Cod
Cod is a cold-water species which prefers water temperatures around 10°C. It used to be common in the North Sea, however it has been seriously overfished since the early 1990s. It can live at depths down to 600 meters, however is usually found between 150 and 200 meters deep. Cod can grow to a length of 1.6 meters and weigh 40 kg. It hunts other fish, crustaceans, cuttlefish and worms. Cod forms schools in shallow water and makes limited migrations in search of food and to spawn. Cod is seriously overfished in the North Sea.
Cod is easy to distnguish from other cod species from the several barbels on its chin and from the light colored line along its side. The colors of cod vary from place to place. In 1895, the largest cod ever caught was by the North American coast: 95 kilograms and 1.8 meters long. The average weight of cod nowadays is one kilogram, the larger specimen are rarely heavier than seven kilograms. This is all due to overfishing.
Reproduction and growth
Female cod can lay 500 eggs per gram body weight. For a fish weighing 10 kilograms, that means 5 million eggs. Cod spawn in the winter months. The transparent 1.5 millimeter large eggs rise to the upper water layers, where they hatch after 10 to 30 days, depending upon the temperature. Extremely young cod eat copepods. After reaching a length of 7 centimenters (within 3 - 5 months), they descend to the water layer lying just above the sea floor. This change in habitat also means a change in diet. Here they eat benthic animals (particularly shrimp and crabs). Adult animals eat mainly other fish (herring species, lesser sandeel, flatfish and other cod species). They do not eat much during the winter or spawning season.
| First and second year cod from the southeastern North Sea gather in the winter in the shallow coastal water and the Wadden Sea. In the summer, they disperse over the deeper parts of the North Sea. Young cod are more tolerant of higher water temperatures than adult animals and profit from the food richness in shallow waters. |  |
Cod as consumption fish
In 1994, cod was the 13th most caught fish species in the world, but it was even higher on the list in previous years. In 1990, 1.5 million tons of cod were caught worldwide; it declined to around 1.2 million tons in 1992 and 1994. Several thousands of tons of cod are farmed for consumption in Norway every year. Farming cod will increase in the future now that a special feed has been developed for young larvae up to forty days old, consisting of shrimp and algae. The young fish eat this food, in comparison to the dry feed they used to get.
In the Scandinavian countries, cod is often sold as dried (stockfish) or dried and salted (dried cod). Even Portugal is fond of salted and dried cod. Cod-liver oil comes from the liver of cod or halibut. Cod wastes are often processed into fishmeal.
Because the cod is so threatened nowadays, the four most prominent restaurants in the Province of Utrecht decided not to offer cod on the menu at the end of 2004. In this way, they hope to make it clear to the client that 'the choice in the future can be even more limited if nothing is doen about fishing the sea until it's empty'.
Distribution of cod
The distribution area of cod contains the Northern Arctic Sea, the northeastern Atlantic Ocean, the Baltic Sea and the North Sea.
The distribution of cod in the North Sea is divided in areas according to age. Most of the cod found along the Dutch coast, in the German Bight and to the east of Northeast England are one year old. Two-year old cod are also swimming around here, but they also migrate further north in the North Sea. Three-year old fish or older can only be found in the northern sections of the North Sea.
Cod fisheries in the North Sea
Around half of the total cod population keeps being fished in years by intensive fisheries, resulting in an age structure of the cod population which is totally out of balance. The fisheries is the number one cause of death for cod, with 90%. Nowadays, North Sea cod does not grow older than 5 years. A large number do not even reach 2 years of age, while cod in the North Sea used to get as old as 25 years and weigh 40 kilograms. Nowadays, they are rarely heavier than 1 kilogram. Cod are only sexually mature after a year, so that this development endangers the reproduction of cod and consequently the species.
Since 1900 up till the 1960s, the total supply of cod from the North Sea was relatively stable around 100,000 tons. Death among the cod due to the fisheries increased since 1963. A peak of 340,000 tons was reached in 1972. Since 1981 (supply 300.000 tons), the supply has continually declined to around 30,000 tons in 2005.
The cod stock in the North Sea
The North Sea cod stock is far below the safe biological minimum. In addition, the stock is consists of a number of poor year classes (as far as numbers go). The last strong year class was born in 1985. The spawning stock in early 1993 was already alarmingly small: around 60,000 tons. At least 150,000 tons of adult cod are necessary in order to obtain reasonably strong year classes. Between 1993 and 1996, the stock seemed to recover but there has been a strong declining trend since 1997. The spawning stock in the North Sea in 2002 was an historic al ltime low with only 30,000 tons. Just like in previous years, fewer cod is allowed to be fished in 2007. There is still no total closure of the cod fisheries in the North Sea. In addition, the European Commission wants to close fishing grounds in order to save the seriously threatened cod. As compensation, fishermen are allowed to catch more herring and blue whiting.
In the period 1994-1996, the cod stock increased from 60,000 to 100,000 tons. This was still under the safe biological minimum of 150,000 tons. Due to the fairly weak year classes which became adults in this period and the continually high fishery pressure, the spawning stock declined further to the present average of around 50,000 tons. The new year class in early 2008 was also too weak.
Will closure of the cod fisheries save the cod?
There is a chance that the cod stock will never be able to recover. The ecological balance in the sea changes due to the immense decline of the population, whereby too little suitable food remains to allow the population to increase again at a later time. Even if there were to be a total stop to catching cod, the numbers can still continue to drop. Research data from Canada show that this irreversible collapse of the population occurs in fish species that reproduce more poorly when there are fewer specimen. This is called the Allee effect.
This effect can be seen in Canada. Cod off the Canadian coast eat primarily one kind of fish, capelin. In fact, they eat mostly only the smaller specimen of capelin. When fewer cod hunt these small capelin, then the fish are able to grow larger. After a period of time, there are more larger capelin (that eat even more) and there is less food available for them. The capelin species reacts to this problem by releasing fewer eggs, whereby there are fewer smaller capelin available for the cod. The food situation for cod by the Canadian coast has changed radically due to a lack of small capelin. The ecological system seems to have two stable situations: one with lots of cod and lots of capelin and one with only a small amount of these species. The ecosystem has gone from the one situation to the other due to overfishing and there is no (natural) way to go back.
The question is whether this development is also taking place in the North Sea and British coastal waters. Firstly, capelin is not the source of food for cod in the North Sea. Secondly, North Sea cod is not dependent upon one type of prey and therefore can switch over to other food if the ecosystem were to change. Furthermore, ecosystems with two equilibriums are a fairly new terrain for biologists to study. There is little known and it is very possible that a factor such as climate change can also influence the fish stock fluctuations in the long term.
Sexual maturity of cod changing?
Because of the fisheries, the age of sexual maturity of cod is continually younger, just like with plaice. The high fishery pressure, whereby large numbers of sexually mature fish are caught, means smaller cod remain. Small (young) fish which are able to reproduce is a particular advantage because they are not caught. During past years, this selection process has decreased the age of sexual maturity of cod from 7 to 4 years old.
Names:
Dut: Kabeljauw (dogger, gul, kolvis, labberdaan, muitje, stockvis)
Lat: Gadus morhua
Eng: Cod (codling)
Ger: Kabeljau (Dorsch)
Fren: Cabillaud, Morue
Dan: Torsk
Source: de Vleet, Ecomare
