TEMPERATURE SELECTION BY YOUNG TOPSMELT 523 



Redondo Beach, Calif. This harbor is a receiving body for thermal 

 effluent and contains many horizontal and vertical temperature 

 gradients. 



Nearshore areas are often nursery grounds for many fishes. 

 Alteration of the physical environment could result in these areas' 

 being less desirable or suitable to many species and, thereby, in 

 reduced recruitment. With this in mind, we initiated our investiga- 

 tion, combining a laboratory investigation with field surveys to 

 provide further understanding of topsmelt's behavioral responses to 

 temperature. 



MATERIALS AND METHODS 



Laboratory Procedures 



We collected ripe adult topsmelt (Atherinops af finis) in June 

 1975 with gill nets placed in King Harbor, Redondo Beach, Calif. 

 Eggs were stripped, fertilized, and incubated according to a modified 

 version of the methods of Ehrlich and Farris (1971) for grunion 

 (Leuresthes tenuis). The incubating eggs remained totally submerged, 

 and gravel was not used since topsmelt do not spawn in sand as do 

 grunion. The incubation temperature was 18° C, and that for rearing 

 was 21°C. We cultured the fish in round, blue, 100-liter polyethylene 

 containers through which water flowed at a rate of 1.5 liters/min 

 from an open seawater system. The salinity was generally in the 

 range of 32 to 34% o, with rare maximum excursions of 1.5% q. 

 Young larvae were fed brine shrimp (Artemia salina) nauplii 

 supplemented with natural plankton seived through a 750-/im screen 

 and collected on a 20-jum seive. As the fish grew, we increased the 

 size of the brine shrimp fed to them, until adult Artemia was the sole 

 food source. The brine shrimp were fed cultured green algae 

 (Brachyamonas sp.) so that they would be of a more uniform 

 nutritive value to the fish. 



The temperature preference of topsmelt larvae was tested 3, 11, 

 and 18 days post-hatching. We also tested juvenile fish 120 days 

 post-hatching and, at this time, transferred three groups of 30 

 individuals each to new containers for acclimation to additional 

 temperatures of 10, 15, and 20° C. The temperature was changed at 

 the rate of l°C/day. The fish were held approximately 60 days at the 

 acclimation temperatures before their thermal preference was tested 

 again. 



We used the two horizontal temperature gradients and the 

 methodology of Ehrlich et al. (1978) to delineate behavioral re- 



