Pearson and Shaw: Age determination errors for Anop/opoma fimbria 



137 



Figure 6 



Image of a baked sablefish otolith with an electronically pasted section taken from 

 an image captured under UV light. The dark OTC mark is clearly located within a 

 hyaline zone, and the hyaline zone persists through the entire otolith. The fish was 

 injected with OTC on 5 October 1991. 



would probably mis-age these fish. Because the age read- 

 ers who examined the otoliths without knowledge of the 

 recapture information were not informed that the point 

 they were counting from was just inside the summer mark, 

 it was interesting to note that all three of them counted 

 the hyaline zone in which the OTC mark had occurred as 

 an annual hyaline zone in all cases. In other words, the 

 summer hyaline zone did not appear to be a check to the 

 readers. The readers indicated that the manner of prepa- 

 ration of the otoliths (embedded and baked) was not the 

 manner in which they were accustomed to view otoliths 

 and may have influenced their results. The fact that the 

 hyaline zones were not as dark with the baking method 

 as opposed to the burning method may have influenced 

 the readers age estimates; however, some otolith burns 

 can be quite light and experienced readers recognize the 

 various levels of burning, particularly when cross reading 

 otoliths from other age readers. Readers sometimes use 

 multiple sections and are free to manipulate the otolith 

 to improve viewing, which was not possible in the present 

 study. Beamish et al. (1983) indicated that when readers 

 knew how many marks to look for, they were able to iden- 

 tify false annual marks (checks). According to their study, a 

 check is not persistent throughout the otolith. In Figure 6, 

 the hyaline zone in which the OTC mark appeared clearly 

 persists throughout the otolith. If the hyaline zone which 

 contained the OTC mark began to be laid down in the win- 

 ter, then there would be very little time for the formation 

 of a wide opaque zone to form after injection in the fall. 

 Because the age readers counted the hyaline zone in which 

 the OTC mark occurred, they clearly assumed that it was 

 not a check. If the age readers had known that the hyaline 



zone (in which the OTC mark occurred) had formed in the 

 summer, then they presumably would not have counted 

 it. It is therefore of interest to see the effect on agreement 

 between reader counts minus the hyaline zone where the 

 OTC mark occurred and the actual number of hyaline 

 zones that should have been present. When we adjusted the 

 reader counts by subtracting one year from their original 

 counts and compared their adjusted counts to the expected 

 number of annual marks (Table 11 ), agreement for readers 

 1 and 2 improved, whereas it decreased for reader 3 (the 

 least experienced reader). 



Also of importance is the fact that on some otoliths, even 

 after eight months at liberty, no growth had occurred, as 

 evidenced by the fact that the OTC mark was on the edge. 

 For example, otoliths from two fish, recaptured after eight 

 months at liberty showed marked differences in otolith 

 growth ( Fig. 7 ). On otolith A there was no detectable growth 

 with the OTC mark on the edge, whereas on otolith B there 

 was substantial growth. The OTC marks on both otoliths 

 were very prominent. These otoliths came from similar fish; 

 that is, otolith A came from a 597-mm female fish caught 

 in 680 meters of water at 40°52' latitude, and otolith B 

 came from a 610-mm female fish caught in 480 meters of 

 water at 41°54' latitude. This provides strong evidence 

 that otolith growth, and presumably fish growth, varies 

 greatly among individual sablefish. Beamish et al. (1983) 

 reported that the OTC mark was on or near the edge in 

 28 otoliths (18.1%) of 154 fish which had been at liberty 

 for two to three years. In a similar time interval, we found 

 that 34 of 126 (27.0%) had the OTC marks on or near the 

 edge. Both the finding of a summer hyaline zone and the 

 differences in growth of the otolith among individual fish 



