I 



desirable to count only a part of the residue of the washed sample and 

 apply a factor to determine the total number. In such exceptional 

 cases, care was taken to obtain a representative portion of the residue. 

 This was done by adding water to the residue in a large beaker, mixing 

 completely, and dipping up a sample before there was time for settling. 



Because of the difficulty in getting samples on hard bottom and 

 in getting the active organisms which live on or just above the bottom, 

 the dredge hauls were supplemented by samples taken with a bottom sled 

 (Helgoland trawl). The sled consisted of a sheet of heavy galvanized 

 iron about 6 feet long and 3 feet wide, with a metal arch attached at 

 the front end, to which was laced a bag of No. 0000 bolting cloth of the 

 same shape as the sled. The front end of the metal sheet was turned up 

 slightly to prevent the catching of snags. The sled was usually towed 

 for a period of ten minutes at a speed of five miles per hour. The 

 organisms taken were identified but not counted. 



The reader will find considerable difference in the exactness of 

 identification in different groups of organisms; some were identified 

 to species while others were taken no further than the order. The policy 

 in this regard was determined by expediency, that is, identifications 

 were carried out as far as the available time, importance of the organ- 

 isms, and knowledge of the staff members appeared to justify. Certain 

 forms were submitted to experts for identification. 



One group of organisms, the Nematoda, because of their small size 

 and transparency, present a difficult problem in counting. At times they 

 appeared in the samples in rather large numbers but it was found inad- 

 visable to devote the time necessary to obtain accurate counts. For 

 that reason, counts of nematodes ha-ve not been included in the tables of 

 this paoer. No attempt was made to §tudy the numerous microscopic 

 organisms which live on and in the bottom deposits. 



Criteria of pollution 



A study of the bottom deposits and the organisms living in or on 

 them is essentia] in any investigation of the suitability of a body of 

 water for fishes. The importance of the bottom and its associated 

 organisms arises from two facts: first, that the eggs of most fresh- 

 water fishes develop in contact with the bottom, and second, that the 

 bottom organisms are used as food by a number of species of fishes. One 

 of the essential needs of a developing fish egg is a constant supply of 

 oxygen. If the bottom on which the developing egg lies contains a large 

 amount of decaying organic matter, the available supply of oxygen will 

 be usurped by the organic matter in the process of decay, and the egg will 

 die. On such a bottom, too, there is danger of the egg's becoming 



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