Two seabass populations barely mix, and nobody knows why
European seabass, sold as branzino in North America and also known as loup de mer, is the most important farmed fish in the Mediterranean. Yet its wild stocks hide a puzzle: Atlantic and Mediterranean populations are genetically distinct despite being separated by only a narrow stretch of sea off southern Spain.
The species belongs to a small family of just six temperate basses. It lives mostly at sea off western and southern Europe and northern Africa, from southern Norway to Senegal, throughout the Mediterranean and into the southern Black Sea, but not the Baltic. It has even slipped through the Suez Canal into the Red Sea, moving against the usual direction of such migrations. In summer and late autumn it enters shallow coasts, lagoons, estuaries and river mouths, and in winter it heads offshore to spawn.
Growth is slow and adulthood takes years. A typical adult weighs about 2.5 kilograms and is silvery grey, sometimes dark blue along the back; the biggest reach a metre and 12 kilograms, though half a metre is more usual. Young fish school and eat invertebrates, while adults become loners that prefer fish. They hunt by day and night on sardines, sprats, sand smelts, sand eels, worms, squid, prawns and crabs, and fish over 4 kilograms feed mainly after dark. Spawning runs from February to June, mostly near shore, and the young spend a year or two in estuaries.
The genetic split sits at the Almería-Oran front, east of Almería. It is hard to explain: larvae drift for up to three months as weak swimmers, juveniles tolerate swings in temperature and salinity, and adults can travel hundreds of kilometres, so some mixing would be expected.
Linnaeus named the fish Perca labrax in 1758; after a century and a half of shifting synonyms, Dicentrarchus labrax was settled in 1987. The genus name is Greek for two stings at the anus, after its paired anal spines. It goes by dozens of names, including capemouth, king of the mullets, white salmon, sea dace and levrek, and in Britain and Ireland restaurant sea bass always means this species. Caught commercially and prized by anglers, its wild numbers are falling, and the IUCN lists it as near threatened.
Source: European seabass
