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Tuesday, April 1, 2014

Herring Connoisseurs Switching?

source: Finnish Game and Fisheries Research Institute



(Baltic herring)

(Clupea harengus)

Order: Clupeiformes Family: Clupeidae

Foto: Lauri Urho
Description: The Baltic herring is sometimes considered as a subspecies of the Atlantic herring (Clupea harengus), adapted to brackish water. The Baltic herring grows more slowly and has a lower fat content than the Atlantic herring. The back of the herring is bluish green and its flanks have a silvery sheen. It can be distinguished from the sprat (Sprattus sprattus) by its skin-tight stomach scales, which lack the sharp edges of sprat scales. The ventral fins of the adult herring are also clearly behind the anterior edge of the dorsal fin, whereas those of the sprat are in front of the dorsal fin.
Origin and distribution: The Baltic herring is adapted to low salinity and occurs in the whole Baltic Sea. Reproduction takes place along almost the entire coast.
Reproduction: In the Finnish coastal area, herrings spawn in May or June, with autumn spawners accounting for only a very small proportion in the northern Baltic Sea. Herrings spawn in large, dense schools close to the coast, usually at a depth of 1−5 m. The Baltic herring spawns for the first time at 2−3 years of age, when it is 13−15 cm in length and about 20 g in weight. The newly hatched 6−7-mm-long larvae are translucent and very thin.
Diet, growth and migrations: Zooplankton at different developmental stages are important food for herring larvae. Young herrings also feed on various plankton crustaceans. At a length of 15−20 cm, they start to feed on small fishes, too, and once they have reached a length of 25 cm, fish are their main food. There are marked local and temporal differences in growth rate. The average weight of an 8-year-old herring in the Gulf of Finland and main basin is 50−100 g, whereas in the northern Gulf of Bothnia it is 40−60 g, or even less. Fast-growing Baltic herrings were caught especially in the early 1980s, but in the1990s there were numbers of very thin fishes. Among the normally growing Baltic herrings, mainly in spring, there are also “gigantic herrings”, which have an average weight of about 400-500 g. The biggest herring ever caught in Finland weighed 1.1 kg.
The herring is a school fish, migrating annually between spawning and feeding areas. Finnish tagging experiments have shown that feeding migrations are relatively long, but that the fish still tend to return to the same area to spawn. Most herrings spawning on the south and southwest coasts of Finland migrate afterwards to the middle or even southern part of the Baltic Sea. Diurnal vertical migrations are also typical of herring behaviour. In summer, the schools of adult herrings remain near the bottom during the day, only rising to the middle water after sunset. In winter, shoals swim in deep waters, even at depths of over 100 metres.
Fishing and catches: Measured by both the amount and value of the catch, the herring is the most important target of commercial fishing in Finland. The total annual catch of all Baltic Sea countries was less than 100 000 tonnes until the mid 20th century. Catches began to increase in the 1950s along with the expansion of trawl fishing, and in the 1979s the catch amounted to 450 000 tonnes. Catches decreased in the late 1980s and 1990s. In 2003, the herring catch in the Baltic Sea totalled 223 000 tonnes, which was the smallest total in the observation period 1974−2003. Finland took 28% of the total catch. The Bothnian Sea has been the most important fishing area for Finland since the 1990s. In 2003, about 71% of the Finnish herring catch was taken in the Bothnian Sea, 86% by trawl and 13% with trap-nets.
Vulnerability, threats and management: Atlantic herring stocks have suffered heavily from over-fishing. Baltic herring stocks have, however, not collapsed despite the high fishing effort. Stocks have probably been protected by the fact that the Baltic herring spawns widely in shallow water and also that the catch consists mainly of adult fish. The increasing abundance of sprat stocks in the late 1990s, together with the persistently low salinity levels in the Baltic Sea have contributed to a decrease in the growth rate of herring individuals. The effects of eutrophication on herring stocks are not known very well, though severe eutrophication has been found to impair reproduction success, and mass occurrence of plankton algae may have an effect on growth rate. On the other hand, environmental changes in the Baltic Sea may threaten use of the herring, because, as well as healthy substances, herrings have been found to contain substances harmful to humans. The Food Agency has been concerned about the high dioxin concentrations, in large herrings in particular. Some herring connoisseurs have switched to small and medium-sized fishes.

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