Drifting in the ocean, afloat in the midst of a sea of plankton, the life cycle of a cod begins with an incubation period designed to ensure that every fish has a really good chance of survival. If the temperature is hot, ensuring a good supply of phyto (plant) plankton and corresponding zoo (animal) plankton, then the tiny larvae, measuring only 4mm in length, can hatch after only ten or eleven days. If the weather is colder this incubation period is longer, thus giving the tiny larvae the chance to wait for better conditions. In the North Sea incubation usually takes about twelve days.
Once hatched the young cod quickly give a hint as to what their appetites will eventually become, consuming planktonic creatures before their yolk-sac is even absorbed. They then stay at this top, or pelagic, layer for about two and a half months, packing on weight and increasing their size until they attain a length of between 1.5 and 2.2 cms. Then they start heading for the ocean floor with some entering shallow waters around the coast while others go for somewhat deeper venues.
It is thought that the fry in shallow water grow faster than their cousins in the depths, attaining lengths of 35 cms compared to, in some deep water cases, only 18 cms.
The shallow water codling remain inshore throughout the Summer and Autumn but then, in Winter, move out into deeper water, quickly showing a preference for strong tidal currents and bottoms composed of sand, mud or gravel in the vicinity of rocks, presumably to give them some cover if they are in danger from predators.
At this time they feed mainly on shrimps, tiny crabs, worms and, as they grow bigger, small fish such as gobies. Then, as the fish continue to grow, so too does their appetite, taking larger prey items instead of consuming lots of smaller items like sand shrimps or shellfish such as whelks.
Looking back through some of the early studies, I found some of the dietary articles quite interesting. In the North Sea, for example, one study showed a massive preference for Norway lobsters, varying in size from a large prawn to specimens which were considerably larger. Nearly three quarters of the cod investigated had eaten these Norway lobsters so the implications for anglers are really quite interesting, especially since I have just been browsing a catalogue of latex lures! Those cray bugs look like they might do the business! Humm... The next largest category was the unassuming sea mouse, which was found in a fifth of all the cod examined.
By the age of three a lot of the young codling, weighing in the region of 4¼ pounds, are mature. A year later most of them, weighing now around 7¼ pounds, are ready to move into deeper water to spawn. (The peak months for spawning are March and April but some breeding takes place during the period from January to June. ) A large cod, say around 21½ pounds in weight, can lay some 6 500 000 eggs, which just goes to show that the species should, if given the chance, be able to stage a revival from even the drastically low levels to which human greed has currently reduced their numbers. We can always hope!