TEMPERATURES OF LAKE MICHIGAN, 1930-32 



U, S, 



by 



John Van Oosten 

 Fish and Wildlife Service 

 Ann Arbor, Michigan 



ABSTRACT 



Lake Michigan temperatures were obtained at 136 stations, June 18-November 14, 

 1930, May 8-November 2, 1931, and April 19 -September 12, 1932. 



One hundred and fifty (ISO) series of vertical temperatures were obtained south of 

 the Frankfort -Algoma line, 58 above this line in the lake proper, and 39 in Green Bay. 



The text presents the data of relatively shallow and deep stations in nine regions 

 covering the range of dates and depths, the seasonal changes of surface, subsurface, 

 and bottom temperatures, the shifting of S-degree water, and the depth intervals and 

 thermal gradients of thennoclines. General compansons were made of the temperatures 

 of the various regions. 



INTRODUCTION 



This report presents all records of 

 the Lake Michigan temperatures obtained 

 during the 1930-32 investigation described 

 by Van Ot^ten and Eschmeyer (1956). This 

 investigation was conducted primarily to 

 determine how the destructiveness of small- 

 mesh gill nets to immature lake trout might 

 be reduced by such means as mesh size ad- 

 justments and the restriction of fishing to 

 certain depths of water or areas. However, 

 considerable information was also obtained 

 on the other species of fish taken with the 

 trout in the experimental nets. Only when 

 time was available, the weather favorable, 

 the lake relatively calm, and the currents 

 not too strong, could samples of the bottom 

 fauna, plankton, and bottom sediments be 

 collected at the fishing stations, water 

 temperatures recorded, and drift bottles re- 

 leased for the purpose of studying currents. 

 Temperatures were not obtained at all of 

 the 178 fishing stations or on every date 

 at the 136 localities listed in table 1 and 

 shown on the map (fig. 1). 



Of the 247 series of vertical tem- 

 peratures 150 were obtained in the area 



south of the Frankf ort-Algoma line, 58 

 above this line in the lake proper, and 39 

 in Green Bay. Nearly all were taken be- 

 tween the hours of 7 a.m. and 12 noon: 97 

 percent in 1930 and 1931 and 89 percent in 

 1932. Because the temperatures were not 

 uniform throughout the lake at all of the 

 same depths, the data were compiled by 

 local areas. Even in the same local region 

 the temperatures may differ significantly 

 on the same day. For example, on May 12, 

 1931, off the St. Joseph-South Haven shore 

 a temperature of 5.2° C. (41.4° F.) was 

 recorded at a depth of 37 meters (121 ft.) 

 in shallow water and was observed also at 

 the surface of a deep station, even though 

 the two stations were only 8.2 miles apart 

 on the vessel's course. Unlike in the 

 small lakes, the thermocline of Lake Michi- 

 gan was not always present in all areas at 

 the same time or same depth. In this study 

 a thermocline was considered a stratum of 

 water where the centigrade temperature 

 decreased at leg.st one degree per meter. 



The temperatures were recorded with 

 Negretti-Zambra and Richter-Wiese reversing 

 thermometer? (equivalent Fahrenheit tem- 

 peratures are shown in parentheses). These 



