FISHERY BULLETIN: VOL. 82, NO. 1 



200-1 



• =14L10D/CT 

 a = 24L/14Ti:10T 2 



40 

 AGE (DAYS) 



50 



70 



FIGURE 2.— Total otolith increment count as a function of age for 

 plainfin midshipman from two cyclic experimental environments. A 

 straight line has been fitted to the data, although the relationship is 

 probably curvilinear. N = 5 for each data point. 



Criteria for distinguishing daily from subdaily 

 increments have been reported previously (Taubert 

 and Coble 1977; Campana and Neilson 1982; 

 Marshall and Parker 1982). Nevertheless, no objec- 

 tive criteria have yet been defined which can be 

 applied to all otoliths. In this study, I have used visual 

 prominence and increment width as guides for dif- 

 ferentiating daily and subdaily increments. In- 

 crements assigned as daily were 1) of similar visual 

 prominence (contrast) to adjacent daily increments 

 (±30%), 2) of similar increment width to adjacent 

 daily increments (±50%), 3) not merged with adja- 

 cent daily increments in the nearest radial groove of 

 the sagitta. Some increments met only some of the 

 criteria and were subjectively assigned as daily or 

 subdaily. The observed widths of daily increments, 

 as classified above, were similar to those expected on 

 the basis of otolith growth calculations (see 

 previous paragraph). 



Diel Light Cycle 



Otoliths offish reared under a diel photoperiod and 

 constant temperature ( 1 4L: 1 OD/CT) produced clear 

 daily growth increments from the time of hatch. 

 Regression of major increment number against elapsed 

 time produced a slope not significantly different 

 from 1.0 (P> 0.05); a slope of 1.0 would indicate that 

 one increment was formed every day. 



Increment width varied with location on the otolith 

 and fish age (Fig. 3). Subdaily increments were com- 

 mon at all ages, numbering up to 5 between adjacent 

 daily increments. They were most abundant in the 

 first month after hatch. The distinction between 

 daily and subdaily increments was generally clear; 

 however, increments produced 5-20 d after hatch 

 were the most irregular on the otolith, and were 

 sometimes difficult to interpret. Subdaily incre- 

 ments tended to be prominent in this region, so that 

 distinction was a matter of degree (Fig. 4A). 



5 

 I 



H 

 Q 



i 



5 



cr 

 U 



z 



z 

 < 



16 



12 



4- 



24L/14T, 10T 2 



24L/CT 



14L 10D/CT 



i 



10 



i i 



20 30 



AGE (DAYS) 



— r~ 

 40 



- 1 

 50 



Figure 3. -Daily increment width as a function of age for otolith 

 samples of plainfin midshipman from each of the three experimental 

 environments. At a given age, mean widths do not differ significant- 

 ly among environments, with the exception of values at age 40 d (P 

 < 0.05). 



Fish transferred to a constant environment (24 L/ 

 CT) as juveniles produced posttransfer increments 

 that were very different from those produced prior 

 to transfer. Posttransfer increments were visually 

 faint and, in some cases, virtually invisible (Fig. 5A). 

 Subdaily increments were also present. Transfer to a 

 constant environment was not associated with a 

 recognizable lag period during which increments 

 gradually shifted their appearance. Increments pro- 

 duced within 1-2 d of transfer were virtually nonexis- 

 tent. Nevertheless, posttransfer increments were 

 daily in nature, as indicated by increment counts 

 similar to those expected of daily increment produc- 

 tion (Table 1). Daily increments gradually became 

 more prominent after about 15 d posttransfer, their 

 visual contrast improving until the end of the 

 experiment. 



168 



