Atlantic Cod Gadus morhua; Linnaeus, 1758. Family: Gadidae
Cod can be found in almost any sea area around Scotland from the shoreline to depths in excess of 200 metres. They can be found living a solitary existence or they may occur in large shoals. Generally, they spend their first winter in the shallower inshore waters before moving offshore into deeper waters. Adult fish are generally concentrated in the northern and more offshore regions of the North sea and west coast.
Growth rate differs by area but cod generally grow quickly and can reach a length of 20 cm after one year, 50 cm after two years and 80 cm by the time they are four years old. They can live for many years but they are big enough to be caught as early as age one. By the time they reach two years old, young cod are fully exploited by the commercial fishery and many are caught long before they have the opportunity to spawn.
A small number of cod will reach maturity at two years old but it is not until they reach four years old that a large proportion of the population is capable of spawning. By the time they reach six years, all cod are mature and an adult female of around 100 cm length will produce around five million eggs in a season. Spawning takes place between January and April and occurs in most offshore areas. The eggs will take 10 to 30 days to hatch depending on the temperature of the water. Newly hatched cod spend their first few months living and feeding in the surface water layers before eventually adopting the bottom living way of life. Seasonal migrations of cod populations can take place but in many areas feeding and spawning movements are limited.
The picture sequence below shows a cod egg from pre-hatch through three stages of hatching.
Cod do not usually browse for food on the bottom but are active feeders. By weight, around three quarters of the food of all sizes of cod consists of a variety of fish and crustaceans (shrimps and crabs). The rest is made up of smaller quantities of molluscs and worms. As they grow, cod eat an increasing amount of fish. Sandeels, Norway pout, whiting, herring, dab and smaller cod are the main fish species eaten.