^.HERY BULLETIN: VOL. 82, NO. 1 



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Fic.i'RE 6. — Daily growth increments produced on the sagittae of plainfin midshipman after 15-25 d of rearing under a diel temperature cycle. 

 The increments were visually prominent and sharply delineated relative to those produced under other environmental regimes. Bar = 20 

 /i in. 



O 5i 



O 4- 



o 



24L/14T, 10T ; 



24L CT 



14L 10D CT 



10 



20 



i 



30 



40 



50 



AGE (DAYS 



FlGi RE 7. Index of daily increment width regularity as a function of 

 age for otolith samples of plainfin midshipman from each of the three 

 experimental environments. Bars represent ±1 SE. 



differentiated at first, suggesting that the circadian 

 deposition rhythm was not yet mature. Maturation 

 apparently occurred by days 10-20. Early larval 

 increments were only indistinct temporarily in the 

 14L:10D/CT fish, suggesting that the cyclic 

 photoperiod entrained the maturing rhythm fairly 

 quickly. In addition, very young animals may be more 

 responsive to a diel light cycle, due to age-related 

 characters of the rhythm cycle (Sacher and Duffy 

 1978). For instance, the metabolic rate of newly 

 hatched rats is very sensitive to changes in light level, 

 while older rats are less affected. In this study, larval 

 fish exposed to a constant environment took longer to 

 produce daily increments than did juvenile fish, sug- 

 gesting an analogy with the rat study. Similar age- 

 related results were reported by Gibson et al. (1978) 

 in an ontogenetic study of flatfish activity cycles. A 

 constant photoperiod eliminated a diel activity cycle 

 in larval plaice (Pleuronectes platessa), but had no 

 such effect on juveniles of the same species. 



Increasing age of midshipman was correlated with 

 decreasing increment width and fewer subdaily 



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