DESCRIPTION OF THE SPECIES AND ITS USE IN AQUACULTURE

Cod (Gadus morhua L.), the most important member of the Gadidae family, was formerly abundant on both sides of the Atlantic. Due to over fishing and environmental changes there has been a substantial reduction in stocks over the past decade. A fishing moratorium was introduced for the west Atlantic stocks (George Banks, Grand Banks, New Foundland – Nova Scotia, Canada) in the early 1990s and these stocks have not yet recovered. East Atlantic stocks are in better shape – ranging from South-East Greenland, Iceland, the Faroe Islands, the North Sea, the Baltic Sea, along the Norwegian coast and in the Barents Sea. The North East Atlantic cod stock in the Barents Sea is by far the largest and is managed within the limits of sustainable harvesting. It is mainly fish from this stock that are used for CBA in Norway. In total, cod harvest in the Atlantic has been reduced from 2 500 000 tonnes to less than 1 000 000 tonnes during the last 20 years.

 

Spawning, eggs, larvae and juveniles

Spawning takes place along the northern part of the Norwegian coast from February to May. The main area is Lofoten and the peak spawning period is in early April. From the age of six to seven years the cod recruit into the spawning part of the population. The females ovulate eggs every second day for five to six weeks. The spawning behaviour ends with male and female swimming belly to belly shedding eggs and sperm. The amount of eggs shed by the females equals approximately 500 000 eggs per kilograms of bodyweight. The pelagic eggs hatch two to three weeks after spawning, depending on the temperature, and the larvae are 4–5 mm in length. Within a week the larvae start eating phyto- and zooplankton. The nauplii of the Copepode (Calanus finmarchicus) are the most important prey. At 10–12 mm the larvae go through metamorphosis and become juveniles. Transported by the coastal and Atlantic current, the juveniles become demersal at 5–15 cm in length. This takes place along the coast of Finnmark County and in the Barents Sea. The juvenile cod are called the “0-group” during their first year and the varying size of each year-class is mainly decided by the conditions during the first autumn.
In the late 1980s the artificial production of cod fingerlings was limited and the number available for the aquaculture industry was unpredictable (Olsen and Soldal, 1988). In 1988, 600 000 0-group juveniles were captured for aquaculture purposes. The juveniles were caught in shallow water (10–20 m) with a small-meshed seine net. Capture mortality was low (<1 percent) and the cod were easily weaned to a moist pellet. Unfortunately, the juveniles were contaminated with a bacterial disease (Vibrio salmonicida) and only a small portion of the fingerlings were grown to slaughter size (Jorgensen et al., 1989). The capture of wild fingerling was not repeated in Norway, but similar experiments have been conducted in Iceland (2003–2005).

Capelin cod

At the age of 3–5 years, immature cod follow the capelin (Mallotus villosus) on their spawning migration from the Barents Sea to the coast of Finnmark County (see Figure 6). The cod feed on capelin for several weeks and are therefore referred to, during this period, as the “capelin cod”. The capelin cod are characterised by a very low terminal post-mortem pH, resulting in a very poor processing quality. Their loose muscle structure gives low yields in mechanical operations such as filleting or splitting. It is mainly this part of the North-East Arctic cod stock that is used for CBA. Capelin cod has shown great potential for growth and quality improvement due to controlled feeding in captivity.