lu the surface layers of the open sea (catadromous habit), the all essential end is secured and 

 the young fishes hatch out under the beneficent rays of the sun. 



The second kind of seasonal movement is a feeding migration. In order to cover a wide 

 area in their search for food a race of fishes will scatter over the entire geographical extent of 

 its habitat. Certain kinds set out on their own to seek their chosen food, while others disperse 

 in small schools. Finally, others, especially those that live on the microscopic algae of the 

 plankton, keep together in dense shoals, to graze on these floating pastures. 



These yearly movements are directly subject to conditions of temperature and salinity of 

 the water masses, most species having a habitat strictly determined by precise physico-chemical 

 laws. Concerning temperature conditions, this habitat is more often fixed over a range of 

 several degrees centigrade, including the minimum temperature at which the species can live. 

 With regard to salinity, the variations that can be tolerated are even slighter. Fishes submitting 

 to these severe rules are called stenothermal and stenohaline in habits — habits that can change 

 according to the age and sexual state of a species. But in each biological phase the rules are 

 faithfully followed. 



The sedentary, shallow-water, fishes are subjected to great seasonal \arialions in temper- 

 ature and salinity and tolerate them fairly well. These are eurythermal and euryhaline in 

 habit. But their resistance is limited; for very cold winters, hot summers and persistent 

 downpours can cause a heavy mortality. 



One of the consequences of a stenothermal and stenohaline habit is that fishes are inescap- 

 ably confined to those water masses essential for the life of their kind and must follow such 

 waters in movement. Their habitat is no longer geographical ; it is uniquely determined by 

 hydrological conditions — by temperature and salinity. In this way the laws directly governing 

 a species pass into those presiding over the immense circulation of oceanic waters. 



Within the limits of this book it is impossible to give even a concise summary of oceanic 

 water movements. We can merely point out that sea-water is not homogeneous and that 

 two great categories can be recognised : waters of polar origin; cold, not very saline (less than 

 35 "/oo), heavy, passive and very rich in plant and animal plankton : waters of equatorial origin ; 

 warm, quite saline (more than 35 "/oo), light, active, mobile and rather poor in pelagic life. 



There is unceasing interplay between these waters, but they retain their imlividuality. 

 As a matter of fact, the principle of immiscibilily of waters, established in 1868 by the British 

 scientists Wyville-Thomson and Carpenter, shows that waters of difTorent temperature and 

 salinity do not mix when in great masses. Waters of polar origin, sinking towards the depths, 

 occupy the bottom of the ocean in the abyssal zone. Like all water masses these are influenced 

 by the force of the earth's rotation and drift westwards to hug the continental masses. In 

 the North Atlantic the Labrador Current, the outcome of the Arctic pack-ice, comes to skirt 

 the east Greenland coast, to run down past the Newfoundland Banks and along the American 

 coast as far as Cape Hatteras. In the Northern Pacific, the cold Oyashio Current emerges 

 from the Bering Straits to fill the Okhotsk Sea and iiathe the Kurile Islands and the Japanese 

 island, Hokkaido. The Falkland Current of the South Atlantic moves north from Tierra del 

 Fuego along the Patagonian and Argentinian coasts as far as Brazil. In certain seasons the 

 drift of polar waters leaves a space, as it were, in the eastern part of the oceans and equatorial 

 waters moving into this region flow northwards in the northern parts of the Atlantic and Pacific 

 Oceans. More than 3U years ago I traced this yearly encroachment of warm salty water over 

 waters of polar origin and gave to all such water movements the name of oceanic transgressions. 



These transiiressions stem from the tropical regions. In the Atlantic they move by .Morocco 

 in the winter, following the Iberian coast in spring to enter the Bay of Biscay in summer. Then 

 skirting the British Isles they enter the North Sea from July to September. Continuing their 

 northward drifl, by nutiiinn Ihoy are foumi off Scandinavia, while the Barents Sea is reached 

 at the onset of the winter. These splendid, clear, deep-blue waters thus encroach on the greasy 

 yellow-grey turbid polar waters in their own domain, seemingly bringing with them the rays of 

 the tropical sun to the dark and misty latitudes of the northern worlii. Along with these 

 waters ('onie fine swift fishes. With iridescent gleams they swim in these azure waves and 

 leave a luminescent wake by night. 



14 



