The Fishing Industry

by William E. Gibbs

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.

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.