The tides of the North sea are of great complexity; their nature has been widely investigated during the last 20 years. On the north-east coast of Great Britain the tide runs southward, so that the co-tidal lines, or the lines of equal time of high water, go from north to south; e.g., high water is at 12 o'clock in the Moray firth, it is high water in the Humber five hours, and at Lowestoft nine hours, later. A second tide comes through the Strait of Dover into the southern part of the North sea, and moves to the right side, i.e., the Dutch-German coast, towards the north and the east. A third weak tide flows south-east towards the Nor wegian coast. The three systems—the Scottish, the Channel and the Norwegian—meet to the east of the Dogger, so that there the co-tidal lines radiate from this centre. The range of the tides on the Scottish coast is mostly 4 metres and over, on the Dutch German coast from 2 to 3 metres, and on the Norwegian coast less than 1 metre. (See also maps in the article TIDES.) Fisheries.—The North sea is particularly rich in organisms of all kinds, and the abundance of food attracts fish in such quantities that the North sea fisheries are some of the most productive in the world. Flat fishes, and those feeding at the bot tom on smooth ground, are chiefly caught by means of the trawl. The favourite trawling-grounds are the Dogger Bank in winter, and the shallow waters off the Continental coasts in summer. In
rough ground, where the trawl cannot be used, hook-and-line fishing is carried on most successfully, and "mid-water" fish are also taken in this way, although the trawl and line fishing overlap considerably. Herring and mackerel are caught by means of drif t nets ; herring are also caught by trawling, and in greater quantities than by drifting. The herring fishing off the British coasts exhibits a remarkable variation during summer and autumn, beginning in Shetland in June, and becoming progressively later southwards, until it ends off the Norfolk coast in November.
The approximate landings of the North sea fisheries amounted in 1924 to 1,069,000,000 kg., consisting of herrings 683,500,000 kg., or 64% of the total catch. Then came haddock 0%, cod 6%, plaice 4%, whiting 3%, mackerel i%, turbot, sole, halibut, etc. As regards the value of the catch, the North sea herring also came first, but only with 37% of the total value. One of the chief objects of the International Council for the Study of the Sea is to discover the true connection between the varying results of the high seas fishing in the North sea and the changing physio chemical nature and movements of the waters of the North sea.