58 W. W. Reynolds, M. E. Casterlin, D. G. Lindquist 



fishes tested correspond fairly well to normal maximum summer temper- 

 atures in the lake. This is a typical result, in that freshwater and marine 

 fishes often prefer temperatures (final preferenda) that are normal sea- 

 sonal maxima in their habitats and geographic ranges (Reynolds & 

 Thomson 1974; Reynolds et al. 1977; Casterlin & Reynolds 1979a, 1982). 



Field observations of fish distributions and movements appeared lar- 

 gely consistent with our laboratory measurements. For example, as 

 inshore lake temperatures over a two-week period warmed from 27.4 ° to 

 29.4 °C at one location, fewer Fundulus and Menidia could be seen or 

 captured by seine inshore during the day, although they moved inshore at 

 night as some cooling occurred. Enne acanthus chaetodon was captured 

 in the cooler depths of backwater canals, which had lower temperatures 

 than occurred in the lake at that time. We did not see this species in the 

 lake, where temperatures were above its mean final preferendum. 



A more dramatic field example of apparent behavioral thermoregula- 

 tion occurred during the unusually dry and hot summer of 1980. Diurnal 

 temperatures in the lake shallows reached 40 ° C, and all fish then vacated 

 these lethally hot regions, at least during daylight hours (Lindquist and 

 Yarbrough, unpubl.). 



We often found males of both Noturus species guarding nests under 

 the same spawning cover, and their thermal preferenda (Table 1) appar- 

 ently differ little. The striking difference in their diel activity patterns (one 

 being nocturnal, the other crepuscular and mainly active at dawn) sug- 

 gests a temporal niche partitioning between these otherwise closely sim- 

 ilar species. 



The very high activity level of Lepomis marginatus (Table 1) is note- 

 worthy. It apparently reflects territorial or reproductive aggression, 

 which was occurring at that time, and may not be typical of other sea- 

 sons. For example, intense agonistic interactions, with resultant body 

 damage, was seen among individuals in holding tanks, although this 

 could not occur during the testing of single individuals in the Ichthyo- 

 tron. It is also interesting that L. marginatus preferred lower tempera- 

 tures than Lepomis macrochirus Rafinesque, the bluegill sunfish, which 

 prefers about 31 °C (Reynolds & Casterlin 1979) but ranges much farther 

 north. Enneacanthus chaetodon prefers temperatures similar to those 

 preferred by Enneacanthus gloriosus (Holbrook), the bluespotted sunfish 

 (Reynolds & Casterlin 1980; Casterlin & Reynolds 1979b). 



Etheostoma perlongum is only the third darter species to have been 

 successfully tested for thermoregulatory behavior in the laboratory. Hill 

 & Matthews (1980) reported thermal preference data for Etheostoma 

 spectabile (Agassiz) and Etheostoma radiosum (Hubbs & Black) from the 

 Blue River in Oklahoma, and found a correlation between thermoregula- 

 tory precision and the thermal stability of microhabitats occupied by 



