36 DEPARTMENT OF THE NAVAL SERVICE 



species, among those pre-eminently which on the eastern side of the Atlantic are 

 counted as spring-spawners. A prominent example is that of the cod (in addition 

 to haddock and JJrepanopsetta) which spawns in considerable numbers in the gulf of 

 St. Lawrence right on into the month of August, whereas in the Norwegian waters, 

 its spawning- is over in May. As a consequence of this, we may, in the gulf of St. 

 Lawrence, iind one and the same plankton haul to contain both newly-spawned cod 

 eggs and newly-hatched mackerel young. 



That cod and mackerel should find suitable spawning conditions in the same 

 water and at the same time is remarkable indeed; in European waters, the spawninjj 

 of these two species differs widely, both as regards time and place. What is more, it 

 is only exceptionally that the mackerel come so far north as to touch the principal 

 spawning grounds of the cod at all. The explanation may not unnaturally be imagined 

 to lie in the abrupt changes of temperature encountered in the Canadian waters. The 

 same natural conditions which cause Atlantic warm-water forms to meet, off the 

 coasts of Newfoundland, with representatives of the arctic fauna, might well account 

 for the fact that in the gulf of St. Lawrence, the cod of the far north are able to 

 live and propagate there simultaneously with the more southerly mackerel. If we 

 take a temperature series from the surface downwards in the gulf of St. Lawrence 

 during summer, we find, within a vertical range of less than 100 metres, a difference 

 in temperature equivalent to that encountered outside Newfoundland by sailing from 

 the Gulf Stream over into the Labrador current, while on the eastern side of the 

 Atlantic, such difference will involve a distance as from the North Sea to Spitzbergen. 



In the gulf St. Lawrence, then, we find the different water layers varying so greatly 

 in temperature as to afford favourable conditions both for cod and mackerel. That 

 both species should live in the same stratum . there is no reason to believe; on the 

 contrary, it must be presumed either that the fish are themselves able to seek out the 

 more suitable water layers, and avoid those less favourable, or that they are, owing 

 to certain biological features, more or less passively led to frequent such strata as 

 are by nature best adapted to their needs. 



It is a well-known fact that certain species of fish, such as cod, herring, and 

 mackerel, vary their habitat according to the temperature of the water. Cod and 

 herring may at times move in shallow, at others in deeper, water, and in the southern 

 Norwegian waters the mackerel seems rarely to move in towards the coast until the 

 surface temperature is at about 10° C, a fact so generally recognized that some of the 

 drift-net fishermen even carry a thermometer in order to make sure of not laying out 

 their nets in a cold current. 



Among the biological features which might be thought to act in this direction, 

 we should first of all remember that the cod is a bottom fish, belonging essentially to 

 the strata nearest the sea floor, where its food is found, and where its spawning takes 

 place. The mackerel, on the other hand, is a pelagic species, frequenting — at any rate 

 during the spawning season — the upper-water layers. Indeed, until the roe is spent, 

 the mackerel hardly seems able to penetrate down beyond some 20 metres depth. 



On glancing at a temperature series from the Princess station 30, where ova of 

 cod and mackerel were found in one and the same sample, the phenomenon will easily 

 be explained. 



Om t= le-l" ot = 20-27 



10 " IS-T" 20-33 



25 •' 10-S" 21-97 



4n " 7-9" 22-99 



50 " ; .. 5-65° 23-97 



fiO " 0-45° 25-40 



If we now apply to these figures what we know of the biology of the cod and 

 mackerel, it will be justifiable to conclude that the mackerel keep to the upper layers — 

 down to about 25 m. — where the temperature is favourable to its needs, -while the cod 

 seek the lower strata, finding there more suitable conditions in this respect. 



