These fall into two categories. Some drains are simply small rivers which have been straightened, widened and dredged, whilst others are completely man-made. They are created to drain water away from flat agricultural land and often have little or no flow. The water levels in the drain are controlled by sluice gates which are opened to allow excess water to run off to sea. The banks of these drains are usually very uniform with little or no cover. The species of fish present are those which prefer slow water, such as bream, tench, roach and perch. Large predators, for example pike and zander, often thrive in these drains. Fish location in a drain is difficult because the fish rove about a great deal. Bream shoals will often move several kilometres along a drain in a few days. Zander shoals tend to follow the small fry about, and the key to zander location is to first find the fry on which they feed.