The parturition season for S. diploproa off 

 California is February through July (Phillips 

 1964), with possible limited year-round spawning 

 ( Boehlert 1977 ). This results in poorly defined year 

 classes and length-frequency distributions, yet a 

 size threshold and distinct season exist for migra- 

 tion from the surface to the benthic habitat 

 (Boehlert 1977, 1978). Zamakhaev (1964) 

 suggested that size discrepancy within an age- 

 group may be minimized through compensatory 

 growth. The observed pattern of reduced growth at 

 sizes >40 mm SL may serve as a variant on the 

 phenomenon of compensatory growth, consolidat- 

 ing the O-group fish at sizes near 50 mm SL prior to 

 the migration, which occurs from May through 

 September (Boehlert 1978). At the onset of migra- 

 tion, the oldest and largest fish would migrate 

 first, as observed in sockeye salmon ( Foerster 1937) 

 and Atlantic salmon (Elson 1957). Smaller fish 

 would continue rapid growth, and as they reached 

 the size threshold, would also migrate. 



Initiation of the surface-to-benthic migration of 

 juvenile S. diploproa may be dependent upon 

 photoperiod or rate of change of photoperiod sub- 

 ject to an endogenous program which depends 

 upon a size threshold (Boehlert 1978, 1981). The 

 temperature change between surface and benthic 

 habitats is about 12° C, suggesting that tempera- 

 ture is an important consideration in the migra- 

 tion. Although there is no change in critical 

 thermal maximum for juvenile S. diploproa ac- 

 climated to the same temperature but different 

 photoperiods (Boehlert 1981), prejuveniles from 

 the field are metabolically preadapted for the 

 lower temperatures during the migratory season 

 (Boehlert 1978). The size dependence of growth in 

 the present experiments suggests a downward 

 shift in the temperature of optimum growth with 

 increasing size in 16L:8D but not in 12L:12D. 

 Photoperiod may thus interact with size, resulting 

 in an ontogenetic change in thermal require- 

 ments. 



Acknowledgments 



The laboratory growth experiments were con- 

 ducted at the Southwest Fisheries Center, La 

 Jolla, Calif., and were partially supported by the 

 Institute of Marine Resources and by the Hubbs- 

 Sea World Research Institute. Analysis of daily 

 growth increments was conducted while the au- 

 thor held a National Research Council Postdoc- 



toral Associateship at the Northwest and Alaska 

 Fisheries Center, Seattle, Wash. I thank an 

 anonymous reviewer for valuable suggestions on 

 the manuscript. 



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