=The Migration of Fishes.= Fishes, like birds, migrate over great distances at certain seasons of the year. In most cases, this migration occurs just before spawning, and is evidently connected directly with the spawning instinct. True marine fishes, such as the herring, haddock, plaice, cod, associate in vast numbers at spawning time, choosing a locality in which the temperature and food supply will be favourable to the development of the young larvae. Generally, the spawning ground is in deep water. The eggs are buoyant, and drift up to the warm surface water and hatch out amongst the plankton. The herring differs from most other pelagic fish in laying its eggs in relatively shallow water, over a rocky bottom covered with seaweed. The eggs are denser than sea-water and are covered with an adhesive substance, so that they sink to the bottom and become attached to the stones and seaweed. It is at the time of this annual migration to the spawning grounds that the fish are most profitably caught, for not only are they gathered together in large numbers, but, just before spawning, the fat content and general condition of the fish, and therefore its food value, reach a maximum. After spawning, the food value of the fish is at a minimum, and remains comparatively low until a few months before the next spawning. The plaice migrates in the autumn from the feeding grounds in various parts of the North Sea to the spawning grounds near the Straits of Dover. Spawning takes place between December and March. In the spring and summer it returns northwards to the feeding grounds in the centre of the North Sea. In the Irish Sea, there are two distinct annual migrations of plaice. The first occurs in summer (from June to September), the larger plaice moving from the warmer, shallow water inshore to the deeper, cooler waters offshore. In winter and spring, (from October to May), the mature plaice migrate from Morecambe and Liverpool bays to the spawning ground in deep water to the North-East of Douglas (Isle of Man). In winter, also (from November to January), a large number of plaice gather in Red Wharf Bay, off the north coast of Anglesey, probably because it is sheltered from the prevailing south-east winds. In February they commence their spawning migration round the coast of Anglesey to Cardigan Bay. Certain species of fish, instead of migrating from one part of the sea to another, migrate from the sea to rivers (anadromous), or from rivers to the sea (katadromous). Thus, in the spring or autumn, according to species, the anadromous salmon and shad ascend rivers to spawn. The eggs are deposited on clean gravel in clean water, where they are likely to remain undisturbed. The salmon does not feed when in the river, and after spawning, becomes very thin and in poor condition.