Kerr et al.: Age validation for Sebastes maliger with bomb radiocarbon 



103 



200- 

 150- 

 100- 



_ 50 



o 



* -50 



-100 



-150 



-200 



• Yelloweye rocktish (n=43) 

 □ Quillback rockfish (n=15) 



1930 1940 1950 



1960 1970 

 Birth year 



1980 1990 2000 



Figure 3 



Radiocarbon (4 14 C) values from quillback rockfish [Sebastes 

 maliger) otoliths and the yelloweye rockfish (S. ruberrimus) 14 C 

 time series for the waters of southeast Alaska. The yelloweye 

 rockfish 14 C data were divided into three intervals (prebomb, 

 bomb rise, and postbomb) and fitted with confidence intervals. 

 The prebomb era 14 C values (1950-571 were fitted with an 

 average (±2 SD), and the bomb rise (1959-71) and postbomb 

 era values (1966-85) were fitted with a linear regression and 

 corresponding 95% prediction intervals. 



minimum number of samples required to achieve the 

 desired degree of precision, the present study takes a 

 step toward reducing the number of prescribed samples, 

 (i.e., 20-30 otoliths; Campana, 2001), effectively making 

 age validation more affordable. 



To determine the number of samples necessary for 

 an age validation study, an assessment of the degree 

 of precision is required. The degree of precision may 

 be defined by the level of variation in the chronology 

 or the uncertainty associated with age estimates. It 

 can also be dependent on the estimated longevity of the 

 fish and the resolution of age that is sufficient for the 

 purposes of the study. For example, a resolution of ±5 

 years may be sufficient for a species estimated to live 

 100 years, but would not be satisfactory for a species 

 estimated to live 20 years. The higher degree of preci- 

 sion, the greater the cost will be for a study. However, 

 a maximum precision can be attained at a minimum 

 cost by taking into consideration the precision of the 

 14 C time series, the error associated with age estimates, 

 and the age resolution necessary to accomplish the 

 goals of the study. 



In our study, the ±2-year variation of the yelloweye 

 rockfish 14 C time series limited the precision to which 

 the age of the quillback rockfish could be determined 

 through comparison. Stratified sampling of nine quill- 

 back rockfish 14 C values between 1956 and 1971 re- 

 vealed an average year of initial rise in 14 C of 1959 

 (±1 year) that was in close agreement with the year of 

 initial rise determined for the yelloweye rockfish time 

 series. Thus, given the unique circumstances for this 



species, we have quantitatively reduced the number 

 of samples required for age validation to 9 (given that 

 some sampling or additional information is used to es- 

 tablish prebomb levels). It can also be envisioned that 

 14 C analysis of a single fish otolith could establish a 

 minimum longevity for a species if the 14 C levels mea- 

 sured in the otolith core of an adult fish with a known 

 capture year were consistent with established prebomb 

 14 C levels for the regional waters in which that fish 

 spent its first year. This exercise illustrates the neces- 

 sity of defining precision on a species-by-species basis 

 prior to beginning a 14 C study. Despite the high cost 

 of AMS analyses, the overall project cost may be lower 

 and of shorter duration than traditional age validation 

 studies because of the relatively short time required to 

 prepare and process the minimum number of otoliths. 

 Currently, the 14 C technique is considered one of the 

 most effective methods for age validation of long-lived 

 fishes (Campana, 2001) and as costs are minimized, 

 future application of the bomb 14 C age-validation tech- 

 nique of marine fishes should increase. 



Radiocarbon analysis 



To interpret radiocarbon values recorded in marine or- 

 ganisms it is essential to put them in the context of 

 the regional oceanography. The Alaska coastal current, 

 driven by wind stress and enriched with freshwater 

 runoff, is the driving force behind the coastal dynamics 

 off southeast Alaska (Royer, 1982). The coastal environ- 

 ment off southeast Alaska is characterized by significant 



