In the English Channel and Celtic Sea mackerel spend the winter months on the sea floor densely packed in places where its level is interrupted by banks and gulleys.In the early spring the fish rise to the surface and migrate to a common spawning ground that lies far out to the westward of the Scilly Islands in the vicinity of the 100-fathom contour.The very localized positions in which mackerel spend the winter are widely distributed throughout the area in both deep and shallow water. Large schools of migrating fish converge upon the spawning ground from many directions, therefore, in the spring for spawning.Fish that have wintered near the land must migrate offshore to reach the spawning ground; those that spend the winter on the bottom to seaward of the spawning ground must migrate shorewards to reach it. Off the south-west of England there is no single shoreward migration to spawn in shallow water as has previously been thought.In the North Sea the chief spawning grounds of the mackerel are also near the 100-fathom contour which, in that region, happens to lie very close to the land in the Skagerrak and along the Norwegian coast. The chief spawning migration of the North Sea mackerel is therefore towards the coast from offshore localities.This migration is, at the same time, chiefly from shallow to deeper water.Existing information concerning the mackerel populations on the westernside of the North Atlantic points to the probability that their spawning habits and migratory movements do not differ greatly from those of the mackerel in North European waters.
Lowest Astronomical Tide in the North Sea derived from a vertically referenced shallow water model, and an assessment of its suggested sense of safety
