28 U. S. BUREAU OF FISHERIES. 



certain Americans made the same claim. Later the subject was 

 engaged from a biological standpoint. Recently Shira." having cal- 

 culated the possible reproductive capacity of certain forms, stated 

 that to determine the possibilities of raising quantities of those organ- 

 isms it was only necessary to multiply the number indicated as the 

 average reproductive capacity by thousands. So, basing his con- 

 clusion upon the calculations and upon certain experiments in raising 

 crustaceans, he pronounced the culture of Daphnia and " fresh- 

 water " shrimp, in quantities to furnish an adequate supply of fish 

 food, as perfectly feasible. 



One of the reasons previously given by Embody (1911) for the 

 selection of Gammcums fasdatus as the particular form likely to give 

 the best results if propagated as fish food was its greatest reproduc- 

 tive capacity. Observations upon fecundity or calculations of pos- 

 sible reproductive capacity of crustaceans alone do not afford any 

 basis for positive conclusions regarding the practicability of raising 

 them, or the selection of species to be raised. Prolific offspring pro- 

 duction in organisms indicates only a provision of nature for the 

 perpetuation of the species, not the " geometrical progression " of the 

 number of individuals. It is an adaptation tending to maintain a 

 natural balance ; in other words, an offset to adverse conditions which 

 naturally beset the species. Expectancy of multiplication and dura- 

 tion of life of aggregations of offsprings can not be based upon the 

 number of eggs or young produced without taking other factors into 

 consideration. For instance, this expectancy is greatly reduced as 

 respects organisms which are limited in powers of accommodation to 

 changes of conditions and as applies to those fonns which are eaten 

 by other organisms or eat each other. 



On the other hand, when favorable predominate over unfavorable 

 conditions, the expectancy rises above normal in direct accord with 

 the degree of predominance of favorable conditions. In the culture 

 of crustaceans, then, it should be expected that the nearer the arti- 

 ficial conditions are to natural conditions the nearer will be the ap- 

 proach to a balance. The degree of removal of unfavorable condi- 

 tions,* or protection against them, and the degree of maintenance or 

 increase of favorable conditions will determine the degree of in- 

 crease in numbers and quality of the organisms. Even if an in- 

 dividual Baphnia pulex should, as has been calculated, produce 13,- 

 000,000,000 progeny in 60 days, the assum])tion is not warranted that 

 there will be that many individuals at the end of 60 days to survive 

 and breed in the next breeding period. The same may be said re- 

 garding the reproductive capacity of any of the other crustaceans. 

 As previously indicated, any calculations based upon such figures 

 alone yield no positive results, are mere mathematical gymnastics, 

 and are more entertaining as curiosities of figures than of practical 

 value. However, should it ever happen that a crustacean-raising 

 establishment should be devised, whereby unfavorable conditions are 

 greatly reduced or wholly eliminated, then the known reproductive 

 capacity of the organisms in a given time would indicate Avhich form 

 would the most quickly yield the largest number, the approximate 

 number of individuals that would have to be provided for, and the 

 capacity of the rearing inclosures. 



« Unpublished manuscript, 1918. 



