Herring Clupea harengus; Linnaeus, 1758. Family: Clupeidae
Atlantic herring are widely distributed throughout the north-east Atlantic, ranging from the Arctic ocean in the north to the English Channel in the south. Young herring are found close inshore or in sea lochs whilst the adult shoals generally occur further offshore.
Herring stocks can be categorised by their different spawning areas and times. Different stocks tend to mix together for most of the year but during the spawning season they migrate to their separate spawning areas. Although herring can be found spawning in almost any month, around Scotland the majority spawn in the autumn, between August and October. In the North Sea and to the west of Scotland more than 60% of two years old herring will be mature and able to spawn. At three, around 95% are mature and, by the time they are four years old, most herring are able to spawn. The number, size and weight of eggs produced by an average sized female varies between stocks.
Herring are demersal spawners, depositing their sticky eggs on coarse sand, gravel, small stones and rock. Shoals of herring gather on the spawning grounds and spawn more or less simultaneously. Each female releases her eggs in a single batch and the resulting egg carpet may be several layers thick and cover a considerable area. The eggs take about three weeks to hatch depending on the sea temperature. The newly hatched larvae follow the current systems. Around the North Sea many autumn spawned larvae spend their first winter drifting towards the major nursery areas on the eastern side of the North Sea; there are smaller nursery areas around the Moray Firth, the Firth of Forth and along the east coast of Scotland. Larvae from the west of Scotland spawning grounds may be retained on the west coast but a large proportion are carried through the Fair Isle channel and penetrate well into the North sea. The drift rate of the larvae is variable and it is probable that in some years many of them do not reach the nursery areas.
In the North Sea, as they grow, the juvenile herring move offshore, eventually joining the adult populations in the feeding and spawning migrations to the northern and western areas of the North Sea. Some of the herring in the Moray Firth, German Bight and Skaggerak nurseries originate from the spawning grounds on the north and west coasts of Scotland. There is evidence from tagging experiments and from using parasites as biological markers to suggest that as they mature, a proportion of the east coast population makes the journey back to the west of Scotland spawning areas. Juvenile herring on the west coast of Scotland spend time in inshore areas and sea lochs before they move offshore to join the adult population in the deeper waters of the continental shelf.
During daytime, herring shoals remain close to the sea bottom or in deep water. At dusk they move toward the surface and disperse over a wide area.
Generally, crustacea (shrimps and copepods) and juvenile sandeels are the main components of the diet of herring. There are no marked differences between the diets of small and large herring; only the proportions of the different food items change with size.