Cod (Gadus morhua)

 

Cod is overall the most important species in the commercial fisheries in the Baltic Sea. The abundance and distribution of cod has varied considerably over time due to biological as well as anthropogenic causes. Two populations are present in the area: eastern and western Baltic cod. These cod stocks have different morphometric characters and population genetics. They overlap in the area near the Bornholm Island. The eastern cod occurs in the central, eastern and northern part of the Baltic but not in significant amounts north of the Aalands Islands. The western cod inhabits the areas west of the Bornholm island including the Danish Straits. The eastern population is the largest (90% eastern -10% western population) , but some fluctuations in the relative proportions of the cod stocks occur due to differences and changes in exploitation level and recruitment.

 

The main fisheries on Baltic cod are with demersal trawls, high opening trawls (operating both pelagically and demersally) and gillnets. There has been an increase in gillnet fisheries in the 1990s and up to 50 % of the total catch of cod is taken by gillnets.

 

The availability of suitable habitats for cod varies between areas and years depending on the prevailing 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 content or absence of oxygen.

 

The preferred habitat of Baltic cod is the saline water, mainly the water layer defined by the base of the halocline as the upper limit and the lower limit determined by the oxygen content. The reproductive success also relates to the hydrographic conditions.

 

Spawning in the eastern Baltic is confined to areas at least 60 to 90 m deep. The main spawning areas are in the Bornholm Deep, the Gdansk Deep and Gotland Deep. However, successful egg development also require an oxygen level of minimum 2 ml/l in the layer of neutral buoyancy. In periods without major inflows, oxygen depletion of the saline water affects the survival of eggs. This implies that the reproductive success in the Gdansk Deep and the Gotland Deep is more variable than in the Bornholm Deep and Western Baltic spawning areas.

 

The western cod stock lives in more saline waters than the eastern stock and its reproduction is therefore less influenced by the salinity conditions.

 

Cod eggs and larva drift with the wind forcing currents from the spawning area to more shallow nursing areas, where the 0-group cod settle.

 

General characteristics of cod

 

There are two separate cod stocks in the Baltic: the western and the eastern stock (see below). There are indications that the main difference between the Baltic and North Sea cod stocks is the ability of the first to spawn successfully at lower salinity levels. Baltic cod also grow slower than North Sea cod and reach maturity around the age of 3 to 5 years, maturation being later in the Eastern stock.

 

Cod is the most important predator in the Baltic ecosystem. The adult cod have few natural enemies mainly seals. Cannibalism of juveniles occurs. Crustaceans and other invertebrates constitute a major part of the food of young cod. With increasing size the amount of fish in the diet increases with sprat and herring being the major food components.

 

Western Baltic cod

 

ICES regards the stock in 2000 to be outside safe biological limits. The quality of the landing statistics has been low especially in the early 1990s and though data has improved there is still some uncertainty in the estimates of stock size and in the level of fishing mortality. The stock size and spawning stock biomass is rebuilding from a historically low level in 1992. The exploitation rate of the stock is estimated to be high. The fishing pressure on the young ages has increased and the fishery is at present very dependent on recruitment. There is discarding of cod below the legal minimum size and therefore also prerecruit cod are exposed to some mortality due to fishing. However, the quantities of discards are still largely unknown. More than one third of the landings in 1996 consisted of 2 year old cod of which only 22% had attained maturity.

 

Eastern Baltic cod

 

ICES considers the stock to be outside safe biological limits in 2000. The spawning stock declined from historically high levels during 1980-1984 to the lowest level on record in 1992. The spawning stock biomass increased thereafter (1994-95), but remains below the long term average.

 

Due to favourable hydographic conditions, unusually strong year classes in the late 1970s and early 1980s formed the basis for an increase in the stock and an expansion in the fisheries. Catch levels more than doubled and the fishery attracted vessels from other Baltic fisheries and from fleets normally operating outside the Baltic. (In the beginning of the 1980s a marked pelagic fishery on the spawning aggregations was initiated.)

 

In almost all years landings have been far above the levels recommended by ICES. The decline in spawning stock size and landings started around 1984 and continued up to 1992. The fleet capacity and fishing effort have not been reduced accordingly and the fishing mortality has increased during the stock decline.

 

After moderate intrusions of saline water in 1993 and 1994, the hydrographic conditions and recruitment have improved slightly and have resulted in an increase in spawning stock biomass, but the recovery rate of the stock is slow. After a drop in the fishing mortality from the high levels in the 1980s the fishing mortality has increased since 1993 and is now above the F _med . Fishing is not sustainable under the present environmental regime. As in the fisheries for western cod there are discarding of cod below the minimum legal landing size, however also here the quantities of discards are still largely unknown.

 

Management of the cod stocks in the Baltic Sea

 

ICES assesses the western Baltic cod and the eastern Baltic cod separately, while IBSFC manages the Baltic cod as one unit covering the whole Baltic. This is mainly due to traditional fishing patterns as a number of coastal states fish on both stocks due to their geographical position. Also there is annual mixing of the two stocks around Bornholm Island, which is a transition area between stocks. Enforcement of two separate Total Allowable Catches (TACs) would therefore be very difficult.

 

In 1999 IBSFC adopted Resolution on the Long Term Management Strategy for the Cod stocks in the Baltic Sea which shall be reviewed not later than 2003. It foresees to maintain a minimum level of Spawning Stock Biomass (SSB) greater than 160 000 tonnes for the eastern stock and 9 000 tonnes for the western stock.

 

In order to protect the stocks IBSFC has reduced the Cod TAC from 1997 (180 000 tonnes) to 2000 (105 000 tonnes) by 42%.