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Fishery Bulletin 92(3), 1994 



from the primordium (the mean distance to the tenth 

 ring from the ten larval otoliths). The juvenile otoliths 

 were transversely sectioned and glycerin-cleared. This 

 method was verified by examination of the innermost 

 rings from exceptionally clear juvenile otoliths. 



We examined sagittae, lapilli, and asterisci to de- 

 termine the best ageing structure for juvenile red 

 drum by comparing ring counts for each otolith with 

 known ages in hatchery-reared fish. We also evalu- 

 ated the relation between fish length and otolith di- 

 ameter for use in backcalculating lengths at age from 

 each otolith. We report the size at which otoliths are 

 formed to evaluate possible biases in age estimations. 



passing through the primordium. The relationship 

 between standard length and otolith diameter was 

 examined by plotting otolith diameter on standard 

 length and calculating the regression. 



We examined larvae and juveniles to determine 

 the size at which otoliths were formed. Larvae were 

 translucent under transmitted polarized light, so 

 otoliths were observable without dissection in fish 

 as small as 4.0 mm SL. To precisely determine fish 

 size at asteriscus formation, five larvae between 2.7 

 and 3.5 mm SL were dissected and the disrupted otic 

 capsules mounted on glass slides with polymer 

 mounting medium and examined microscopically. 



Materials and methods 



Results 



Wild-caught and known-age (laboratory-reared) ju- 

 venile red drum were preserved in 70% ethanol. Stan- 

 dard lengths of fish were measured to the nearest 

 0.1 mm prior to the removal of otoliths. We made no 

 attempt to measure shrinkage due to preservation 

 or to adjust our measurements accordingly. Otoliths 

 were extracted between crossed polaroids by using a 

 dissecting microscope at 6-12x magnification and 

 transmitted light. This procedure exploited the bire- 

 fringence of the crystalline structure of otoliths and 

 greatly aided otolith location and removal. Extrane- 

 ous tissue was removed, and the otoliths were 

 mounted concave side up on glass slides. Two mount- 

 ing media were used, thermoplastic cement and a 

 polymer mounting medium. The thermoplastic ce- 

 ment allowed easier otolith manipulation and the 

 polymer provided a less brittle and more transpar- 

 ent mount. Otoliths were ground in the sagittal plane 

 with 600-grit carborundum paper and polished with 

 0.9-pm aluminum oxide sheets prior to examination 

 under a compound microscope. Otoliths mounted 

 with thermoplastic cement were ground to the pri- 

 mordium, flipped convex side up, and again ground 

 to the primordium. Polymer mounted otoliths were 

 ground to the primordium from the concave side. The 

 polished otoliths were illuminated with plane-polar- 

 ized transmitted light to optimize ring resolution. 

 Ring counts were determined at 125-250x for 

 sagittae and at 250-500x for lapilli and asterisci. To 

 avoid bias from sequentially ageing all three otoliths 

 from the same fish, all sagittae were aged first, fol- 

 lowed by all lapilli and all asterisci. After ageing, 

 mean ring counts, standard deviations, and coeffi- 

 cients of variation were calculated. 



Otolith diameters (urn) were measured with a digi- 

 tal image analysis system (Optimas ver 3.01, Bioscan, 

 Edmunds, WA). Otolith diameter was defined as a 

 chord from the anterior rostrum to the posterior edge. 



Wild-caught fish ranged from 15 to 50 mm SL, and 

 laboratory-reared fish ranged from 18 to 25 mm SL. 

 Fifty laboratory-reared and 70 wild-caught juveniles 

 were processed; six otoliths were removed from each 

 fish. The sagittae were used as landmarks to locate 

 the smaller lapilli and asterisci. The lapilli were an- 

 terior and distal to the sagittae, and the asterisci 

 were posterior and proximal to the sagittae. The 

 asterisci were often found attached to the saccular 

 tissue removed with the sagittae. To determine the 

 size at otolith formation, we examined 35 wild-caught 

 red drum larvae ranging from 1.3 to 7.0 mm SL. 

 Sagittae and lapilli were present in all larvae exam- 

 ined. Asterisci were found in all dissected fish >3.0 

 mm SL (n=5) and not detected in fish smaller than 

 2.8 mmSL(«=3). In specimens that were not dissected, 

 asterisci were first observed in larvae 3.8 mm SL and 

 were present in all larvae larger than 4.0 mm SL. 



Considerable differences were found in the gen- 

 eral size and shape of the otoliths (Fig. 1 ). All otoliths 

 were spherical or slightly ovoid in larvae <5 mm SL. 

 Sagittae in larvae >5 mm SL began to develop a ros- 

 tral process, and sagittae of 10-mm larvae were oval, 

 laterally compressed, and had developed a prominent 

 rostrum. The general shape of sagittae did not change 

 in fish 10-50 mm SL. Lapilli followed the same ini- 

 tial development, but the posterior margin of these 

 otoliths became scalloped when larvae were approxi- 

 mately 15 mm SL owing to the formation of numer- 

 ous accessory primordia. Initially asterisci were also 

 spherical, but the axis of growth changed, with sub- 

 sequent development resulting in a kidney-shaped 

 appearance by 15 mm SL. Accessory primordia were 

 seldom observed in sagittae and were not observed 

 in asterisci. Lapilli and asterisci were similar in size; 

 however, the diameter of sagittae was approximately 

 three times larger and grew approximately five times 

 faster than asterisci and lapilli (Fig. 2). 



