Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 



47 



increments in otoliths. Direct validation of age has 

 not yet been attempted for P. leopardus. 



Fish population models usually require a general 

 description of the growth process by means of an ap- 

 propriate mathematical function. The von 

 Bertalanffy (1938) growth model is the most stud- 

 ied and the most frequently used, since its applica- 

 tion by Beverton and Holt (1957) to the yield-per- 

 recruit problem (Kimura, 1980; Gallucci and Quinn, 

 1979). Many alternative growth curves have been 

 proposed (see Moreau, 1987) as well as the use of 

 polynomial functions (Chen et al., 1992). In this 

 work, Schnute's (1981) formula was used to find the 

 model that best described the growth of P. leopardus. 



For several species of fishes, otolith growth has 

 been found to continue with age, independent offish 

 size (Boehlert, 1985; Casselman, 1990; Beckman et 

 al., 1991). Boehlert (1985) suggested the use of 

 otolith weight as a non-subjective, cost-effective 

 methodology for age determination that would de- 

 crease variability among age estimates. 



The aims of this study were to obtain direct vali- 

 dation of age-at-length information and to find the 

 model that best described the growth of the common 

 coral trout from Lizard Island, Northern Great Bar- 

 rier Reef, Australia. In addition, the relationship 

 between otolith weight, body size, and age of the 

 coral trout was studied to understand the mode of 

 growth of the otolith and to assess the usefulness 

 of otolith dimensions in predicting age. 



Materials and methods 



Coral trout (t?=310) were sampled in the Lizard Is- 

 land area (lat. 14° 40' S, long. 154° 28' E) from March 

 1990 to February 1992. Fishes were caught by rec- 

 reational and commercial fishermen using hook and 

 line (77 = 184) and by recreational spearfishermen 

 (n=94). Individuals smaller than 20-cm total length 

 are usually not vulnerable to line fishing, so they 

 were caught around Lizard Island by scuba divers 

 using fence nets (77=32). Fork length (FL, cm), stan- 

 dard length (SL, cm) and total weight (TW, g) were 

 measured for each fish. FL is defined as the length 

 from the front of the snout to the caudal fork, and 

 SL is defined as the length from the front of the 

 upper lip to the posterior end of the vertebral col- 

 umn. A simple linear regression of the form FL= a 

 + 6*SL was used to describe the relationship be- 

 tween FL and SL. To describe the relationship be- 

 tween FL and TW the variables were logarithmically 

 transformed and the linearized version of the power 

 function TW(g)= a*FL(cm)b was fitted to the data. 

 In the coral trout, the sagittae are the largest of 

 the three pairs of otoliths and were used for read- 

 ings. Sagittae were removed, cleaned, weighed, and 



stored dry. Left and right sagittae, when intact, were 

 weighed to the nearest milligram. Otoliths were 

 prepared and read as described by Ferreira and 

 Russ (1992). To increase contrast between bands, 

 whole otoliths were burned lightly on a hot plate at 

 180°C (Christensen, 1964). Both right and left 

 sagitta were read whole under reflected light with 

 a dissecting microscope at 16x magnification. The 

 otoliths, with the concave side up, were placed in a 

 black container filled with immersion oil. Subse- 

 quently, the left sagittae was prepared for reading 

 by embedding in epoxy resin (Spurr, 1969) and sec- 

 tioning transversely through the core with a Buehler 

 Isomet low-speed saw. Sections were mounted on 

 glass slides with Crystal Bond 509 adhesive, ground 

 on 600- and 1200-grade sand paper, polished with 

 0.3— u alumina micropolish and then examined un- 

 der a dissecting microscope at 40x magnification 

 with reflected light and a black background (Fig. 1). 

 Annuli were counted from the nucleus to the proxi- 

 mal surface of the sagitta along the ventral margin 

 of the sulcus acousticus. 



Terminology for otolith readings followed defini- 

 tions of Wilson et al. (1987). Two experienced read- 

 ers independently counted opaque zones (annuli) in 

 each whole and sectioned otolith of a random 

 subsample (77 = 136) to assess the precision and ac- 

 curacy of countings obtained by the two methods. 

 The precision of age estimates was calculated with 

 the Index of Average Percent Error (IAPE), (Bea- 

 mish and Fournier, 1981). Results obtained from 

 whole and sectioned otoliths were compared by plot- 

 ting the difference between readings obtained from 

 whole and sectioned otoliths (Section Age- Whole 

 age) against Section Age. The results of this com- 

 parison indicated that whole otolith readings tended 

 to be lower than readings from sectioned otoliths 

 when more than six rings occurred in the otolith. 

 Therefore, remaining otoliths were read whole first 

 and, if the number of rings was higher than six or 

 the whole otolith was considered unreadable, the 

 otolith was sectioned and counts were repeated. The 

 results were accepted and used in the analysis when 

 the counts of the two readers agreed. If the counts 

 differed, the readings were repeated once and if the 

 counts still differed, the fish was excluded from the 

 analysis. 



Ages were assigned based on annulus counts and 

 knowledge of spawning season. The periodicity of 

 annulus formation was determined with the use of 

 tetracycline labelling. From August 1990 to Febru- 

 ary 1992, 80 fishes were caught in a trapping pro- 

 gram at Lizard Island fringing reeflDavis, 1992 2 ), 



2 C. Davies. 1992. James Cook University, Townsville, Q4811, 

 Australia, unpubl. data. 



