468 V. S. BUREAU OF FISHERIES 



ments of 1930 at Sandusky, Ohio, on sauger grounds and during the 

 Sanger run in the spring of 1931. 



Accordingly two investigators were detailed at Sandusky and car- 

 ried on these experiments during the period March 12 to May 20, 

 1931. Length and weight measurements were secured for some 33,500 

 fish, including perch, saugers, and yellow and blue pike. It was 

 found that though the percentage of legal fish released by our 

 experimental net was greater in the case of the sauger than in the 

 case of any other species studied, yet the benefits secured by the 

 release of illegal fish would vastly compensate the fishermen for the 

 loss of any legal fish. On the basis of our experiments we expect 

 to recommend to the State of Ohio that the lifting crib of all trap 

 nets be provided with a large mesh (not less than 3 inches as used) 

 in the back and in one-third (or for a distance of not less than 3 

 feet) of the sides, top, and bottom adjacent to the back, and with a 

 small mesh (not more than 2 inches as used) in the remaining two- 

 thirds of the crib, including the tunnel. 



In addition to the field work, much has been accomplished in the 

 laboratory in a study of the life history of the yellow and blue pike 

 perch, the sauger, the yellow perch, the whitefish, and the cisco of 

 Lake Erie. The data for these studies were collected during our 

 investigations of experimental fishing gear. 



Limnological studies. — No field work was done in 1931 on the 

 limnology of Lake Erie, but the time of one investigator was devoted 

 entirely to the compilation of data collected over a period of five 

 years and to the preparation of the final report. As stated in pre- 

 vious reports, the limnological investigation of western Lake Erie 

 was carried on by the Ohio Division of Fish and Game, in coopera- 

 tion with the bureau, under the immediate supervision of Dr. Still- 

 man Wright. The principal objective was to determine the degree 

 and extent of pollution and the effect of pollution on conditions of 

 existence in the lake. The problem was attacked by making nearly 

 simultaneous studies of the chemistry, plankton, and bottom organ- 

 isms in the open lake, far from sources of pollution, and at the mouths 

 of important rivers. 



Conditions in the open lake were found to be apparently normal. 

 The only evidence of pollution was the occasional presence of B. coli. 

 At the mouths of the Maumee and Raisin Rivers there was consider- 

 able chemical evidence of pollution. However, the zones of oxygen 

 depletion and carbon dioxide increase were limited to a small area in 

 each case. Large amounts of soluble nitrogen compounds were added 

 to the lake by the Maumee River. There was no oxygen depletion at 

 the mouth of the Detroit River and the nitrogen content was low 

 because of the great volume of water in this river compared to the 

 volume of sewage added to it. 



Maumee Bay was found to be a center of production for both 

 phyto- and zooplankton. The planlrton Crustacea were nearly twice 

 as abundant here as in the open lake, and more than fifteen times as 

 abundant as at the mouth of the Detroit River. The abundance of 

 plankton in Maumee Bay is presumably the result of the large 

 amount of nitrogen compounds available. The paucity of plankton 

 at the mouth of the Detroit River is explained by the facts that Lake 



