TEMPERATURE SELECTION BY YOUNG TOPSMELT 531 



TABLE 2 



OCCURRENCE OF LARVAL ATHERINE FISHES IN 



THE FIELD AT DIFFERENT TEMPERATURES 



DURING A 1-YEAR SAMPLING PERIOD 



Temperature Abundance 



Mean number/ 50 m^ % 



0.8 0.7 



possibly to 26° C, but they generally avoided water warmer than 

 27°C. 



The temperatures preferred by topsmelt, particularly the higher 

 temperatures found in San Diego Bay, are uncommon or do not 

 occur at all throughout most of the geographical range of this fish. 

 The question then arises, Of what value is a preferred temperature 

 that does not naturally occur? It was suggested by Brett (1971) that 

 the final temperature preferendum may be synonymous with the 

 physiological optimum. If fish prefer water warmer than they will 

 ever naturally encounter, does this imply that they have evolved to 

 function below their physiological optimum? There are several 

 potential advantages of selecting a wide range of warm temperatures, 

 such as 19 to 26° C, which will act as a mechanism to direct the fish 

 inshore. First, the warmer nearshore water will reduce the duration 

 of the highly vulnerable larval stages. Second, movement inshore may 



