The Open Sea: The World of Plankton. Alister Hardy

      FIG. 5

      Showing where drift-bottles liberated at A and B were recovered. A map reproduced from one in the museum of the Fisheries Laboratory at Lowestoft showing the results of one of many experiments made by the late Mr. J. O. Borley when investigating the drift of plaice eggs and larvae from the spawning areas to the coastal nursery grounds. The numerals show the number of bottles picked up at each point.

      There are several different kinds of current-meter but most of them are similar in general principle to that described. Dr. Carruthers (1926) has devised a much more robust machine which is used on a number of lightships anchored round our coasts to record automatically the main drift of water for periods of a month at a time. It records the to-and-fro tidal stream as well as the residual current-drift. These instruments have given much valuable information on the variations in the flow of water through the English Channel into the southern North Sea (Carruthers, 1930). It is in the Flemish Bight that so many plaice congregate in winter to lay their floating eggs at the point where the Channel water enters; these eggs, and later the hatched-out fry, are carried by the current and so bring the new generation to settle down as small flat-fish on the nursery feeding grounds in the shallow waters off the Dutch and Danish coasts. Herring fry in vast numbers are also carried by the same current into the North Sea from one of the largest spawning grounds off Cape Gris Nez. The spawning migrations of many fish have been evolved in relation to the prevailing current systems; selection has naturally acted to preserve the offspring of those parents who migrate to lay their eggs at the point up-stream most favourable for their survival; eggs spawned elsewhere are less likely to be successful because they are carried to less suitable nursery grounds. Fig. 5 shows the results of two of many experiments made with drift-bottles during the English fishery investigations into the life-history of the plaice. Those liberated at a point in the main spawning area in the Southern Bight have all been picked up on the Dutch coast, whereas those from the lesser spawning area off the Yorkshire coast are carried into the Wash, which is another smaller ‘nursery’ ground; the number against each point on the coast indicates the number of bottles found there. In some years a marked variation from a normal current-flow may well have a considerable effect on the relative success of a particular brood of fry; if the flow is weaker than usual they may not reach the best ground, if it is too strong they may be carried beyond it. The speed of flow of Channel water into the North Sea may be affected by the wind at a critical time apart from any variation in the fundamental current system; a prolonged westerly wind may accelerate the flow, and an easterly one may have the reverse effect. These are just some of the many factors we must take into account in puzzling out the possible causes for this or that unusual event in the fisheries; it is so often that such factors have an effect which is most marked in the fishery several years later, i.e. when the young fish of that brood will have grown to maturity—or failed to.

      Without the use of drift bottles or current-meters some indication of the direction of current-flow may also be got by mapping the contours of varying salinity (measurements of salt-content) obtained by the analysis of water-samples collected at a number of different points in the area; for instance, a tongue of very salt water projecting into a less saline area might indicate a flow of ocean water into a more coastal region where the salt water has been diluted by drainage from the land. Indeed modern oceanography has developed an elaborate mathematical system for estimating the direction and relative speeds of water movements from a knowledge of the varying densities of the water at a number of different points. We shall later see how at times certain planktonic animals and plants characteristic of one particular type of water may be used as indicators of the incursion of such water into other areas: in fact one such example, that of the tropical Physalia and Velella reaching our coasts, has already been mentioned, and we shall discuss others at the end of the chapter. We are apt to think of the use of plankton animals as current indicators as rather a modern idea; I was interested to find that Alexander Agassiz in 1883 was emphasising the importance in this respect of the animals just mentioned. “This group of Hydrozoa,” he wrote “is eminently characteristic of the Gulf Stream, and wherever its influence extends these Velellae and Physaliae have been found. In fact these surface animals are excellent guides to the course of the current of the Gulf Stream—natural current bottles, as it were.”

      In the Department of Natural History in the University of Aberdeen there is a cabinet containing a remarkable collection of South American and West Indian seeds picked up on the shores of the Outer Hebrides. They were gathered from 1908 to 1919 by William L. MacGillivray who was a nephew of a former Regius Professor; most of them he found on the West Sand of Eoligarry, Barra, where he lived, but some came from Lewis and the Island of Fudag. There are Brazil nuts, the seeds of a leguminous liana Diodea, the Virgin Mary nut, palm seed of different kinds, the pecan nut, the Calabar bean, nutmeg, the seeds of the Central American soapberry tree and many others. Altogether seventeen tropical species are represented. Just as these seeds are drifted to our shores, so also are the baby eels carried round by the ocean circulation from where they were spawned—from a small area situated between Bermuda and the Leeward Islands—and eventually scattered by the Gulf Stream to enter the rivers along the whole seaboard of Europe. This was the amazing discovery made by the famous Danish oceanographer Professor Johannes Schmidt. On many special voyages he plotted the distribution of the tiny eel fry all over the Atlantic until at last he could show that there is only one limited area where the very smallest and newly hatched young are to be found—a breeding ground some 3,000 miles from the rivers in which they grow to maturity. In Fig. 6, I reproduce his map showing the spread of the fry of different sizes. Their drift round the ocean to Europe takes from 2 to 2½ years, and during this phase they are little flat and quite transparent creatures having the shape of a willow leaf. They used to be thought to be a separate species of fish, called Leptocephalus brevirostris, until the Italian naturalists Grassi and Calandruccio kept some in an aquarium and were surprised to find they turned into the common elvers, as the young freshwater eels are called when they ascend the rivers from the sea. The story is now very well known and I only recall it here because it is, to use Agassiz’s simile, nature’s greatest drift-bottle experiment and demonstrates so clearly the constancy of this vast current system; year after year, for many millions of years, the eel-fry must have been transported in this way with never a break in the sequence. How the adult eels navigate back to breed in this one place is one of the most profound mysteries of the sea; a discussion of this, however, belongs to a chapter on fish and must await the subsequent volume.