CRUSTACEA AS FOOD FOR YOUNG FISH.. 7 



those recorded at Plon. He had built a dam across a brook to obtain 

 a head of water to use in a hatchery. The ground was part of an old 

 pasture and cows waded freely about the brook and, for a time, in 

 the small pond formed by the completion of the dam. A portion of 

 ground thus flowed was a small alder swamp. Late the following 

 winter there came down into the hatchery great numbers of living 

 Entomostraca, mostly daphnids. At that time it was the practice to 

 filter the water through a flannel screen set across the head of each 

 trough, and so great was the number of Entomostraca that they often 

 completely clogged up the screens, causing the Avater to overflow. 

 This continued for several weeks. The pond was covered with ice 

 all winter and the mean temperature of the water was as follows: 

 In November, 39.1° F.; in December, 35.17° F.; in January, 34.5° F. 

 Atkins remarked that whether these Entomostraca subsisted on their 

 normal food (Protozoa) or not, it is evident that they found sufficient 

 nourishment even in that very cold water, and the conclusion was 

 warranted that daphnids might not only be bred in water cool enough 

 for salmon and trout, but that they prob- 

 ably might be brought out at any season of 

 the year by proper management. 



COPEPODA. 



The copepods are the perennial Entomos- fig. 3.— c ope pod. cuciops 

 traca of open water. They are of extensive ffiS''' '^"" ''"'' 

 geographical distribution both in salt and 



fresh water and are present summer and winter. Deep lakes and shal- 

 low lakes have their characteristic copepod faunas, but this distinction 

 does not rigidly hold, for frequently the species show a great deal 

 of flexibility in adapting themselves to changed conditions. Ex- 

 cepting the few winter forms, the. maximum numbers of any species 

 occur in the months from May to September or early November. 

 Sometimes there are two maxima, one in the spring and one in the 

 fall (C. Dwight Marsh, 1918). Atkins (1894) stated that many 

 copepods were found in early spring in very cool pools, sometimes 

 in Craigs Brook, even anticipating the disappearance of the ice. 

 They are small organisms, the different species varjang in length of 

 adults from about 0.5 to 4.5 mm. (0.019 to 0.17 inch). ^ 



The most common species of fresh-water copepods is Cantho- 

 camptus Tiiinutus, and it is found everywhere in the northern conti- 

 nents. Cyclops leuckarti is widespread and common. Cyclops hicus- 

 pidatus is the most common of limnetic species and is characteristic 

 of the Great Lakes. Cyclops viridis is a widely distributed species 

 occurring in pools, ponds, and lakes, and attams a length of 1.25 

 to 1.50 mm. (about 0.045 to 0.059 inch). Copepods feed upon 

 animals, plankton, and algae, especially diatoms. They are them- 

 selves important food for fishes, especially for young fishes. 



OSTKACODA. 



The ostracods are minute crustaceans, the head, body, and append- 

 ages of which are inclosed in bivalve shells, whence the name, 

 meaning shell-like. They are exceedingly common and of extensive 

 geographical distribution. Some are free-swimming, while others 



