Cod

(Gadus morhua)

Order: Gadiformes Family: Gadidae

Foto: Lauri Urho

Description: The cod is distinguished from Finland's other fish species by having three dorsal fins and two anal fins. Under the lower jaw, which is a bit shorter than the upper one, there is a chin barbel, which serves as a sense organ. The cod's colour varies depending on its habitat, from greenish brown in shallow coastal waters to greyish in deep open water.

Distribution: The range of the cod extends from the Bay of Biscay to the North Sea, Skagerrak, Kattegat and the Baltic Sea. In the north, the cod occurs in the White Sea and Barents Sea and in the Arctic Ocean up to Svalbard. It is also quite common in the waters of Greenland and Iceland. The cod is also encountered in coastal waters of Canada and the United States. The Baltic Sea supports two cod stocks with different spawning grounds. The western stock ranges from the Danish straits eastwards to the western side of Bornholm island, and the eastern stock eastwards and northwards from Bornholm island, all the way to Finland's marine areas. The boundary between these two stocks is not clear, and mixing is common. In Finnish sea waters, the cod occurs between the cities of Kotka and Vaasa. In years of abundant cod, it occurs throughout Finland's sea area.

Reproduction: Cod spawn in the deeps of the Baltic Sea, far from Finnish waters. Successful reproduction requires a salinity of at least 10−11‰, because at lower levels the fertilized eggs sink to the bottom and do not develop. Likewise the oxygen level must be sufficiently high, and this is only possible when a large pulse of more saline North Sea water enter the Baltic Sea through the Danish straits. Reproduction of cod is currently restricted by the unfavourable saline and oxygen conditions prevailing in the Baltic Sea.

Diet, growth and migrations: Cod caught in Finland's coastal waters usually weigh 0.5−5 kg. The cod is the fastest growing fish species in Finland after the salmon, and actually the species that ultimately attains the biggest size. A 2-year-old is about 25 cm long and weighs 0.5 kg. At 5 years of age it weighs 2−4 kg and at 10 years 8−15 kg. The biggest individuals ever caught in Finland have weighed 30−38 kg. Adults migrate long distances during the growth period in their search for food. At spawning time they gather into large shoals in the deeps, at the edges of deeps and in shallow parts of the open water area. Migrations back to breeding grounds have extended for as much as 1000 km, even in the Baltic Sea. The cod is a strong and fast predator that often preys in the open water area. It feeds mostly on Baltic herring and other fish species, but sowbugs (Saduria entomon and other invertebrates are also ingredients of its diet. A rapidly growing species, the cod has a voracious appetite, which it satisfies with a very diverse diet. In the western Gulf of Finland, for example, its stomach has been found to contain eelpouts, three-spined sticklebacks, fourhorned sculpins, bullrouts, ruffes, smelts and roaches.

Fishing and catches: Cod fishing in the Baltic Sea increased and fishing methods changed in the early 1980s, when the cod stock strengthened as a result of successful spawning. The peak catch was taken in 1984, when it totalled almost half a million tonnes. The cod catch in Finnish waters was also at its highest, almost 10 million kg, in 1984. After that catches declined rapidly, and in 1992 the total catch was down to 73 million kg. The changes in catches and collapse of the stock in the late 1980s were mainly due to unfavourable reproducing circumstances, eutrophication and excessive fishing.

Vulnerability, threats and management: Gadid fishes are among the economically most important fishes in the world, and almost all cod stocks are heavily exploited. Cod fishing in the Baltic Sea, too, has been so efficient that it has affected the stock and increased the likelihood of weak age groups. The poor water quality in the spawning deeps makes stock recovery more difficult, as successful reproduction requires better oxygen conditions than those currently prevailing and also sufficient salinity.

Text version