Mackerel Scomber scombrus; Linnaeus, 1758. Family: Scombridae
With an annual value of over £89 million, mackerel is a very important species for the Scottish fleet. By weight, it is by far the most abundant pelagic species landed. The bulk of the catch is taken by pelagic trawlers and purse seiners. Each year, the number of mackerel in the sea depends on the number of young fish which survive from spawning to enter the adult fishery as recruits. The large annual change in the number of recruits is one reason for managing this stock.
The mackerel caught in the North Sea belong to two different stocks - the North Sea and the western. This separation is based on differences in the timing and the areas used for spawning. North Sea mackerel overwinter in the deep water, to the east and north of Shetland and on the edge of the Norwegian Deep. In the springtime, they migrate south to spawn in the central part of the North Sea from May until July. The western mackerel stock is found near to the continental slope, over a vast area. These fish spawn between March and July, mainly to the south and west of the British Isles. When spawning is finished, most of the spent fish move to the feeding grounds in the Norwegian Sea and the northern North Sea where they mix with the North Sea stock. Some western stock mackerel, predominantly small individuals, also enter the North Sea through the English Channel. The western stock mackerel travel long distances between the feeding grounds and the spawning areas. Over the past 20 years, the pattern of their southerly migration has changed dramatically in both timing and route. In the 1970s and 1980s this movement occurred in late summer and autumn with the fish passing through the relatively shallow waters of the Minch. Now the migration occurs gradually later in the year and is further offshore. The pattern of the return northerly journey, after spawning, has remained relatively constant. The boundaries of the spawning areas have also slowly changed, with an increase in spawning activity in the north of the area and to the west of the shelf edge.
At one year old, only a small proportion of females are mature and able to spawn, while more than half can spawn at two years old. By the time they reach three years old, most mackerel are mature. Female mackerel shed their eggs in about 20 separate batches over the course of a spawning season. An average-sized fish produces around 250,000 eggs. Juvenile mackerel grow quickly and can reach 22 cm after one year and 30 cm after two years.
The diet of mackerel can vary with the area and the season. By weight, almost half of the food consists of crustacea (shrimps). The remainder is made up of juvenile fish such as sandeel, herring and Norway pout.