abundant on the bottom at other times. Tlie ranges 

 of all the ciscoes mentioned here, especially the 

 kiyi, extend into water deeper than 50 fathoms in 

 southeastern Lake Michigan. 



Lake whitefish (5 collected) were taken at 3 to 

 20 fathoms. Seven carp were caught at 5 to 7 fath- 

 oms. Ninespine sticklebacks (3 collected) were at 

 depths of 20 to 50 fathoms. This species is more 

 common in bottom catches in most other parts of 

 the Lake and is occasionally taken at midlevels. 

 Johmiy darters (19) were collected at 7 to 35 

 fathoms (mostly 12-17) and logperch (12) at 5 to 

 12 fathoms. Single individuals of gizzard shad, 

 white sucker, and quillbaek were caught at 3, 5, 

 and 15 fathoms, respectively. 



EFFECTS OF TEMPERATURE 

 ON DISTRIBUTION 



Temperature was the most obvious factor in the 

 depth distribution of fish in this study. Even in 

 winter, temperature influenced the distribution of 

 some of the species. Alewives and bloatei-s, for ex- 

 ample, congregate in the deeper areas to avoid the 

 colder inshore waters. Numbers of both species 

 increased significantly at 15 to 20 fathoms between 

 February 13 and March 11 as bottom temperatures 

 rose from 0.3-0.7° C. to 1.3-2.1° C. The movement 

 toward shore in the spring of all species except 

 tlie sculpins is closely correlated with a warming of 

 inshore waters. Tlie shift of all species except the 

 fourhorn sculpin into deeper water in the fall also 

 is obviously related to temperature. As warm water 

 steadily encroaches on deeper bottom areas warm- 

 water species spread with it, and cold-water forms 

 move deeper to avoid it. The control of temperature 

 over depth distribution is bast seen in summer when 

 thermal changes are most pronounced. 



After thermal stratification, bottom tempera- 

 tures in water shallower than 12 or 15 fathoms in 

 southeastern Lake Michigan are seldom stable for 

 any length of time. Direction and speed of wind 

 influence water temperatures strongly. Onshore 

 winds thicken the epilimnion near shore and in- 

 crease the depth to which warm water extends on 

 the bottom ; olf shore winds have the opposite effect. 

 Exceptionally strong, persistent northeast or north 

 winds cause extensive upwellings along a large por- 

 tion of the east shore of the Lake and hypolinuiial 

 water of 5 to 6° C. may replace water of 20° C. in 



1 or 2 days. At such times cold water from surface 

 to bottom may extend from shore several miles 

 out into the Lake, and no warm bottom water is 

 available to the fish. 



Catches of the standard series illustrate the in- 

 fluence of thermal conditions on the depth distri- 

 bution of fish in summer. A comparison may be 

 made between the catches of July 7, when epilim- 

 nial waters near shore were relatively cool and the 

 hypolimnion extended into shallow water, and 

 July 28, when epilimnial waters were much warmer 

 and reached the bottom to greater depths. The 

 thermal conditions of July 28 were more common 

 for sunnner ; those of July 7, though less frequent, 

 were by no means unusual. Species in the shallowest 

 water had a deeper distribution on July 28. 

 Bloaters and slimy sculpins showed changes only 

 in the shallow extremity of their depth range be- 

 cause the temperature in the remainder of their 

 range was unchanged. The fourhoni sculpii^ was 

 too deep to be atfected. 



The standard series were too infrequent to show 

 the rapidity of movement in response to tempera- 

 ture change ; however, evidence from other trawl- 

 ing indicates that appreciable movement may oc- 

 cur in a few hours, and that major redistribution 

 may take place in 2 or 3 days. On August 24, 1963, 

 tows were made in the morning (0807-1001 e.s.t.) 

 and afternoon (134-J-1541 e.s.t.) at 7, 10, and 12 

 fathoms off Saugatuck during a period of rather 

 rapidly changing bottom temperatures. The tem- 

 perature remained steady at 16.5° C. at 7 fathoms 

 but rose from 12.6 to 14.0° C. at 10 fathoms and 

 from 12.4 to 13.3° C. at 12 fathoms (table 11). At 

 the same time several species moved into deeper 

 water. Spottail shiners decreased drastically at 7 

 fathoms and increased greatly at 10; several ap- 

 peared in the afternoon catch at 12 fathoms where 

 none had been taken in the morning. Catches of 

 yellow perch decreased slightly at 7 fathoms and 

 strongly at 10 fathoms and increased drastically 

 at 12. No other species were present in significant 

 ninnbers at 7 fathoms in either period. Trout- 

 perch decreased appreciably at 10 fathoms and in- 

 creased correspondingly at 12. Bloaters decreased 

 at both 10 and 12 fathoms. Smelt showed only a 

 slight change at 10 fathoms and none at 12. Ale- 

 wives did not change significantly. Sculpins were 

 not caught in any of the tows. These changes do 

 not necessarily indicate that individual fish 



FISH DISTRIBUTIOX IN SOUTHEASTERN LAKE MICHIGAN 



13 



