When Atlantic cod vanished, lobsters, crabs and shrimp took over the seafloor
In 1992 Canada banned cod fishing after a thousand years of Europeans chasing the fish across the Atlantic. Several stocks had crashed by more than 95 per cent, and even without fishing they never fully bounced back. With the top predator gone, lobsters, crabs and shrimp multiplied and spread, showing just how much cod had held them in check.
Cod are big, long-lived hunters. They can reach 25 years of age and usually grow to 100 to 140 centimetres, although fish over 180 centimetres and 50 kilograms have been landed. Small cod eat mostly crustaceans, large ones mostly fish such as herring, capelin and sand eels, and they will eat their own young: in the northern North Sea juvenile cod made up about 10 per cent of stomach contents by weight. Young cod respond cleverly. With no threat around they settle on sand and gravel, but when an actively hunting predator appears they squeeze into the gaps of a cobble bottom, or into kelp if no cobble is available.
They are finely tuned to temperature. A change of a few degrees shifts their heart rate sharply, and a drop of only 2.5 degrees Celsius raised their metabolic rate by 15 to 30 per cent, a costly jump. So cod tend to sink into colder, deeper water by day and rise into warmer, shallower layers at night, conserving energy.
Shoals move in size order. Larger fish scout at the front and eat more varied and plentiful food, while smaller fish trailing behind get less but still do better than they would alone. Courtship resembles a lek: males gather, establish rank and make sounds, and females choose partners. A female releases eggs in 5 to 20 batches a few days apart, between two hundred thousand and 15 million eggs in total, which drift with the currents toward nursery areas.
Modern trawling from the 1950s sent catches soaring. Eastern Atlantic landings still often exceed a million tonnes a year, largely from the Northeast Atlantic stock and Iceland, while the western Atlantic has usually landed under 50,000 tonnes since the moratorium. A 2013 assessment found European cod had rebounded enough to be listed as least concern, but the species remains at risk in many places.
Source: Atlantic cod
