— 156 — 



which may be of shorter or longer duration, but which in the end passes over into a 

 bottom stage (exception perhaps the Pou tas sou). Even if the cessation of the pelagic 

 stage is far from being so well-marked in morphological regard as for example in the 

 eels and flat-fishes, which undergo a complete transformation, yet there are often outer 

 signs in the gadoids which indicate that the pelagic stage is over and the bottom stage 

 reached. Thus, the extremely elongated, postlarval, ventral fins in Molva, Brosmius and 

 others become reduced, and the cod ((?. callarias) assumes the well-known and charac- 

 teristic tesselated marking. 



The duration of the pelagic stage is very various in the different species. It may 

 be said in general that it lasts longest in the species spawning in deepest water (e. g. 

 Molva species, G. Poutassou, Brosmius), but this rule is by no means without exceptions. 

 The duration is longest in Gadus Poutassou which is pelagic throughout almost the whole 

 of its life, shortest most probably in the distinctly shallow-water species Gadus lusctis, 

 and all the other species are intermediate in this regard. The duration of the pelagic life 

 is a matter of great importance, as the longer it is in a species the greater is the chance 

 of its fry being carried passively for long distances away from the spawning places (see 

 later Part D on the drift of the pelagic fry). 



It seems to be a general rule that the fry of the same species have a longer pe- 

 lagic life in the northern than in the southern part of its distribution 

 (cf. the cod at Iceland and S. W. of Ireland). This often has the result that the fry of 

 the same species reach a larger size in northern waters before seeking the bottom than is 

 the case in southern waters. 



We have already seen in several cases that the young bottom stages can live under 

 very different conditions from the adult fish. With regard to depth, for example, 1 may 

 just call attention to the fact that amongst gadoids (as amongst flat-fishes) there is a 

 group of species in which the young bottom stages live chiefly within the tidal 

 margin close to the shore in quite shallow water, independent of whether the 

 species spawns in deep or shallow water. To this group, which thus stands in contrast to 

 all our other gadoids, belong the cod (G. callarias), coalfish (G.virens), pollack {G- 

 pollachius) and polar cod {G. saïda), thus just the most nearly related species of the 

 genus Gadus, which in systematic regards constitute a sharply defined group in them- 

 selves (see p. 46). 



D. Drift of the pelagic gadoid fry 



Whether the young fishes during their pelagic life will be carried far away from the 

 places where they were spawned and where they will be carried, depends naturally on the 

 strength and direction of the movement in the water-masses in which they occur. If 

 these tend in the direction from the great oceanic depths in towards the land the fry of 

 the deep-sea fishes will be carried towards the coastal banks, and on the other hand the 

 fry of the coastal fishes will be carried out over great depths if the waters move in op- 

 posite direction. 



On account of the general direction of the currents within the regions of the At- 

 lantic investigated by us the pelagic fry will be carried mainly in a direction parallel to 

 the curves of depths, or if this does not hold good from deeper towards shallower water. 

 Thus there is in general no great drift of the pelagic fry from the coastal banks out over 



