PLANTS IN PONDFISH CULTURE. 11 



Plankton catches showed a hirger percentage of Scapholeberis in 

 pond ID than in all others. Attention was then concentrated upon 

 this pond. Plankton studies were made from various regions in the 

 pond to determine the specific plant habitat of the cladoceran. This 

 proved to be the scum or film produced by the alga, Mougeotia 

 sphce7'ocarpa^ in the quieter portions of the pond. In these regions 

 the algal complex teemed with Scapholeberis, the feeding grounds 

 being conspicuously localized in the region of the Mougeotia fila- 

 ments. 



The examination of the food content in the digestive tracts of 

 Scapholeberis revealed a miscellaneous diet of small unicellular and 

 palmelloid algse which abounded in the Mougeotia complex. The 

 most common forms were small diatoms, and such green algae as 

 Sphserocystis, Oocystis, Cselastrum, and Schizochlamys. 



The association of this cladoceran w'itli the Mougeotia complex 

 adds another point in favor of the introduction and cultivation of 

 Mougeotia to supply a natural forage in fishponds. 



The Scapholeberis pulse in pond ID attained its maximum in late 

 July and early August. At that time it supplied a fair percentage 

 of the food of the largemouth black bass (Table 2) and a high per- 

 centage of the food of the bluegill (Table 3). 



Ponds ID and 2D have differed in the contribution they made to 

 the food supply of the young fish in the matter of quantity as well 

 as variety. This was particularly true of the Bosniina pulse which 

 occurred in both ponds simultaneously but to a lesser degree in pond 

 ID. Birge and Juday (1911) state: "The answer to the question 

 of why different bodies of water differ so widely in productivity is 

 wholly beyond our knowledge." In these ponds, however, a partial 

 explanation may be sought in the difference in kind and quantity of 

 living plants as well as in the accumulations of debris during suc- 

 ceeding seasons. 



In physical features the ponds are similar. They are equally deep, 

 and they are surrounded by a similar vegetation. They have been 

 treated alike, that is, they have been wintered full, not dry. Such 

 draining as has been necessary to conduct the seining operations has 

 been temporary only. Physiologically, however, they are more or less 

 distinct because of the dissimilar character of the vegetation in them. 

 Pond ID has been richly stocked with floating algse which at times 

 have covered the surface. Few of the larger rooted aquatics are 

 present. Pond 2D has no algal mats or blankets, but fully one-tenth 

 of the surface area has been covered by the large-rooted aquatic, 

 PotaTYiogeton iUinoensis, interspersed in places with the nonrooted 

 Ceratophyllum, or hornwort. In these ponds are present, apparently, 

 the requisite conditions to produce a conspicuous plankton pulse of 

 great economic value in terms of the natural forage of fish fry by 

 supplying in quantity organisms which are preferred by the fry 

 during the early days of feeding, 



DAPHNIA AND APHANIZOMENON ASSOCIATION. 



Data in the tables show that a high percentage of food is supplied 

 by the daphnids. They are recognized herbivores in the ponds, and 

 it remains to explain their particular plant preferences and associa- 

 tions. 



