Table 11. — Trawl catches in identical morning (0807-1001 e.s.t.) and afternoon {134^.-15^1 e.s.t.) tows at various depths 



off Saugatuck, Mich., August H, 1963 

 [Bottom temperatures (° C.) with each tow are in parentheses) 



Species 



Alewift" 



Bloater 



American smelt. 

 Spottail shiner__ 



Trout-perch 



Yellow perch 



traveled a great distance (e.g., the 0.6 nautical mile 

 from 10 to 12 fathoms), but may reflect a moi-e 

 modest .shift of tlie entire populations of various 

 species. 



A pronounced shift in deptli distribution of fish 

 during a few days occurred between August 14 

 and 17, 1962, when trawl tows were made at 7, 

 10, 12, and 1.5 fathoms off Saugatuck. Bottom tem- 

 peratures in this jjeriod rose from 7.9 to 17.7° C. 

 at 7 fathoms and from 5.9 to 11.2° C. at 15 fathoms. 

 Spottail shinere were rare as deep as 7 fathoms at 

 the cooler temperatures on the earlier date, but 

 were common out to 12 fathoms when the water 

 warmed. Trout-perch were caught only at 7 fath- 

 oms on August 14, Ijut were concentrated at 15 

 fathoms on the I7th. Yellow perch, ])loaters, and 

 smelt moved similarly. Alewives, which character- 

 istically responded less to sudden temperature 

 changes than other species, showed little move- 

 ment. Sculpins %yere not in the catches. 



Shifts in the dejith distribution of fisli in shal- 

 low water in summer, although not usually as 

 abnxpt as the examples cited, are frequent if not 

 almost continuous in southeastern Lake Michigan 

 (and probably along other exposed shorelines of 

 the Lake as well ) as bottom temperatures fluctuate. 

 During upwellings, however, warm-water species 

 may suddenly be exposed to temperatures much 

 colder than those preferred from which they can- 

 not escape. Such conditions can oceur at crucial 

 times, such as the spawning .season or during hatch- 

 ing. It would seem then that temijeratures for 

 these species would be much more suitable in terms 

 of growth rates and spawning success in smaller 

 bodies of water or protected bays, where thermal 

 stability is greater. 



TEMPERATURE PREFERENCES 



Certain generalities regarding temperature pref- 

 erences of the various S{>ecies are possible from 

 tlie standard series and other trawling at Sauga- 

 tuck, although unstable thermal conditions and 

 other factors prevented any determination of either 

 exact temperature preferences or ''final pref- 

 erenda" (see Fry, 1947). These observations apply 

 to adult fish during i)eriods when the range of 

 temperatures on the bottom was wide. 



All species except fourhorn sculijins were in 

 water with a temperature range of several degrees. 

 Alewives apparently had the least temperature 

 specificity. At times they were simultaneously in 

 water from 5° C. or less to 20° C. or more, but most 

 were in water warmer than 8° C. Evidence sug- 

 gests that they tend to shun temperatures greater 

 than 22° C, although in some sjiawning streams 

 they must occasionally endure temperatures of 

 25° C. or more. Bloaters generally inliabited water 

 from 4 to 11° C, although a few usually were 

 present at teinijeratures se\eral degrees higher: 

 greatest concentrations often were at 6 to 10° C. 

 Smelt were mostly in water of intermediate tem- 

 peratures, about 6 to 14° C. Spottail shiners with 

 few exceptions were in water from 13° C. to the 

 warmest available. Ti"out-perch were mainly in 

 water of 10 to 16° C. Yellow perch were usually 

 in water above 11° C. and were occasionally abun- 

 dant in water as warm as 22° C. ; substantial num- 

 bers, however, were found at temperatures as low 

 as 8° C. Slimy sculpins were seldom taken in water 

 above 10° (\, and most were at 4 to 6° C. Occasion- 

 ally in fall overturns, however, when moderately 

 warm water reached bottom at gi-eater depths. 



14 



U.S. FISH AND WILDLIFE SERVICE 



