CAMP ANA: DAILY GROWTH INCREMENTS IN OTOLITHS OF PLArNFIN MIDSHIPMAN 



TABLE 1 .—Growth increment counts in otoliths of plainfin midshipman, Porichthys notatus, in rela- 

 tion to elapsed time for various experimental environments. Fish were transferred to new environ- 

 ments (or kept in the original environment as a control) on 10 August, 



'This value was derived from 14L: 10D/CT otoliths sampled 10 September; counts were made from the hatch check to the 

 prominent subdivision/transfer check 



Diel Temperature Cycle 



Fish hatched under a 24L/14T 1 :10T 2 regime 

 deposited growth increments that differed in many 

 respects from those produced under a cyclic 

 photoperiod (14L:10D/CT). Increments produced 

 up to 8 d medial and distal of the peripheral nuclei 

 were characterized by a high incidence of prominent 

 subdaily increments (Fig. 4B), more so than was the 

 case under a cyclic photoperiod. Daily/subdaily 

 similarities are reflected in the data of 10 August 

 (Table 1), where the observed major increment count 

 was significantly different from that expected of daily 

 increments (P < 0.05). The high increment count 

 indicates that some subdaily increments were promi- 

 nent enough to be classified as daily. 



Increments produced in the 15-20 d before transfer 

 were generally distinct and regular in appearance. 

 Increment width and the incidence of subdaily 

 increments were similar to those observed in the cor- 

 responding region of the cyclic photoperiod otoliths 

 (Fig. 3). However, the appearance of the major 

 increments was unusual in that the opaque portion of 

 each increment was relatively broad and sharply 

 delineated (Fig. 6). 



Fish maintained in the 24L/14T,:10T 2 environ- 

 ment after 10 August were overcrowded and did not 

 grow well. As a result, posttransfer otolith growth was 

 limited, increments were very narrow, and reliable 

 counts could not be made. However, increment 

 counts of representative otoliths suggested that daily 

 increments were deposited after the transfer date. 



Juvenile fish transferred from the fluctuating tem- 

 perature regime to a constant environment (24L/CT) 

 produced posttransfer increments similar to those of 

 fish transferred from 14L:10D/CT to 24L/CT (Fig. 

 5B). The difference between August and September 

 increment counts corresponds to that expected of 

 daily increment deposition (P > 0.05) (Table 1). The 

 first five posttransfer increments were faint and vir- 

 tually nonexistent; subsequent increments became 



more distinct and regular with time. Opaque regions 

 within each increment never became as broad and 

 discrete as was observed prior to transfer. 



Constant Environment 



Otoliths of fish hatched under constant conditions 

 (24L/CT) initially resembled those of the other two 

 environments (with respect to the first 5-8 

 increments). The subsequent region resembled that 

 of 24L/14T,: 10T 2 fish in that subdaily increments 

 were prominent (Fig. 4C). Although the difference 

 was not significant (Scheffe's test, P = 0.07), incre- 

 ment widths tended to be more irregular than those 

 of 1 4L: 1 OD/CT fish of similar age (Fig. 7) . The confu- 

 sion of daily and subdaily increments in the early lar- 

 val region resulted in a high variance and a mean 

 increment count that was significantly higher than 

 would be expected of daily increments (P < 0.05) 

 (Table 1). After age 10-25 d, daily increments de- 

 creased in width (Fig. 3) and became more regular in 

 width (Fig. 7) and appearance, although subdaily 

 increments were still present. Increments with 

 broad, discrete opaque portions were not observed in 

 the 24L/CT fish, as they were in the fluctuating tem- 

 perature regime. For an unknown reason, otolith 

 growth (but not fish growth) under a 24L/CT regime 

 significantly exceeded that observed under 

 14L:10D/CT(P<0.05). 



Fish remaining in a constant environment after the 

 10 August transfer date continued to produce dis- 

 tinct increments, although daily and subdaily 

 increments were occasionally difficult to differen- 

 tiate. Increment width was significantly more 

 irregular than in the posttransfer region of 1 4L: 1 0D/ 

 CT fish (<-test, P < 0.05) (Fig. 7). Major increments 

 in the posttransfer region were daily; the regression 

 of increment number against elapsed time resulted in 

 a slope not significantly different from unity (P > 

 0.05). 



Posttransfer increments of fish hatched and reared 

 under constant conditions were prominent, although 



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