The Alaskan salmon, from which the bulk of American canned salmon comes, exists in five species. It has a similar spawning habit to the British salmon, except that the same species always tends to use the same rivers. Once having spawned, the fish dies, so that the parents never see their offspring. The young larvae hatch out in the fresh water and make their way to the sea, where they pass the whole of their lives until they are mature, some years later, and then, in their turn, ascend the rivers to spawn. Eels are normally fresh-water fish. After living for six or seven years in rivers and ponds and streams, they become mature and migrate to the sea to spawn. This spawning always takes place in deep water (over five hundred fathoms), the particular region chosen depending upon the species. Eels from the British Isles and North-West Europe spawn in deep Atlantic, some hundreds of miles west of Ireland. In the autumn, the mature eels move down the rivers to the sea. When approaching maturity, the yellowish coat of the eel changes to silver. These “silver” eels pass into the sea and are never seen again. It is probable that the eel only spawns once in its life and then dies. The spawn floats to the surface and hatches out into curious little transparent, leaf-shaped larvae. These larvae develop rapidly into elvers and commence the return journey to the shores and rivers. In the spring, the young eels ascend the rivers in enormous swarms. Many of them leave the rivers and travel over damp ground and grass to isolated pools and lakes. It is probable that the eels that are found in the Thames travelled overland from the Severn. The Baltic flounder migrates in winter from rivers and estuaries to the open sea, and spawns in spring in deep water. It returns in the summer when the spawning is over. By observing the movements of marked fish, it has been shown that the fish move at an average rate of from three to four miles per day. During its seaward migration, the flounder takes no food, but uses the material stored up in its tissues for the development of its reproductive organs. In addition to these spawning migrations, there are migrations that are prompted by a search for food, or for warmer or colder water. In northern and temperate seas, the surface water grows warmer with the spring. This warming influence spreads northwards from the equator, producing what is known as the annual wave of sea temperature. A direct result of the rise of temperature and the increased sunshine is a rapid increase in the amount and quality of the plankton. It is not surprising, therefore, that fish migrate in the wake of this annual wave of sea temperature, attracted by the increased food supply, and possibly, also, by the warmer water.