Cod

In the strict sense, cod means the fish of the family Gadidae, which includes the Atlantic cod, the Pacific cod, haddock, pollock, whiting, and the freshwater burbot. The Atlantic cod, Gadus morhua, is the fish most people mean by the word, a heavy-bodied predator of the cold North Atlantic seabed with three dorsal fins and a chin barbel. In the fish trade, though, cod is used much more loosely, and fillets sold as cod may be Pacific cod, Alaska pollock, saithe, ling, hake, or several other white-fleshed species, which is why labelling rules in many countries now require the specific species to be named.

Chiefly because fishing removed adult cod faster than the population could replace them, for years on end. The arrival of large factory trawlers in the mid-20th century allowed catches far beyond what smaller boats could take, and quotas were repeatedly set too high, partly because managers overestimated how many fish were left. As the big old females that produce the most eggs were stripped out, recruitment of young fish fell. Cold conditions and heavy predation on the survivors made recovery harder. By the time Canada closed the Grand Banks fishery in 1992 the northern cod stock had fallen by about 99 per cent, and it has barely rebuilt since.

Yes, which is the more hopeful part of the story. The cod stock around Iceland was heavily overfished by the 1980s, but after Iceland took full control of its waters and imposed strict, science-based catch limits and gear rules, the stock rebuilt and now supports a stable, profitable fishery. The Barents Sea cod, jointly managed by Norway and Russia, is currently one of the largest cod stocks on record. The pattern is that recovery is possible where fishing is cut hard and early and kept low for long enough, but stocks that are pushed too far, like the Grand Banks, may shift into a new state that does not bounce back on any useful timescale.

Sea water freezes at about minus 1.9 degrees Celsius, colder than the freezing point of a fish's blood, so a cod in far-northern water is at constant risk of ice forming inside it. To prevent this, cod produce antifreeze glycoproteins, molecules that circulate in the blood and gut fluid and latch onto the surface of any tiny ice crystal that starts to form, blocking it from adding more water molecules and growing. This lowers the effective freezing point of the fish's fluids below that of the surrounding sea. Several other polar fish have evolved similar proteins independently, one of the best-known examples of convergent evolution at the molecular level.