MARINE PISCICULTURE 319 



ductive, and the same fishing operations which 

 formerly caught a reduced number of fish will 

 now catch more at the same expense. We must 

 remember, however, that the whole number of 

 fry turned out from the hatchery does not re- 

 present clear gain, but the gain is the difference 

 between the assumed (greater) number of fry 

 produced in the hatchery and the (lesser) number 

 which would have been produced from the same 

 parent fishes if they had been allowed to remain 

 in the sea, and this difference must be the basis of 

 the profit and loss calculation. There would be 

 a difference, (i) because of the advantages gained 

 in hatching under artificial conditions, and (2) 

 because some of the parent fish would, presumably, 

 have been caught if allowed to remain in the sea ; 

 but the gain resulting from (2) is to be discounted, 

 because if some of the parent fish had been caught 

 fishing operations would have been more lucrative 

 than if there had been no hatching. Again, it is 

 tolerably certain that all the marketable fishes 

 added to the sea by hatching would not have 

 been caught, any more than all the fishes of the 

 same stage present in the sea apart from those 

 artificially added. The argument, indeed, cannot 

 be considered to be a sound one, but I mention 

 it because it has occasionally been used. 



The argument a posteriori is much more difficult 

 to discuss, and it is very probable that the materials 

 for its proper consideration do not exist in the 



