3O V.CTOR E. SHELFORD. 



ponds may be taken as an index of plankton production. It 

 appears that this must be true on tin- l>a-i- of the conclusions 

 of Pond ('05) no matter what factor is of greatest importance in 

 controlling the quantity of plankton. Johnstone CoS) pointed 

 out that the plankton production follows Liebig's law of mini- 

 mum i. e., quantity is determined by the food substance present 

 in minimal quantity. If rooted vegetation is the controlling 

 factor a deficiency in one food substance in the soil would show 

 itself in the rooted vegetation and through this affect the plankton 

 production of the pond. 



The question of the general application of the principle of 

 quantitative increase with age is important. It seems probable 

 that in all bodies of water with small outflow organisms increase 

 with age because, in addition to the effect of rooted vegetation, 

 inwash continuously brings food substances which are tied up 

 if not carried away by extensive outflow. 



Experimental study of the quantitative problem is possible on 

 the basis of such a set of ponds as those at the head of Lake 

 Michigan. From such a set all the organisms can be trans- 

 planted and most of the conditions duplicated where closer control 

 would be possible than in the natural ponds. There appears 

 to be no difficulty in such experimental study except that it 

 requires extensive facilities and institution or government sup- 

 port. Such ponds as ours and such ponds as may be constructed 

 with them as a basis give promise of throwing more light on the 

 factors controlling the quantity of life than do the large and 

 complex bodies of water. 



2. Economic. The writer has no practical knowledge of fish 

 culture and only the knowledge which has been acquired by reading 

 some of the characteristic literature. Apparently the economic 

 problems in fishes are concerned with questions of the preserva- 

 tion of fishes in natural waters, and their increase and main- 

 tenance against the removal tor tood, which makes them ot 

 economic importance. With these ends in view efforts have long 

 been made mainly to increase fish by increasing food suppK , to 

 care for fish during the critical reproductive season by artificial 

 hatching and pond culture, and to decrease enemies by de^t ruc- 

 tion of objectionable fish and fish parasites. The preservation 



