Figure l. — Locations of the intake and outfall of the Morro Bay 

 power plant in California and the collecting sites for mussels in 

 this study. In addition to outfall samples of both species, control 

 samples of Mytilus eduUs were taken from site 1, and control 

 samples of M. caltforntanus were taken from site 2. 



face area depending on plant load and weather 

 conditions. The mean temperature differential be- 

 tween the intake and outfall was about 7" C and 

 ranged from about 3° C in spring to about 15° C in 

 late summer and fall (Figure 2). The intake 

 showed a seasonal cycle of low temperatures 

 around 11-12° C in winter and high temperatures 

 around 14"-15°C in summer and fall. Mean outfall 

 temperatures exceeded 20° C from May through 

 January and varied seasonally from around 18° C 

 in spring to around 26° C from July through Oc- 

 tober. Daily temperature fluctuations in the out- 

 fall were much greater (up to 11°C) than those in 

 the intake (up to 3°C). Also, heat treatment every 

 few weeks to kill organisms fouling the cooling 

 tubes raised outfall temperatures in places to as 

 high as 36° C for about an hour. 



Mytilus californianus and M. ediilis, were col- 

 lected from the warmwater outfall and from 

 nearby control areas of normal temperatures at 

 about monthly intervals from November 1972 to 

 November 1973 (Figure 1). Mytilus edulis were 

 collected from shallow ( 1-2 m) subtidal rocks mid- 

 way along the discharge canal and from the un- 

 dersides of floats near the intake. Mytilus califor- 

 nianus were collected intertidally at 1-3 ft above 

 mean lower low water at the mouth of the dis- 

 charge canal and at equivalent tidal heights from 

 the jetty at the Morro Bay harbor entrance. High 

 surf made collecting impossible on the jetty and 

 difficult at the mouth of the discharge canal at 



Figure 2. — Temperature records from the outfall and intake of 

 the Morro Bay power plant. Weekly mean temperatures were 

 calculated from continuous temperature recorders positioned at 

 mean lower low water near the power plant's intake screens and 

 discharge tubes. The dashed line marks the temperatures above 

 20' C. which are energetically stressful to Mytilus edulis. (Re- 

 drawn from Hines 1978.) 



times during the winter. Neither outfall nor con- 

 trol M. edulis were exposed to significant surf at 

 any time, but the outfall had much stronger cur- 

 rents (up to 0.7 m/s) than the control areas. 

 Salinities in the control and discharge areas did 

 not differ significantly from seawater. 



The temperature records closely reflect the 

 thermal environments of the samples of M. edulis, 

 because they were collected at nearly the same 

 locations as the recorders. However, the records do 

 not represent as closely the thermal regimes of the 

 samples of M. californianus. which were collected 

 from intertidal positions above the recorders and 

 were therefore exposed to air temperatures part of 

 the time, or which were collected at locations dis- 

 tant from the recorders. Seawater temperatures 

 for the control sampling site for M. californianus 

 were sometimes l°-2° C lower than intake temper- 

 atures, and temperatures at the mouth of the dis- 

 charge canal where outfall samples were taken 

 were often 2°-4° C lower than the records show due 

 to dilution of the warmwater discharge by incom- 

 ing surf. 



Monthly samples of 12 mussels 70-1 10 mm long 

 from outfall and from control populations of each 

 species were processed. For each mussel, the shell 

 length and the internal shell volume determined 

 by the volume (milliliters) of water required to fill 

 the empty valves were recorded. Total wet tissue 

 weight (grams) and wet weight of the gonad tissue 

 dissected from the mantle and body mass were 

 recorded for each mussel. From these data the 

 gonadal index was calculated as: (gonad wt x 



499 



