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Fishery Bulletin 92(2). 1994 



Materials and methods 



Otolith collection 



Sablefish were collected along the Eastern Pacific 

 Continental Shelf during two research cruises by the 

 AFSC. The first collection was made from the re- 

 search vessel Alaska on 8 August 1986 and con- 

 tained 61 fish captured at 135 m in Morro Bay, 

 California. The second collection was made from the 

 vessel American Viking between 23 September and 

 31 October 1986 and contained 423 fish captured at 

 depths of 246-1426 m off the California coast be- 

 tween 32°23'N and 42°23'N. Both sagittal otoliths 

 were removed at sea. The sacculus membranes were 

 removed and otoliths were stored in 50% ethanol. 



Ages were estimated by the first author (Reader 

 1) from one otolith per fish applying the break-and- 

 burn method, and the other otolith was used in the 

 radiometric study. The criteria used to count annuli 

 for this study were similar to those typically used 

 by experienced age readers at the AFSC and Tiburon 

 Laboratories of the National Marine Fisheries Ser- 

 vice (NMFS), and were the same used by Beamish 

 and Chilton (1982) and Beamish et al. (1983). By 

 using these criteria, an age range of up to ±5 years 

 for older fish was often possible. The oldest age con- 

 sistent with these criteria was often assigned as the 

 most probable age. When a fish was aged as 1 year 

 old the otolith surface was usually adequate and no 

 break-and-burn was done. The otoliths were subse- 

 quently pooled into four age categories (1 year, 9— 

 11 year, 14—23 year, and 24-34 year), on the basis 

 of Reader- 1 ages. 



For comparison, a subsample of otoliths initially 

 aged >14 years (re=186) was read by a second expe- 

 rienced sablefish age reader at the AFSC (Reader 

 2). Additionally, all fish initially aged >14 years 

 (rc=266) were read by an experienced sablefish age 

 reader at the Pacific Biological Station (PBS), Cana- 

 dian Department of Fisheries and Oceans (Reader 3). 



In the radiometric dating procedures, we used 

 that part of the otolith which was deposited during 

 the first year of life (i.e. the first year core). For age 

 category 1, whole otoliths representing the first 15 

 to 18 months of growth were used. These otoliths 

 were those classified as age 1 by Reader 1 and were 

 intact and unburnt. For other age categories, the 

 first year cores were isolated by grinding away ex- 

 cess otolith material. The grinding was done with a 

 Buehler metallurgical polishing machine equipped 

 with Buehler wet and dry #600 or #900 paper. 

 Otolith material representing the first year was 

 readily identifiable in older fish from the distal sur- 

 face of the ground otolith, and from the broken-and- 



burnt section, viewed with a dissecting microscope 

 (25x) as a guide. Removal of material was done 

 slowly, with frequent viewing of the otolith during 

 the grinding process (Kastelle, 1991). The position 

 of the first annulus on the otolith has been con- 

 firmed by several authors in age validation studies 

 of young sablefish (Beamish and Chilton, 1982; 

 Beamish et al., 1983; McFarlane and Beamish, 

 1983). Average measurements of the core dimensions 

 were not used as an aid in the grinding process. 

 Instead, grinding was completed when the contours 

 of the first year were approximated. Small inaccu- 

 racies in the grinding were inconsequential because 

 samples were pooled into four categories based on 

 age ranges. 



All otolith cores were cleaned with an ultrasonic 

 cleaner in distilled and deionized water for a mini- 

 mum of 30 seconds. Any soft tissue remaining on the 

 otolith after collection was visibly broken down by 

 the ultrasonic cleaner. The goal of the cleaning was 

 to remove any contamination from soft tissue or 

 grinding paper. After cleaning, the otoliths were 

 stored again in a fresh 50% ethanol solution prior 

 to analysis for radioisotopes. 



Approximately one gram of material was neces- 

 sary for radioisotope activity to be measurable above 

 background levels, which meant that 83 to 141 

 otoliths were used for each age category. To increase 

 the weight of category 1 ( 1 year olds), an additional 

 unburnt half of the aged otolith from some speci- 

 mens was included with the intact otoliths (83 speci- 

 mens total: 83 whole otoliths plus 65 half otoliths 

 from some of the same fish). 



Activity measurements 



The methods employed in the chemical separation 

 and counting techniques for Ra-226 and Pb-210 were 

 similar to those used by Bennett et al. (1982) and 

 are detailed in Kastelle (1991). 



In general, the activity of Pb-210 in the otoliths 

 was determined by counting the alpha decays of Po- 

 210 (the granddaughter-proxy of Pb-210 with which 

 it is in secular equilibrium, Eq. 1) by using a yield 

 tracer of Po-209. Reagent blanks, with and without 

 yield tracers, were processed with each age category 

 as follows: category 1, one blank without yield 

 tracer; categories 2 and 3 processed simultaneously, 

 one blank with a yield tracer and one without; cat- 

 egory 4, one blank with a yield tracer and one with- 

 out. The counting time for each sample or reagent 

 blank was approximately three weeks. 



The activity of Ra-226 was determined by count- 

 ing alpha decays of Rn-222 (the daughter-proxy of 

 Ra-226 with which it grows into secular equilibrium, 



