /181//182/. Since the mid-1980s, cod reproduction of the eastern stock has only been successful in the southern spawning areas, mainly in the Bornholm Basin /175/. The Arkona Basin is used for spawning by both the western and eastern cod stocks.

In the mid-1990s, despite improved hydrographical conditions for egg development, there was a lack of recovery in recruitment related to poor larval survival, apparently due to a lack of food availability. A decline in the abundance of the copepod Pseudocalanus spp., related to lower salinity, limited the food supply of first-feeding cod larvae /161/.

The key factors governing the spawning time are water temperature during the period of gonadal maturation, density-dependent processes related to the size of the spawning stock and food avail-ability. The age structure of the spawning stock is also suggested to have an additional effect. The western Baltic cod spawns in the period of January-April, whilst the eastern Baltic cod spawns in the period of April-September. The spawning time of the eastern Baltic cod is subject to inter-annual variations in the Bornholm Basin, and has been thoroughly studied /173/. During the 1970s and late 1980s, peak spawning took place between the end of April and mid-June, and in the 1990s a shift in the spawning time to the end of July was observed.

Conservation status

Cod is classified as vulnerable on the HELCOM Red List (see section 7.9.4).

7.9.2.2 Herring

The Baltic herring (Clupea harengus membras) is a subspecies of the Atlantic herring (Clupea ha-rengus). Herring feed primarily on zooplankton, although older herring may also feed on fish eggs and fry.

Distribution

Herring occur in large schools throughout the entire Baltic Sea, with clearly distinct stocks in differ-ent areas. Herring tend to make seasonal migrations between coastal archipelagos and open sea areas, staying close to the coast during spring and autumn, while spending the summer in produc-tive open sea areas. Older herring move into deeper waters of the open sea during the winter, whereas younger individuals tend to remain close to the coast. The abundance and biomass of Baltic herring generally has decreased during the last 40 years owing to changes in the amount and composition of zooplankton and overfishing /162//183/. However, this general trend has been re-versed, albeit slowly, since the beginning of the year 2000 /175/.

Spawning

Different herring stocks have different spawning periods, and herring populations in the Baltic Sea include both spring and autumn spawners. Previously, autumn-spawning herring dominated the general herring population, but this changed in the 1960s. Since then, spring spawners have dom-inated the population, with spawning from March-June (Table 7-17). Herring spawn in coastal areas in most parts of the Baltic Sea /183/, see Figure 7-36.

Figure 7-36 Herring spawning areas and migration routes in the Baltic Sea /183/.

Conservation status

Herring is not classified as threatened on the HELCOM Red List (see section 7.9.4).

7.9.2.3 Sprat

Sprat (Sprattus sprattus) eat zooplankton as well as cod eggs and fry /183/, and sprat larvae have a strong preference for the copepod Acartia spp. as their main food source.

Distribution

Sprat live in schools throughout the Baltic Sea. Sprat is an open-sea species that is rarely found along the coast. Sprat migrate to open water areas, seeking out warmer water layers during differ-ent seasons and avoiding areas where the water temperature drops to less than 2-3°C. During harsh winters, the distribution of sprat shrinks, entailing an increase in density in some distinct regions /183/.

Spawning

Figure 7-37 shows the spawning area for sprat. The spawning of sprat and the distribution of their planktonic eggs are restricted to the central part of the deep basins inthe Baltic Sea, with the highest concentration in the upper part ofthe halocline, typically between 45-55 m. Spawning oc-curs from April-July or November-January, depending on the geographical area (Table 7-17). Winter spawning (Nov-Jan) of sprat is observed 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. Years of strong larval displacement towards the southern and eastern Baltic Sea coasts indicates weak recruitment conditions, while years of retention within the deep basins are associated with relative recruitment success /170/.

Figure 7-37 Spawning areas of sprat /184/.

The Bornholm Basin is an especially important spawning ground for sprat /184/. As egg-specific gravity changes during the season, there is accordingly a shift in vertical egg distribution in the Bornholm Basin, i.e. from 50-80 m in April to 25-65 m in May/June /185/.

The abundance of Acartia has drastically increased since the 1990s in parallel with the increase in temperature. This may have led to generally higher sprat larval survival /172/.

Conservation status

Sprat is not classified as threatened on the HELCOM Red List (see section 7.9.4).

7.9.2.4 Flounder

Flounder (Platichthys flesus) is a demersal flatfish, which feeds on bivalves (e.g. blue mussel), other benthic invertebrates (polychaete worms, gastropod molluscs) and small fish.

Distribution

Flounder inhabit most of the Baltic Proper, except for the deeper parts of the Gotland Deep, and show a wide tolerance to changes in salinity. Flounder are presently divided into six separate stocks in the Baltic Sea. The stocks are moderately exploited and are stable or slightly increasing in the eastern Baltic Sea /175/.

Spawning

There are two ecological types of flounder in the Baltic: one southern, with pelagic eggs, and one

In the southern Baltic Sea, flounder migrate between coastal feeding areas and spawning areas in the deep basins (spawning in March-July, see Table 7-17). They have larger, pelagic eggs adapted to floating, despite low salinity. Salinity determines the buoyancy of the eggs, and the pelagic eggs require a minimum salinity of 10 psu in order to float. Furthermore, the success of spawning also depends on oxygen content. Oxygen content below 1 ml/l is critical for egg survival /168/.

The other ecological type of flounder occurs in the northern Baltic Sea, where flounder are more stationary and spawn in shallow banks or coastal areas. Their eggs are smaller, more thick-shelled and demersal. The minimum required salinity is lower, only 6-7 psu, and the main spawning period is in May-July. The larvae inhabit the bottom in shallow coastal areas before they metamorphose /168/.

The onset of spawning in the spring is influenced by rising temperatures. Consequently, the spawn-ing period varies across different areas in the Baltic Sea; for example, in the Kattegat, spawnspawn-ing starts in February-April, while in the Gotland Basin, spawning occurs in April-May /168/.

Conservation status

Flounder is not classified as threatened on the HELCOM R ed list (see section 7.9.4).

7.9.2.5 Plaice

Plaice (Pleuronectes platessa) is a demersal flatfish that feeds on molluscs and polychaete worms.

Distribution

Plaice inhabit the western Baltic Sea and are rarely found east of the Bornholm Basin. Plaice are less tolerant to low salinity and low oxygen content than flounder are, which affects their distribution patterns.

Spawning

Fluctuations in abundance are assumed to be mainly caused by migration of plaice from the Kattegat into the western Baltic Sea, but opportunities for successful plaice reproduction exist regularly in the Bornholm Basin /175/. Plaice spawn in December-April (Table 7-17).

There is only limited information about the potential effects of salinity on stock development of the Baltic plaice population, but it has been observed that the stock recovered during the 1950s at the same time as major saline water inflows occurred.

Conservation status

Plaice is not classified as threatened on the HELCOM Red List (see section 7.9.4).