Sprat

(Sprattus sprattus)

Order: Herrings Clupeiformes Family: Clupeidae

Foto: Jussi Pennanen

Description: The sprat bears a close resemblance to the herring (Clupea harengus). It has silvery sides and lives in schools but can be distinguished from the herring by its angular ventral scales, which feel sharp when stroked from tail to head. The ventral fins of the sprat are located forward of the dorsal fin, whereas those of the adult herring are under the dorsal fin. The eyes and also the head of the sprat are relatively smaller than those of the herring.

Distribution: The sprat occurs in the North Atlantic, the Mediterranean, the Black Sea and throughout the Baltic Sea though it is not so common in the northernmost Gulf of Bothnia as it is in the southern Baltic Sea. The sprat spends most of its life in the open sea. In winter, when the water is cold, sprat form schools at depths of 100 metres, whereas in autumn they rarely live below depths of 10−40 metres.

Reproduction: Sprat mature at 2−3 years of age when approximately 12 cm long. In the northern Baltic proper, they spawn in the open sea in summer but relatively close to the surface. Spawning occurs in several periods at intervals of 8-10 days. The eggs drift freely in the water provided salinity is at least 5-6‰. At a lower salinity, the eggs sink to the bottom, and the larvae most probably fail to hatch. The female may produce tens of thousands of eggs in one summer, but the annual spawning success varies greatly.

Food, growth and migrations: Sprat grow rapidly during the first two years, after which growth ceases practically altogether. The sprat catch consists mainly of two- and three-year-olds, with a length of 13−14 cm and weight of 10−13 g. Fishes over 10 years of age are 13 to 14 cm long and weigh 14−15 g. Sprat feed on plankton crustaceans, during cold seasons on copepods, and in summer mainly on cladocera. Sprat themselves are an important food for cod (Gadus morhua). Like those of herring, sprat schools migrate vertically. When the sprat stock is small, the fish tend to remain in the southern Baltic Sea and so are caught less frequently in Finnish waters. When the stock is more abundant, sprat schools may extend their range northwards, and so sprat become more common in Finnish waters, too.

Fishing and catches: As a result of improvements in trawl fishing, sprat catches started to increase in the late 1900s, and by the early 1970s, annual catches in the whole Baltic Sea were exceeding 200 000 tonnes. Catches decreased in the 1980s, coinciding with an increase in the abundance of cod. Since 1990, the sprat catch in the Baltic Sea has increased five-fold, being over 500 000 tonnes in 1997. Finland's sprat catch was about 19 000 tonnes in 2006 and 9000 tonnes in 2003, which was about 3% of the total annual sprat catch in the Baltic Sea. The bulk of the world's sprat catches are taken in the Baltic Sea. In Finland, sprat have until recently been the main ingredient of industrially canned fish products and conserved spicy fish products.

Vulnerability, threats and management: In the Baltic Sea, abundances of sprat stocks and sprat catches vary markedly for reasons that are not clearly understood.

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