Not completley cold blooded
The speediest of the fish species are the mackerel and the tunnys. Members of this family are not completely cold blooded, and can slightly raise their temperature if they want to increase their speed. Fast swimming is something they do very well! When a school of mackerel find a school of prey like small herring or sand eels, they speed up and swim through the schools at random, creating great confusion amongst the prey. When the prey fish leave their relatively safe school, they are singled-out and eaten. In this way a mackerel can grow very fast and build up a lot of fat stores. Of course, this fatty fish is a prey for bigger animals like birds, sharks and dolphins; humans like to eat them too. They are fished quite heavily and since the North Sea is at the northern edge of its range they are vulnerable to overfishing in this area.
Near the edge of the continental shelf
In winter mackerel more or less hibernate near the edge of the continental shelf at a depth of a few hundred metres. In spring they come up to the surface layers and feed on fish again, forming large schools and following the herring and sand eels, even when they come into shallow areas like the Wadden Sea, mostly during the month of July. If you are lucky, in open water you may see sharks feeding on mackerel. I once had this privelage and saw two thresher sharks attacking a school of mackerel in the English Channel. They used the same method that the mackerel uses to hunt herring. They swim at high speeds through the schools splashing around with their long tails and confusing the school so that individual fish get separated and can be picked out and eaten. The fish that try to escape by jumping are of course a target for birds flying over the sea surface and/or diving for prey that is near the surface. During such frenzies, it is as if the the whole sea is on a rolling boil.
For a mackerel, the saying ‘be as fast as a hare or you will be a dead hare’ (and you will end up well smoked on a plate in a restaurant) rings true.