BOEHLERT: AGE DETERMINATION IN FISHES 



first with 400-grit carborundum paper and then 

 polished with 3 ^im jeweler's rouge. 



To compare internal otolith section annuli with 

 surface annuli, 25 whole left otoliths from S. pin- 

 niger and 50 from S. diploproa were selected. 

 Sample size was chosen to represent the range of 

 ages estimated from whole otoliths. I determined 

 the distance from focus to each annulus on the 

 whole otolith along the dorsal- ventral axis from 

 focus to dorsal edge of the otolith using an ocular 

 micrometer on a dissecting microscope. These 

 measurements were used to identify the first sev- 

 eral annuli on corresponding sections. By follow- 

 ing these identified annuli around to the internal 

 dorsal surface it was determined that each small 

 ring in the direction of counting (from focus to 

 dorsal, interior surface) corresponded to a single 

 year of growth (Fig. 1). 



Sections were initially examined under a dis- 

 secting microscope at 30 x magnification with 

 either reflected light and a black background or 

 transmitted light, depending upon the clarity of 

 the annuli. Discerning and counting the narrow 

 zones in otoliths from older fish was facilitated by 

 the use of a compound microscope interfaced with 

 a video camera and television screen. A more ac- 

 curate estimate of age was made possible by the 

 increased magnification and enhanced contrast of 

 the compound microscope, coupled with the ease of 

 viewing annuli on an enlarged screen. 



Sections were aged by identifying the first 

 translucent annulus (winter growth zone) and 

 counting sequential growth zones from the center 



to the dorsal edge. Subsequent annuli were fol- 

 lowed from the dorsal edge to the interior dorsal 

 quadrant (after Beamish 1979b), and counted to 

 the internal surface. In this paper, ages deter- 

 mined by different methods and sources will be 

 discussed; none of these ages is known with cer- 

 tainty. For this reason, given ages will be defined 

 as "standard ages" only for purposes of compari- 

 son. 



Calibration Subsample 



To establish models of age based upon otolith 

 dimension and weight criteria, otoliths from the 

 entire collection were subsampled. Every fourth 

 otolith pair of S. diploproa and every third of S. 

 pinniger were selected to provide roughly equal 

 sample sizes representative of all sizes and collec- 

 tion (latitudinal) areas. These subsampled 

 otoliths were used to develop the multiple regres- 

 sion models (see section on Data Analysis) and 

 were treated as described below. 



Whole otolith ages were determined by an ex- 

 perienced otolith reader to whom fish length re- 

 mained unknown. This practice has been recom- 

 mended by Williams and Bedford (1974), among 

 others, to minimize bias in otolith reading. 

 Otoliths were then dried to a constant weight at 

 58°C and placed in a dessicator for 8 h. Intact left 

 otoliths were weighed to the nearest milligram. 

 Otoliths were measured with dial calipers in the 

 anteroposterior dimension (length) to the nearest 

 0.02 mm and in the maximum dorsoventral di- 



FlGURE 1. — Dorsal-ventral section of the left otolith of a 305 mm FL female Sebastes diploproa. Whole otolith ages are generally 

 determined from the focus (F) to the dorsal edge (A), but often extend to the posterior margin (not shown) which may include additional 

 annuli extending to greater ages (A to B). Section ages are determined from the focus (F) to the internal dorsal surface (C). Note the 

 additional growth zones on axis F-C which have been deposited after the latest visible zones on axis F-A. The otolith section age of this 

 specimen is 40 yr. 



105 



