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Richard W. Brill 



Southwest Fisheries Center Honolulu Laboratory 



National Marine Fisheries Service, NOAA 



P.O. Box 3830. Honolulu. HI 96812 



Present address: Department of Zoology 



The University of British Columbia 



207.5 We.'ibrook Mall. Vancouver, B.C. V6T IWS. Canada 



EFFECTS OF A THERMAL DISCHARGE ON 



REPRODUCTIVE CYCLES IN MYTILUS 



EDL'LIS AND MYTILUS CALIFORSIANL'S 



(MOLLUSCA, BIVALVIA) 



One principal concern about thermal effluents is 

 the effect of altered temperatures on the repro- 

 ductive biology of organisms near the discharge 

 (e.g., Hedgpeth and Gonor 1969). In marine mus- 

 sels of the genus Mytilus. the role of temperature 

 in regulating the reproductive cycle and the ef- 

 fects of temperature stress on the energy budget 

 for growth and reproduction have been particu- 

 larly well studied (Bayne 1975; Gabbott 1976; 

 Seed 1976). Mytilus edulis has a seasonal cycle of 

 gametogenic activity that is conditioned by tem- 

 perature and is linked with the storage and utiliza- 

 tion of reserve materials in the body (Bayne 1975). 

 Metabolism and filtration rate show complete 

 temperature acclimation from 5° to 20° C, and the 

 scope for growth is relatively independent of 

 temperature over this range (Widdows and Bayne 

 1971; Widdows 1973, 1978a). However, above 20" 

 C the mechanisms of temperature adaptation 

 break down, producing an increase in the 

 metabolic rate, a decline in filtration rate, and 

 thus a reduced scope for growth (Widdows 1976, 

 1978a). Above 25° C this scope is so reduced that 

 there is no energy for growth, and energy reserves 

 are depleted in order to survive (Widdows 1978b). 

 This study examined the effect of a thermal dis- 

 charge from a coastal steam-electric power plant on 

 reproduction in M. edulis and M. californianus in 

 central California. The reproductive cycles and 

 gonadal weights of these mussels in the warmwa- 

 ter outfall and in control regions of naturally oc- 

 curring temperatures were compared using body 

 component index methods. Water temperatures in 

 the outfall exceeded 20° C much of the late sum- 

 mer and early fall, while plant intake tempera- 

 tures were usually in the 12-15° C range and 

 rarely exceeded 17° C, 



Methods 



This study was conducted at the Pacific Gas and 

 Electric Company fossil-fuel power plant at Morro 

 Bay, Calif (Figure 1). The 1,030-MW plant used 

 ocean water for once-through cooling and dis- 

 charged warmed water into a canal about 80 m 

 long. The canal released water into the surf, form- 

 ing a plume with an isotherm 5° C above naturally 

 occurring temperatures of about 0.6-3.0 acres sur- 



498 



fi.shery bulletin vol 



. NO, 2, 1979, 



