THE PULSE OF THE SEA
By PROFESSOR C. M. YONGE
HE oceans are far from being stagnant masses of
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These life-bringing currents are not constant : they wax and wane. Like our own life blood, they possess a pulse, but these pulsations, owing to their very magni- tude, are less easy of detection than the steady throb in our arteries. They are of no less significance in the vitality of the sea. They are the cause of those fluctuations in the productivity of our commercial fisheries which render the harvest of the sea, where, without sowing, man garners the surplus of natural abundance, even more uncertain than the harvest of the land exposed to far greater variations in climate and preyed upon by innumerable pests unknown in the sea.
The controlling factor in the fertility of the North Sea, the site of the richest of all the great fisheries of the world, would appear to be the entrance of what is known to oceanographers as Atlantic water. This flows in round the north of Scotland and southward, between the coasts of Norway and Scotland, down into the North Sea. This water is somewhat more saline and also richer in salts essential for the production of plant life than is the water of the southern North Sea. It is true that even with the most accurate chemical methods the differences appear at first sight microscopically small, but they are" nevertheless of overwhelming significance. The sea is a much more constant medium than is the land, and its inhabitants are affected by correspondingly smaller changes.
Those who study the economy of life in the sea, for the great intrinsic interest of the subject or because of its ultimate bearings on the problems of our commercial fisheries, have concentrated their attention more and more on the microscopical plant and animal life which drifts about near the surface. The very nature of this May provide an indication of the origin of the water in which it lives. Despite the excessively minute chemical differences between the waters of the Atlantic and those of the southern North Sea and the Channel, it has been found that, of two closely allied species, one occurs exclusively in the former and the other as invariably in the latter.
By a careful study of its inhabitants, it is becoming possible for the biologist- to determine the nature of the water, hitherto the exclusive province of the chemist. It has been found that in the years when Atlantic water pushes far south into the North Sea there is a corre- sponding extension of Channel water, with its typical inhabitants, westward into the Bay of Biscay. Thus it comes about that examination of the microscopical marine life in this region may indicate what is happening hundreds of miles away in the North Sea.
The southern extension of Atlantic water affects life in a variety of ways. The greater abundance of nutrient substances permits the growth of a more abundant plant life, of the microscopical plants which form the true meadows of the sea. This in turn brings about an increase in the numbers of. animals, only one degree larger, which feed on the plants and are in their turn the food of larger animals, notably food fishes such as the herring. The Atlantic water brings with it its own characteristic inhabitants, which may be of greater food value than those which are displaced. Or, again—and this has been conclusively proved—the presence of Atlantic water may permit the growth of dense swarms of certain plants obnoxious to freely swimming animals such as the herring. The presence of these may deflect the herring shoals from their usual path with sometimes disastrous results to the fishery.
It has long been known that herring broods vary greatly from year to year. In certain years the spawning has such meagre results that it hardly affects the total population. In others the yield is so great as to provide the bulk of the adult herring population for many seasons after the young become mature. Thesegood year classes, as they arc called, appear roughly every three years.
It appears very proEable that these fluctuations in the herring broods—of such overwhelming importance to the fisheries—are connected in some way with the ebb and flow of Atlantic water. We know that when there is a bad herring year there is always a good spawning of the haddock. This is a More northern fish, and it may be that when the Atlantic water dOes not extend far into the North Sea the haddock benefit to a much greater extent than in the years when it extends further south and the herring consume much of the rich food which it brings. In both cases the greater size of the brood may be the direct result of the richer feeding of the parent fish.
We are only at the beginning of the study of these things. Many gaps still require to be filled; new factors, as yet unthought of, may emerge. But there can be little doubt that the pulsations .in the current of Atlantic water round the north of Scotlandre one of the most important factors controlling the fertility of the North Sea. We know little or nothing about the causes of these changes. The study of these takes us beyond these islands to the waters of the North Atlantic Drift and, further still, to the Gulf Stream and the origins of this beyond the Straits of Florida in the tropical Gulf of Mexico.
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