FISHERY BULLETIN: VOL. 73, NO. 1 



mullet (primarily Mugil curema), the creek func- 

 tioning at ambient temperature affords a nursery 

 habitat for a much greater array of utilizable 

 species. In both of the thermally affected creeks 

 there was a marked diminution in the numbers 

 and the biomass of the following seven species of 

 utilizable fishes (listed in order of decreasing 

 abundance in ambient temperature creek): tide- 

 water silverside Menidia beryllina, spot Leios- 

 tomus xanthurus, Atlantic vcvevihsiden Brevoortia 

 tyrannus, silver perch Bairdiella chrysura, Atlan- 

 tic thread herring Opisthonema o^/mum, Atlantic 

 croaker Micropogon undulatus, and pinfish 

 Lagodon rhomboides. 



The shallow-water habitats afforded by marsh- 

 land creeks are especially important to estuarine 

 fishes during the hot season. As shown by McEr- 

 lean et al. (1973), both the number of species and 

 the number of individuals that rely on the shallow 

 shore zone of the estuary are maximal in summer 

 and spring. This is due in part to the time of 

 spawning in many species, frequently in late 

 winter or in the spring, and to the subsequent 

 immigration into estuarine areas of juveniles of 

 the many transient species. The September de- 

 cline in both the number and the biomass of fishes 

 that occurred in the ambient temperature creek 

 (Browns Creek) is similar to the fall decline de- 

 scribed by McErlean et al. for shallow-water sta- 

 tions in the Patuxent River. 



In the review by Wurtz and Renn (1965) on 

 water temperature and aquatic life, the following 

 statements were made: "Although there are his- 

 toric records of fish surviving natural tempera- 

 tures of 100°F, this is unusual. Waters with tem- 

 peratures that regularly exceed 95°F would not be 

 expected to support a large, or diverse, fish popula- 

 tion." It is noteworthy that throughout the period 

 of this study, June through September, daily 

 temperatures in San Carlos Creek routinely ex- 

 ceeded 95°F (35°C). In July and September, the 

 average daytime temperatures recorded at all sta- 

 tions in this creek were, with but one exception, 

 greater than 95°F (see Table 1). A maximum 

 temperature of 102.6°F (39.2°C) was recorded at 

 one station in July. Although the overall popula- 

 tion of fishes found in San Carlos Creek was 

 neither large nor very diverse, 15 species exhib- 

 ited no marked diminution in numbers, and a few 

 species were actually more abundant here than in 

 the ambient temperature creek. Among the 

 species of temperature tolerant fishes recorded in 

 the current study, the gray snapper was reported 



by Nugent (1970) as being a species whose num- 

 bers were not diminished by the thermal effluent 

 of a power plant situated to the south of Miami. 

 Although the gray snapper was the most abun- 

 dant species collected in the gill net samples re- 

 ported by Nugent, it was a very minor member of 

 the fish population sampled in our study. Nugent 

 also reported that the tarpon, Megalops atlantica, 

 was equally abundant at both heated and control 

 stations. In the current study large tarpon were 

 seen during midsummer within a few hundred 

 yards of the outfall of the power plant in San 

 Carlos Creek although no specimens were col- 

 lected. Nugent reported a decrease in the numbers 

 of white mullet (primarily adults taken by gill 

 nets) at stations in the area of elevated tempera- 

 ture during the hot season. In the current study 

 the estimated density of juvenile white mullet was 

 quite similar in all of the creeks. Trembley (1961) 

 reported that mummichogs from the Delaware 

 River, acclimated to 32.2°C, had an LD 50 (mean 

 lethal dose) of 39.4°C. If this LD 50 approximates 

 that in our own study area then the presence of 

 large numbers of this species in San Carlos Creek 

 indicates that it is literally thriving in an envi- 

 ronment in which the temperatures encountered 

 daily during July are very close to the upper limits 

 for survival. 



The current study was directed primarily at an 

 analysis of the effects of a thermal effluent and 

 related conditions upon the population of juvenile 

 fishes utilizing a nursery habitat during the hot 

 season. It might be contended that the effects de- 

 scribed herein for juvenile fishes do not necessar- 

 ily apply to the population of adult fishes using the 

 same heated creeks. However, de Silva ( 1969) sur- 

 veyed literature relating to differential tolerances 

 exhibited by both juveniles and adults of several 

 species. He concluded that many juveniles appear 

 to tolerate eurythermal conditions, while adults 

 tend to be broadly stenothermal. If the above 

 generalization is correct, then one would predict 

 that the effect of the thermal effluent on the popu- 

 lation of adult fishes would be even more extreme 

 during the hot season than the effects we recorded 

 regarding the juvenile fishes. 



We recognize that some of the population 

 changes ascribed to elevated water temperatures 

 in the current study may be due in part to other 

 factors operating in concert with temperature. As 

 was pointed out by Mihursky and Kennedy ( 1967), 

 temperature tolerance limits of aquatic organisms 

 are affected by a number of other variables such as 



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