784 



DEPTHS OF THE OCEAN 



A third and important point is that all the individuals of a 

 species do not spawn at the same time. Hensen himself 

 thinks that each fish spawns several times within a short period, 

 and besides the spawning season of each species varies from place 

 to place. At a definite moment it is thus impossible to find all 

 the eggs in the earliest stage, for as a matter of fact in the 

 Norwegian coast waters the same haul includes eggs in various 

 stages as well as larvse and more advanced young. As regards 

 Norwegian waters it is therefore, as far as 1 can see, at present 

 impossible to realise Hensen's idea of counting the fishes of the 

 sea, or to cope with the problem of calculating the stock arising 

 from the developed larvae. 



It is well known that in many countries a considerable 

 amount of work has been devoted to so-called artificial fish- 

 hatching, which consists in keeping the eggs until the minute 

 larvae have escaped. Hopes have been entertained of increas- 

 ing the fish-supply by means of this hatching, the idea having 

 prevailed that these larvae had a better chance of growing up 

 than the eggs. But when these minute larvae are placed in 

 the sea, where there are already great numbers of them, they 

 disappear from view in a few minutes, and their subsequent 

 fate is entirely unknown. All calculations as to how many of 

 them grow up must be based on unknown and uncontrollable 

 factors, and become all the more doubtful considering there 

 is now ample proof that the abundance of different annual 

 classes varies enormously in nature. 



Quantitative investigations of an entirely different kind have 

 in recent years been started by C. G. J. Petersen,^ who 

 constructed a bottom-sampler, or kind of gripper (see Fig. 575), 

 which, like a dredging apparatus, brought up a large sample 

 from the surface of the sea-floor. The bottom-sampler is 

 intended to cut out a sample of one square foot from the 

 bottom, which is passed through sieves, the sand and mud 

 being sifted off, leaving the animals to be classified, measured, 

 counted, weighed, and finally submitted to chemical analysis. 

 These investigations on the abundance of bottom animals are 

 simpler than those dealing with the pelagic organisms, which 

 move so freely in a horizontal as well as in a vertical direction. 



Petersen has also attempted to solve the problem of the 

 quantity of fishes by experiment. "-' He captured great numbers 



1 Report of the Danish Biol. Station, No. xx., 191 1. 



^ " The Labelling of Fish in the Sea," Fishery Report for the Years i8SS-i88g, and Report 

 from the Danish Biol. Station, No. iv., 1893. 



