Changes

Spawning occurs throughout the geographical range mostly in offshore waters but also in small fjords and bays. The cod along Norway and Iceland both spawn from January through March. In the North Sea spawning continues through April while Baltic cod start spawning late in March but continue through October. Eggs are transparent and measure around 1.5 mm in diameter. Baltic cod produce larger eggs, possibly as an adaptation to float in low salinity waters (10 ‰). The eggs hatch within a month in normal sea temperatures. After another three months, the pelagic larvae settle during the autumn at suitable sea grounds and take up a permanent demersal life. Maturity is attained at around age 3 in the south (Baltic, North Sea, Irish Sea) but not until age 6 in the colder, northern waters (Norwegian, Icelandic waters). A female cod will lay up to 500 000 eggs per kg of her own weight. Consequently, a 3 -year-old female of half a kg can produce 250000 eggs; an 8 -year-old female of 5 kg can produce 2.5 million eggs per year. A cod can live to over 25 years of age and weigh over 90 kg. Although the fecundity decreases at large weights, a heavy female will still produce an impressive number of eggs. A record 9 million eggs was found in a 34 kg female. Virtually all of these eggs, and subsequent larvae, will die during the first 3 months of their life. In theory, only 2 eggs need to survive to maturity if the number of fishes in the sea should remain constant. Spawning sites are generally located upstream of a stable hydrographic gyre that will transport eggs and larvae to their nursery grounds. Tagging studies have shown that mature cod migrate annually from feeding grounds to these specific spawning sites. The migrations can be extensive. Cod that have been tagged in the northern Baltic have been recaptured more than 500 km south at spawning sites in the southern Baltic. Observations and simulation studies indicate that eggs, larvae and juvenile cod generally disperse northward after spawning. Thus eggs and larvae drift to nursery grounds, juvenile cod migrate to feeding grounds and mature cod migrate to spawning grounds. This lifecycle of migration has been the basis used to define spawning stocks and their preferred spawning locations. At present 14 cod stocks are defined in the Northeast Atlantic and around 10 cod stocks in the Northwest Atlantic. Some of these stocks are large in terms of numbers or biomass, like the Arctic cod stock outside Norway. Others are small, like the Rockall cod stock west of Ireland.