Breder. — Marine Fish Culture. 217 



Europeans concerning the causes of fluctuations of fishes from 

 season to season give promise of developing into an interesting 

 and valuable viewpoint bearing on the future of the husbandry 

 of such fishes. It has been known for some time that practically 

 all marine fry of commercial interest reach a very precarious 

 stage of existence in from a few days to two weeks or more 

 after hatching, its time of appearance varying, of course, with 

 each species. In captivity even under the particular conditions 

 possible in an experimental laboratory it has been found next 

 to impossible to carry salt water fish successfully beyond this 

 point; nearly all species dying at the expiration of this definite 

 length of time which is about coincident with absorption of the 

 yolk sac. Apparently at, or some time before, this stage minute 

 forms of plankton as food are necessary in order to wean the 

 fry gently from its earliest mode of obtaining nourishment. At 

 least one reason for such small fry to fail of survival in captivity 

 is found in the difficulty of procuring this microplankton in the 

 proper quantity and quality. As in the final analysis plankton 

 is dependent chiefly on sunlight and temperature, the European 

 oceanographers deduce that both these factors are largely in- 

 volved in the fate of the fry for any one year. It is in this con- 

 nection that polar ice and the conditions of the sky are brought 

 in by them. As what are considered favorable seasons to this 

 microplankton occur but periodically, it follows that larval fishes 

 dependent on such would likewise flourish only periodically. 

 Scale examination substantiates this largely. That is, practically 

 . any school of adults shows a great preponderance of some par- 

 ticular year class. Figuring back this has been found to coincide 

 well v/ith some year especially favorable to these food organisms. 

 These successful years seem to be able to carry over the more 

 numerous poor ones when nearly all of the fry are annihilated. ^ 

 When this matter is better understood it may be turned to good 

 advantage in the matter of the management of marine hatcheries, 

 as it at least partially accounts for the high mortality of marine 

 fry at a set time after hatching. If the proper kind of plankton 

 could be cultivated and fed to the fry in such a manner as 



1 These views have been summarized and are set forth lucidly and without 

 the use of technical terms in the recent publication, "Ocean Research and the 

 Great Fisheries," by G. C. L. Howell, M. A. 1921. The Clarendon Press, Oxford. 

 England. 



