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



99 



the chronology determined for the waters of southeast 

 Alaska (i.e., yelloweye rockfish), and 3) to analyze 14 C 

 in aged quillback rockfish otoliths, spanning the pre- 

 to postbomb era, in order to examine the complete 14 C 

 time series and demonstrate the effectiveness of using 

 the timing of the initial rise in U C as an age valida- 

 tion method. 



Materials and methods 



Sample size assessment 



Because of the considerable cost of accelerator mass 

 spectrometry (AMS) analyses, the minimum number of 

 14 C samples required to validate the aging method of 

 the quillback rockfish was mathematically determined 

 from a previously determined yelloweye rockfish otolith 

 14 C time series for the waters of southeast Alaska (Kerr 

 et al., 2004). To assess minimum sample size, estimated 

 years of initial rise in 14 C levels (and associated errors) 

 were determined for different numbers of data points 

 (n = 3, 5, 7, 9, and 11) subsampled from the bomb-rise 

 region of the yelloweye rockfish data set. The estimated 

 years of initial rise from each subsample set were then 

 compared to the initial year of rise and error deter- 

 mined from all bomb-rise yelloweye rockfish 14 C samples 

 (n=23). Because the error associated with the yelloweye 

 rockfish bomb-rise data set is limited by the uncertainty 

 associated with age estimates for yelloweye rockfish, a 

 maximum error of ±2 years for fish with birth years 

 during the bomb rise (1956 to 1971; Kerr et al., 2004) 

 was our precision criterion for defining the minimum 

 number of quillback rockfish otolith samples. 



Twenty-three yelloweye rockfish otolith 14 C values 

 with birth years from 1956 to 1971 were divided into 

 data sets of 3, 5, 7, 9, and 11 data points. A stratified 

 sampling approach was applied by creating bomb 14 C 

 linear regressions from repeated selection of 3, 5, 7, 9, 

 and 11 data points at uniform intervals from 1956 to 

 1971. Random selection of data points was not practi- 

 cal because it is established that the careful choice of 

 sample year during the rapid rise in 14 C is required for 

 this technique (Baker and Wilson, 2001). The year of 

 initial rise in 14 C, and associated error, was determined 

 from the bomb 14 C regressions. The year of initial 14 C 

 rise was calculated with the following formula: 



x = (y - b)lm, 



where x = year of initial rise in 14 C values; 

 y = average prebomb 14 C value; 

 m = slope of the line; and 

 b = y-intercept. 



The error associated with the year of initial rise in 14 C 

 values (o x ) was calculated by using the delta method 

 (treating 6 as a scaler; Wang et al., 1975): 



°y/° m > 



where a v = error associated with average prebomb 14 C 

 value 

 a m = error associated with the slope of the line. 



Radiocarbon analysis 



Sagittal otoliths of quillback rockfish were collected from 

 a random subsampling of catches from commercial long- 

 line fishing vessels in the coastal waters off southeast 

 Alaska by the Alaska Department of Fish and Game 

 (ADFG), Juneau, AK in 2000 (Fig. 1). A single otolith 

 from a pair taken from each fish was aged by using the 

 break-and-burn method developed by researchers at the 

 Mark, Tag, and Age Laboratory, ADFG in Juneau, AK, 

 and the corresponding intact otolith was analyzed for 

 14 C. Whole and broken-and-burned otoliths were stored 

 dry in paper envelopes. Year of capture, estimated final 

 age, assigned year class, readability code, and reader 

 identification information were archived and provided 

 by ADFG for each sample. 



Fifteen quillback rockfish otoliths, with estimated 

 birth years ranging from the prebomb 1950s to the 

 postbomb mid-1980s were selected for 14 C analysis. The 

 core of each otolith, which constitutes the first year of 

 growth, was analyzed for 14 C. From life history infor- 

 mation, it is known that the core was formed while the 

 fish inhabited the ocean mixed layer during its early 

 growth stage (Yoklavich et al., 1996). To determine the 

 average length and width, and minimum depth of the 

 core, whole and broken-and-burnt otoliths from adult 

 quillback rockfish were examined under a Leica- dis- 

 secting microscope with an attached Spot RT® video 

 camera and were measured using Image Pro Plus® im- 

 age analysis software (version 4.1 for Windows, Media 

 Cybernetics, Silver Spring, MD). Cores were extracted 

 with a milling machine with a 1.6-mm (1/16") diam- 

 eter end mill. To minimize the extraction of material 

 deposited after the first year of growth, length, width, 

 and depth parameters of the otolith core were used to 

 guide coring. Because the first year of growth in quill- 

 back rockfish otoliths is clearly visible from the distal 

 surface of the otolith we were able to visually correct for 

 individual variability in otolith core size. The core (first 

 year of growth in the otolith) was reduced to powder, 

 collected, and weighed to the nearest 0.1 mg. 



For 14 C analysis, otolith calcium carbonate (CaC0 3 ) 

 was converted to pure carbon in the form of graphite 

 (Vogel et al., 1984, 1987) and measured for 14 C content 

 by using AMS at the Center for Accelerator Mass Spec- 

 trometry, Lawrence Livermore National Laboratory. The 

 14 C values were reported as 4 14 C (Stuvier and Polach, 

 1977). 



The 14 C values measured in quillback rockfish otolith 

 cores were plotted with respect to corresponding birth 

 years assessed from break-and-burn age estimates, 

 taking into consideration the potential variation of the 

 age estimate (coefficient of variation=2.6%, rounded to 

 the nearest whole number; Chang, 1982). The 14 C time 

 series for the waters of southeast Alaska established 

 from the otoliths of the age-validated yelloweye rockfish 



