Flatfishes (flounder, plaice, turbot and dab)

In the western and eastern areas of the Baltic there is a directed bottom trawl, gill net and danish seine fishery for flatfishes, but to a large extent the flatfishes are by-catches in the cod fishery. There are no sufficient data available to make analytical assessment and forecasts of flatfish in the Baltic. The biological status is mainly evaluated by using catch curve analysis, yield/recruitment analysis and survey data.

Flounder (Platichtus flesus)

Flounder inhabit most parts of the Baltic Sea and show wide tolerance to changes in salinity. It is in general the most important species among flatfish in the Baltic Sea. The species is divided into six geographical stocks within its European distribution range. Flounder is at present divided into six separate stocks in the Baltic of which three stocks have been preliminary assessed.

Flounder is able to spawn in all deeps of the Baltic Sea and coastal areas in the Northern Baltic, where the oxygen content is not a limiting factor. The eggs float at salinities around 10 psu, however, egg development may be successful at even lower salinities, i.e. at the bottom. The larvae inhabit the bottom in shallow coastal areas before they metamorphose. the food mainly consists of crustaceans and ploychaetes.

Indications from analysis of the catch and survey data available as well as preliminary assessments indicate that in general the stocks are moderately exploited and that the stocks are stable or slightly increasing in the eastern part of the Baltic.However, there are also indications that it some areas of the Eastern part of the Baltic the stocks are exploited more intensively and a slight decrease in catches is observed.

Plaice (Pleuronectes platessa)

Plaice occur in the Western Baltic and is rarely found east of the Bornholm Basin. The abundance is low and the commercial importance is limited. The plaice is less tolerant to low salinity and oxygen content than flounder, which affect the distribution pattern. The eggs, which are pelagic need a salinity of around 12-13 psu to float and hatch successfully. Larvae settle in shallow water, where the juveniles stay, until they are around one year old. The food consists of invertebrates, mainly bivalves and polychaetes.

Sub-divisions 22-24 are the most important areas for the plaice fishery in the Baltic. the catches were high in the 1970s but have decreased since the early 1980s. Catches are now low compared with the level in the 1970s.

Turbot (Psetta maxima)

Turbot occur in large parts of the Baltic Sea Proper, but the abundance has been low and reached mean weights lower than in the Atlantic. Because of its high value, turbot is of some importance for the fisheries. Successful spawning is possible in waters with a salinity of 6-7 psu. Spawning takes place in shallow water at depths between 5 and 40 m. Turbot mainly feed on demersal fish like sandeel and gobies, but bivalves and crustaceans are also important components of the diet.

The total catches of turbot have increased since 1984 to a level above 1,000 tonnes. A directed gillnet fishery is developing in Sub-division 25, 26 and 28.

Brill (Scophthalmus rhombus)

Brill is found sparsely in the western Baltic and is of no commercial importance. It is doubtful whether brill spawns in the Baltic Sea, but it is common in the Belt Sea (Danish Straits), the brill in the Baltic Sea may derive from these sources. Reproduction and habitat is similar to what is described for turbot. Only exist preliminary data on the total catch.

Dab (Limanda limanda)

Dab abundance in the Baltic is low and the species is of limited importance to the fishery. A salinity of minimum 15 psu and a sufficient oxygen level is necessary for successful spawning of the dab, which means that reproduction is restricted to the Western Baltic. The eggs are pelagic, and the larvae and small bottom stages remain in rather deep water. The total catches have been rather stable during 1980 around 2,000 tonnes per year, but have increased slightly in recent years. The majority of the catches are taken in Sub-division 22.