Gillanders et al.: Aging methods for Serio/a lalandi 



813 



from age-length data (from counts of zones in calci- 

 fied structures) to those obtained from length-fre- 

 quency and mark-recapture (tagging) data. We ac- 

 knowledge that the data from these three approaches 

 for estimating growth are not directly comparable 

 (Francis, 1988a), but follow the recommendations of 

 Francis (1995) in interpreting differences. 



Materials and methods 



Fish collection and treatment 



Yellowtail kingfish, Seriola lalandi. specimens were 

 collected from New South Wales, Australia, between 

 August 1995 and July 1996 by commercial or recre- 

 ational fishermen. Fish caught by commercial fish- 

 ermen were obtained after being processed by fillet- 

 ing. Fish were measured (total length, fork length) 

 and sagittal otoliths, dorsal spines, scales, and ver- 

 tebrae removed. Dorsal spines were examined but 

 considered unsuitable for aging because the center 

 (core) region was either occupied by vascular bony 

 tissue or was hollow. The hollow core in large fish 

 was found to have a larger diameter than the whole 

 spine of small fish. For this reason, it was likely that 

 early growth zones were lost in older fish and there- 

 fore spines were considered unsuitable for aging. 



Sagittae 



Whole sagittae were burned for 7 min at 500'C. They 

 were viewed under a low-power dissecting microscope 

 (6x magnification) with reflected light against a black 

 background. Assignments of age were based on 

 counts of opaque (light) zones or ridges (or both) that 

 were usually most visible at the base of the rostrum 

 on the ventral surface (Fig. lA). Sagittae were also 

 embedded in clear resin, sectioned in a transverse 

 plane with a low speed saw, the sections (=350 mm 

 thickness) mounted on glass slides, and viewed un- 

 der a compound microscope (40x magnification) with 

 reflected light against a black background. 



Scales 



Scales were removed from a position anterior and 

 ventral to the pectoral fin. It was necessary to re- 

 move scales from such a position because most fish 

 had been processed prior to the removal of scales. 

 Scales from each fish were soaked in a solution of 

 sodium hydroxide for 3 h, then rinsed and soaked in 

 water for a further 3-12 h. Clean, nonreplacement 

 (i.e. original scales showing typical ctenoid shape) 

 scales were dry-mounted between two glass micro- 



scope slides. Scales were read under a compound 

 microscope (20x magnification) with reflected light 

 against a black background. Presumed annuli were 

 identified by cutting over {sensu Bagenal and Tesch, 

 1978) in the lateral fields or by clear zones, where 

 circuli were more widely spaced, in the anterior field. 



Vertebrae 



The second vertebra of 24 vertebrae present in king- 

 fish was chosen because it was most easily obtained 

 from processed fish. Vertebrae were either stored fro- 

 zen with flesh intact, or the flesh was removed, and 

 the vertebrae were separated from each other and 

 stored dry. The spines were removed and each verte- 

 bra cut in half along the longitudinal— horizontal 

 plane and stained in a solution of alizarin red S (fol- 

 lowing Berry et al. , 1977 ) for 8 h, rinsed in tap water 

 for at least 1 min, and dried at room temperature. 

 Vertebrae were read under a dissecting microscope 

 (6-12x magnification) with reflected light from a 

 blue-filtered, high-intensity bulb against a black 

 background. Age was estimated from counts of ridges 

 on the inner surface of the vertebra from the core to 

 the outer edge of the centrum (Fig. IC). 



Assessment of aging techniques 



To determine whether the zones would be reliably 

 interpreted, two replicate counts of zones were made 

 for each structure by the same person. Counts were 

 usually separated by one month. All readings were 

 done in a random order, with no knowledge of date 

 of collection, size of fish, or knowledge of previous 

 counts. Preliminary investigations of transverse sec- 

 tions of otoliths from 50 fish (ranging from 323 to 

 1090 mm FL) found that growth zones were not in- 

 terpretable for any fish (Fig. 2). 



Multiple counts of zones (two counts for each ag- 

 ing structure ) were used to estimate the probability 

 of assigning an age a to a fish with estimated "true" 

 age b following maximum-likelihood estimation pro- 

 cedures outlined in Richards et al. ( 1992). This pro- 

 cedure requires the estimation of a classification 

 matrix where there are columns for each "true" age 

 and rows for each assigned age, and the entries re- 

 fer to probabilities of assigning age a to a fish, given 

 its true age 6. "True" age is best described as the 

 most probable age, and it does not refer to the accu- 

 racy of the age estimate nor does it substitute for 

 age validation procedures. It is assumed that fish will 

 be assigned to the true (or most probable) age class 

 with the highest probability (Richards et al., 1992). 



The classification matrix is defined by up to four 

 parameters, where the first two parameters, CTj and 



