Several cod stocks collapsed in the 1990s (decline of more than 95% of maximum historical biomass) and have failed to fully recover even with the cessation of fishing.

Larger fish act as scouts and lead the shoal's direction, particularly during post spawning migrations inshore for feeding.

Without apparent risk of predation, juvenile cod demonstrated a preference for finer-grained substrates such as sand and gravel-pebble.

As cod are apex predators, overfishing them removed a significant predatory pressure on other Atlantic fish and crustacean species.

These fine-tuned behavioural changes to water temperature are driven by an effort to maintain homeostasis to preserve energy.

[32] When hatched, cod larvae are altricial, entirely dependent on a yolk sac for sustenance until mouth opening at ~24 degree days.

Based on behavioral observations of cod, the cod mating system has been likened to a lekking system, which is characterized by males aggregating and establishing dominance hierarchies, at which point females may visit and choose a spawning partner based on status and sexual characteristics.

[32] Evidence suggests male sound production and other sexually selected characteristics allow female cod to actively choose a spawning partner.

[40] Atlantic cod act as intermediate, paratenic, or definitive hosts to a large number of parasite species: 107 taxa listed by Hemmingsen and MacKenzie (2001)[41] and seven new records by Perdiguero-Alonso et al.

[41] Parasites of Atlantic cod include copepods, digeneans, monogeneans, acanthocephalans, cestodes, nematodes, myxozoans, and protozoans.

[41] Atlantic cod has been targeted by humans for food for thousands of years,[43] and with the advent of modern fishing technology in the 1950s there was a rapid rise in landings.

[44] Cod is caught using a variety of fishing gears including bottom trawls, demersal longlines, Danish seine, jigging and hand lines.

The quantity of cod landed from fisheries has been recorded by many countries from around the 1950s and attempts have been made to reconstruct historical catches going back hundreds of years.

The Northwest Atlantic cod has been regarded as heavily overfished throughout its range, resulting in a crash in the fishery in the United States and Canada during the early 1990s.

On average, about 300,000 t (330,000 short tons) of cod were landed annually until the 1960s, when advances in technology enabled factory trawlers to take larger catches.

[45]Technologies that contributed to the collapse of Atlantic cod include engine-powered vessels and frozen food compartments aboard ships.

Atlantic cod was a top-tier predator, along with haddock, flounder and hake, feeding upon smaller prey, such as herring, capelin, shrimp, and snow crab.

[16] With the large predatory fish removed, their prey have had population explosions and have become the top predators, affecting the survival rates of cod eggs and fry.

In the winter of 2011–2012, the cod fishery succeeded in convincing NOAA to postpone for one year the planned 82% reduction in catch limits.

The fishery brought in $15.8 million in 2010, coming second behind Georges Bank haddock among the region's 20 regulated bottom-dwelling groundfish.

Data released in 2011 indicated that even closing the fishery would not allow populations to rebound by 2014 to levels required under federal law.

By summer, the young cod reach the Barents Sea, where they stay for the rest of their lives, until their spawning migration.

The North Sea cod stock is primarily fished by European Union member states, the United Kingdom and Norway.

In 1999, the catch was divided among Denmark (31%), Scotland (25%), the rest of the United Kingdom (12%), the Netherlands (10%), Belgium, Germany and Norway (17%).

In 2003, ICES stated a high risk existed of stock collapse if then current exploitation levels continued, and recommended a moratorium on catching Atlantic cod in the North Sea during 2004.

However, agriculture and fisheries ministers from the Council of the European Union endorsed the EU/Norway Agreement and set the total allowable catch at 27,300 t (30,100 short tons).

[49] Decades of overfishing in combination with environmental problems, namely little water exchange, low salinity and oxygen-depletion at the sea bottom, caused major threats to the Baltic cod stocks.

Adaptations include differences in hemoglobin type, osmoregulatory capacity, egg buoyancy, sperm swimming characteristics and spawning season.