NOTE Powell et al : Validation of age estimates from otoliths of Cynosaon nebulosus 



651 



Figure 1 



Photomicrograph of a transverse section of a spotted seatrout otoHth from a 32-d-old laboratory- 

 reared fish (SL=23.6 mm). C = the core; E = edge. The arrows point to daily increments that were 

 determined as those rings that maintained relatively equal spacing as the fine focus was manipu- 

 lated. The rings between the arrows were considered subdaily rings or optical artifacts. These 

 rings did not maintain equal spacing when the fine focus was manipulated, were not continuous, 

 and generally were poorly defined. Scale bar = 10 jim. 



reader without knowledge of age of fish. Two to three 

 readings were made and the counts averaged. Following 

 Rice et al. ( 1985) and Ahrenholz et al. ( 1995). counts were 

 regressed on age (SAS Institute, Inc., 1985), and Student's 

 <-test used to determine if the slope was significantly dif- 

 ferent (a=0.05) than one. The null hypothesis is that the 

 slope of increment counts on known age equals one. The 

 null hypothesis is rejected if the slope is significantly dif- 

 ferent from one. A summary of the increment count data 

 from known-age fish is shown in Table 1. 



In our initial analysis, the slope of increment count 

 regressed on known-age was significantly greater than one 

 (a=0.05), but there were problems in aging the oldest juve- 

 niles (32 d) — problems that we related to variability in fish 

 sizes (16.2 to 34.6 mm). Using linear regression, we exam- 

 ined the relation between increment count on size for all 

 age groups. Only for the 32-d-old juveniles was the slope 

 of increment count on size significantly different from zero 

 (a=0.05), indicating that greater counts were observed as 

 size increased. This result suggested that we interpreted 

 subdaily rings as daily rings. We then examined otoliths 

 from known-age juveniles in great detail to establish cri- 

 teria to separate subdaily from daily rings. Although read- 

 ing the older age (32-d-old) juveniles was difficult, daily 

 rings maintained relatively equal spacing as the fine focus 



was manipulated. Subdaily rings or optical artifacts were 

 not continuous, did not maintain equal spacing when focus 

 was manipulated, and were not well defined (Fig. 1). Juve- 

 nile otoliths (23-d and 32-d-old) were reread (two blind 

 readings per sagitta, with a hand counter). The reader 

 avoided making mentally sequential counts, relying solely 

 on the results of the hand counter. 



