Within European fishing areas there are three horse mackerel stocks, which have received their names according to their spawning areas. The southern stock spawns outside Spain and Portugal, the western stock spawns in the Bay of Biscay, west of Ireland and Great Britain, and the North Sea stock spawns in the southern North Sea. Western horse mackerel are basically spawning in the same area and at the same time as western mackerel. After spawning, the horse mackerel perform a feeding migration in the North Sea and into the southern part of the Norwegian Sea.
Horse mackerel is also managed as three separate stocks. The catches are distributed to each stock, according to when and where the catches are taken. The egg production of western and southern horse mackerel are measured every third year, at the same time as the measurements of egg production / spawning stock undertaken by our scientific surveys.
Investigations of the roe from horse mackerel have shown that based on the techniques available today, it is impossible to find out how many eggs a female fish spawn. It looks like that horse mackerel can adjust its egg production during the course of the spawning season. It is therefore not possible to accurately calculate spawning biomass from total egg production.