Baltic Sea (Central) - Key Species
Cod (Gadus mohua callarias)
Spawning by cod is confined to the deep basins where egg survival depends on oxygen concentration in the deep saline water layer, where fertilized eggs are neutrally buoyant. The total and spawning stock biomass increased by the end of the 1970s and declined from the historically highest level during 1982-1983 to the lowest level on record in 2004 and 2005. The decline of the stock was a result of an increase of the effort in the traditional bottom trawl fishery, introduction of the gillnet fishery, and decreased egg and larval survival due to unfavorable oceanographic conditions (i.e., low oxygen concentrations for eggs and low food supply for larvae). Since the mid-1980s, cod reproduction has only been successful in the southern spawning areas. The stock size has increased recently (Eero et al. in press), the SSB is considered to be above any candidate precautionary biomass reference points and the stock is harvested sustainably (ICES Advice 2011).
Herring (Clupea harengus membras)
The herring stock in CBS comprises a number of spawning components. This stock complex was at a high biomass level in the early 1970s but has declined since then. The proportion of the various spawning components has varied in both landings and in stock. The southern components that grow to a relatively large size have declined. Atpresent, the landings are dominated by the more northerly components that reach a maximum size of only about 18-20 cm.. Recruitment has been below the long-term average since the beginning of the 1990s, the SSB is currently low and the stock is harvested unsustainably (ICES Advice 2011).
Sprat (Sprattus sprattus)
The species is considered to be harvested unsustainably with declining SSB which is above possible reference points (ICES Advice 2011). The spawning stock biomass was low in the first half of 1980s. In the beginning of 1990s, the stock started to increase rapidly and in 1996-1997 it reached the maximum observed spawning stock biomass. The stock size increased due to a combination of strong recruitment and declining natural mortality (effect of low cod biomass). A notable part of the sprat catch is taken in the mixed sprat-herring fishery, and species composition of these catches is very imprecise in some fishing areas/periods (ICES 2011).
Salmon (Salmo salar)
Baltic salmon stocks had already started to decline in the mid-19th century and at about the sametime, artificial stocking activities were started. The decline of natural stocks has been rapid since the late 1940s, owing to the construction of hydroelectric power plants, damming of rivers and potentially also poorer water quality. As a result of human activities, natural salmon production decreased substantially during the 20th century and several natural stocks have disappeared. Baltic salmon catches have continued to decline and they are now at their lowest level since the 1970s, when recording of joint catch statistics started. However, the natural smolt production of salmon populations has improved in the northern Baltic rivers in recentyears (HELCOM 2009a).
Seals
The Baltic Sea is inhabited by three species of seals. Ringed seal (Phoca hispida) is an Arctic species and mainly colonizes the large gulfs in the northeastern Baltic Sea. The main concentrations of grey seal (Halichoerus grypus) are found in the northern part of the Baltic Proper. The harbor seal (Phoca vitulina) is present only in the southern Baltic Sea. Population size of the ringed seal was about 180,000-200,000 individuals in the early 1900s, but declined to about 5,000 individuals in the early 1970s. At the beginning of the past century, the abundance of grey seal reached 90,000 individuals, but had dropped to some 3,000 individuals by the end of the 1970s. This population has significantly grown since then, currently reaching about 22,000 individuals and the area was re-opened for limited national quota-based hunting (HELCOM 2009a, Ojaveer et al. 2010).
Birds
There are several bird species which are of importance from various aspects. For instance, the following species have indicative value for characteristic population developments: cormorant (Phalacrocorax carbo sinensis): representative of species that have recovered after being nearly extinct by persecution; white-tailed eagle (Haliaeetus albicilla): representative of top-predators that have suffered from chemical pollutants; dunlin (Calidris alpina): representative of waders, which are declining in many regions of the Baltic Sea; barnacle goose (Branta leucopsis): a new species which has recently occupied the Baltic Sea as a breeding area. Typical marine and coastal species include: sandwich tern (Sterna sandvicensis), eider (Somateria mollissima) and razorbill (Alca torda). Species of worldwide concern for which the Baltic is of special importance are Stellers eider (Polysticta stelleri) and long-tailed duck (Clangula hyemalis) (HELCOM 2009a).