August 26, 1920] 



NATURE 



817 



tigators — Grassi, Hjort, and others — who have dis- 

 covered and traced the staggs of growth of the Lepto- 

 cephalus and its metamorphosis into the "elvers" or 

 young eels which are carried by the North Atlantic 

 drift back to the coasts of Europe and ascend our 

 rivQrs in spring in countless myriads ; but no man 

 has been more indefatigable and successful in the 

 quest than Dr. Schmidt, who in the various expedi- 

 tions of the Danish investigation steamer Thor from 

 1904 onwards found successively younger and younger 

 stages, and is during the present summer engaged 

 in a traverse of the Atlantic to the West Indies in the 

 hope of finding the missing link in the chain, the 

 actual spawning fresh-water eel in the intermediate 

 waters somewhere above the abysses of the open 

 ocean.* 



Again, take the case of an interesting oceanographic 

 observation which, if established, may be found to 

 explain the variations in time and amount of im- 

 portant fisheries. Otto Pettersson in 19 lo discovered 

 by his observations in the Gullniar Fjord the presence 

 of periodic submarine waves of deeper salter water 

 in the Kattegat and the fjords of the west coast of 

 Sweden, which draw in with them from the Jutland 

 banks vast shoals of the herrings which congregate 

 there in autumn. The deeper layer consists of "bank- 

 water " of salinity 32 to 3'4 per thousand, and as this 

 rolls in along the bottom as a series of huge undula- 

 tions it forces out the overlying fresher water, and 

 so the herrings living in the " bankwater " outside are 

 sucked into the Kattegat and neighbouring fjords 

 and give rise to important local fisheries. Pettersson 

 connects the crests of the submarine waves with the 

 phases of the moon. Two great waves of salter 

 water which reached up to the surface took place in 

 November, 19 10, one near the time of full moon and 

 the other about new moon, and the latter was at the 

 time when the shoals of herring appeared inshore and 

 provided a profitable fishery. The coincidence of the 

 oceanic phenomena with the lunar phases is not, how- 

 .ever, very exact, and doubts have been expressed as 

 to the connection ; yet, if established, and even if 

 found to be due, not to the moon, but to prevalent 

 winds or the influence of ocean currents, this would 

 be a case of the migration of fishes depending upon 

 mechanical causes, while in other cases it is known 

 that migrations are due to spawning needs or for the 

 purpose of feeding, as in the case of the cod and the 

 herring in the west and north of Norway and in the 

 Barents Sea. 



Then, turning to a very fundamental matter of 

 purely scientific investigation, we do not know with 

 any certainty what causes the great and all-important 

 seasonal variations in the plankton (or floating minute 

 life of the sea) as seen, for example, in our own home 

 seas, where there is a sudden awakening of micro- 

 scopic plant-life, the Diatoms, in early spring when 

 the water is at its coldest. In the course of a few 

 days the upper layers of the sea may become so filled 

 with organisms that a small silk net towed for a few 

 minutes may capture hundreds of millions of trdi- 

 viduals. And these myriads of microscopic forms, 

 after persisting for a few weeks, may disappear as 

 suddenly as they came, to be followed by swarms of 

 Copepoda and many other kinds of minute animals, 

 and these again may give place in the autumn to a 

 second maximum of Diatoms' or of the closely related 

 Peridiniales. Of course, there are theories ae to all 

 these more or less periodic changes in the plankton, 

 such as Liebig's"law of the minimum," which limits 

 the production of an organism by the amount of 



5 According to Schmidt'< results, the European fresh-water eel, in order 

 to be able to propagate, requires a depth of at least 500 fathoms, a salinity 

 of more than 35'20 per mile, and a temperature of more than 7* C. in the 

 required depth. 



that necessity of existence which is presenr in least 

 quantity, it may be nitrogen or silicon or phosphorus. 

 According to Raben, it is the accumulation of silicic 

 acid in the sea-water that determines the great in- 

 crease of Diatoms in spring and again in autumn. 

 Some writers have considered these variations in the 

 plankton to be caused largely by changes in tempera- 

 ture supplemented, according to Ostwald, by the 

 resulting changes in the viscosity of the water; but 

 Murray and others are more probably correct in 

 attributing the spring development of phyto-plankton 

 to the increasing power of the sunlight and its value 

 in photosynthesis. 



Let us take next the fact — if it be a fact — that the 

 genial, warm waters of the tropics support a less 

 abundant plankton than the cold polar seas. The 

 statement has been made and supported by some 

 investigators and disputed by others, both on a certain 

 amount of evidence. This is possibly a case like some 

 other scientific controversies where both sides are 

 partly in the right or right under certain conditions. 

 At any rate, there are marked exceptions to the 

 generalisation. The German Plankton Expedition in 

 1889 showed in its results that much larger hauls of 

 plankton per unit-volume of water were obtained in 

 the temperate North and South Atlantic than in the 

 tropics between, and that the warm Sargasso Sea 

 had a remarkably scanty microflora. Other inves- 

 tigators have since reported more or less similar 

 results. Lohmann found the Mediterranean plankton 

 to be less abundant than that of the Baltic ; gatherings 

 brought back from tropical seas are frequently very 

 scanty, and enormous hauls, on the other hand, have 

 been recorded from Arctic and Antarctic seas. There 

 is no doubt about the large gatherings obtained in 

 northern waters. I have myself in a few minutes' 

 haul of a small horizontal net in the north of Norway 

 collected a mass of the large Copepod, Calanus fin- 

 marchicus, sufficient to be cooked and eaten like 

 pKJtted shrimps by half a dozen of the yacht's com- 

 pany, and I have obtained similar large hauls in the 

 cold Labrador current near Newfoundland. On the 

 other hand, Kofoid and Alexander Agassiz have re- 

 corded large hauls of plankton in the Humboldt cur- 

 rent off the west coast of America, and during the 

 Challenger Expedition some of the largest quantities 

 of plankton were found in the equatorial Pacific. 

 Moreover, it is common knowledge that on occasions 

 vast swarms of some planktonic organism may be 

 seen in tropical waters. The yellow alga, Tricho- 

 desmium, which is said to have given its name to the 

 Red Sea, and has been familiarly known as " sea- 

 sawdust " since the days of Cook's first voyage,* may 

 cover the entire surface over considerable areas of 

 the Indian and South Atlantic Oceans; and some 

 pelagic animals, such as Salpae, Medusae, and Cteno- 

 phores, are also commonly present in abundance in 

 the tropics. Then, again, American biologists ' have 

 pointed out that the warm waters of the West Indies 

 and Florida may be noted for the richness of their 

 floating life for periods of years, while at other times 

 the pelagic organisms become rare and the region is 

 almost a desert sea. 



It is probable, on the whole, that the distribution 

 and variations of oceaniQ currents have more than 

 latitude or temperature alone to do with any observed 

 scantiness of tropical plankton. These mighty rivers 

 of the ocean in places teem with animal- and plant- 

 life, • and may sweep abundance of food from one 

 region to another in the open sea. 



But even if it be a fact that there is this alleged 

 deficiency in tropical plankton, there is by no means 



" See "Journal" of Sir Joseph B.mks. This and o'her swarms were 

 also noticed by Darwin during the voyace of the Bcag/e. 

 1 A. Agassiz A. G. Mayer, ami H. 15. Hiijelow. 



NO. 2652, VOL. 105] 



