Roberson et al Indirect validation of the age-reading method for Gadus macrocephalus 



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under- or overestimated ages). The key difficulty of the 

 cod otolith patterns is differentiating the translucent 

 zones that are annual from the translucent zones that 

 are checks, particularly in young fish. It is necessary to 

 have validated criteria in order to confidently eliminate 

 checks without under-aging the fish. In our study, we 

 have given two examples of indirect validation for Pacific 

 cod age determination by using otoliths from marked 

 and recaptured fish. 



In the first example, we used back-calculations to test 

 our reading criteria, which exclude counting lighter 

 translucent zones. Early in the study, we found a strong 

 relationship between otolith size and fish length, which 

 supported using back-calculations as a vehicle to test 

 accuracy. Overall, using strong translucent zones to 

 back-calculate fish length at tagging gave fairly accu- 

 rate results. This finding supports the assumption that 

 translucent zones are laid down on an annual basis. 



An ancillary finding was that otolith area measure- 

 ments provided more accurate estimates of fish length 

 than otolith lengths. Although back-calculations are 

 typically performed by using radial or diametral mea- 

 surement, the more accurate estimates of fish length 

 from otolith area measurements are not surprising in 

 that otolith area is a more comprehensive measure of 

 otolith three-dimensional growth. 



A second indirect validation of reading criteria was 

 possible by estimating how much each tagged fish grew 

 (millimeters) between tagging and recapture by its 

 estimated age at recovery and von Bertalanffy growth 

 parameters (derived only from tagging data). When 

 compared to the observed growth increments, we found 

 that the results support our proposed aging criteria 

 (which exclude lighter translucent zones) because these 

 criteria give the best fit to growth increments based on 

 the mark-recapture growth increments. Aging the fish 

 older (by counting light translucent zones) or younger 

 (counting less annuli by banding translucent zones to- 

 gether) increases the residual fit to the mark-recapture 

 growth increments. Large growth increments of fish 

 length were difficult to estimate (Fig. 4). A possible 

 explanation is that the longer a fish remains at liberty, 

 the more likely that the growth becomes asymptotic, 

 making the relationship between the growth increment 

 and time at liberty less exact. 



The final test for reading criteria was performed 

 through a more direct comparison: simply "aging" the 

 tagged fish by its length-at-release plus its time at 

 liberty after tagging and comparing that age to the 

 otolith-based age at recovery. Dwyer et al. (2003) also 

 used this method in their study of yellowtail flounder 

 (Limanda ferruginea). Average deviation from tag-based 

 age was -0.075; 75% of these fish were found to be 

 within one year of our age according to otolith readings, 

 and 94% were within two years. These results provide 

 further evidence that the current criteria do not result 

 in the underestimation of the age of the fish and sup- 

 port the practice of not counting checks. 



We found that growth information residing in oto- 

 liths from tagged and recovered Pacific cod provided 



200 400 



600 



600 -, 



400 



™ 200- 



200 400 



600 



600 



400 



200 



200 400 600 



Observed growth Increments (mm) 



Figure 4 



Three plots comparing predicted and observed 

 fish-length growth increments by using recap- 

 tured fish from tagging experiments (n = 97). 

 Estimated ages at recovery (B) were scaled 25% 

 smaller (A) and 25% larger (C). The lines indi- 

 cate theoretical 1:1 line of perfect agreement. 



significant information applicable to indirectly validat- 

 ing otolith aging criteria. Therefore, it seems that oto- 

 liths from other species that were tagged and recovered 

 might be useful for indirect age validation as well. The 



