Age Determination in Larval and 



Juvenile Sheepshead, Archosargus 



prohatocephalus 



Pannella (1971) first demonstrated the existence 

 of daily growth increments in the otoliths of sev- 

 eral species of teleosts. Enumeration of these 

 daily increments is particularly useful for age 

 determination in larval and juvenile fishes. Veri- 

 fication of daily growth rings in individual fish 

 species has generally been approached in one of 

 two ways: laboratory rearing (Brothers et al. 

 1976; Struhsaker and Uchiyama 1976; Taubert 

 and Cable 1977; Barkman 1978; McGurk 1984; 

 Davis et al. 1985;) or by using chemical marking 

 techniques either in the wild or in the labora- 

 tory. 0.xytetracycline hydrochloride has been 

 used to produce a fluorescent mark for examina- 

 tion of daily growth increments in otoliths of the 

 stan-y flounder, Platkhthys stellatus (Campana 

 and Neilson 1982), and juvenile Hawaiian snap- 

 per, Pnstipomoides filanientosus (Ralston and 

 Miyamoto 1983). Otolith microstructure and its 

 use in fish age determination was reviewed by 

 Campana and Neilson (1985). 



The early life history of the sheepshead, 

 Archosargus prohatocephalus, is poorly known. 

 In this study, daily growth rings in the otoliths 

 of larval and juvenile sheepshead were examined 

 and vaUdation was accompUshed using tetra- 

 cycline-marked specimens held in the labora- 

 tory. The daily rings of wild caught specimens 

 were counted to obtain information on the age at 

 transition from larva to juvenile. 



Materials and Methods 



Sheepshead larvae and juveniles were col- 

 lected from Bayboro Harbor, St. Petersburg, 

 FL, between 21 April 1983 and 7 May 1985. 

 Larvae (5-8 mm SL) and juveniles (gi-eater than 

 about 8 mm) were collected from a seawall using 

 a dip net fished at the surface. All specimens 

 were preserved in 95*?^ ethanol. Subsamples of 

 larvae were measured (SL) using an ocular 

 micrometer, and the sagittal otoliths were 

 teased out and mounted on microscope slides 

 using Pro-texx' mounting media. In the largest 

 individuals (about 10 mm SL), one otolith was 

 mounted on a microscope slide in thermoplastic 

 cement and poHshed with 3 \y grit microtome 



'Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



paper. The otoliths were poUshed until the rings 

 near the primordium were clearly visible (Fig. 

 1). 



The daily nature of the growth increments was 

 validated using 29, (7-10 mm SL) wild-caught 

 larvae collected on 1 May 1984. Larvae were 

 placed in 38 L aquaria containing oxytetracycline 

 hydrochloride at a concentration of 10-15 mg/L, 

 left for 7 hours, and then were removed to un- 

 treated tanks. Fish were fed Arionia salina 

 nauplii and allowed to grow for 6 or 15 days, 

 after which they were preserved in 95% ethyl 

 alcohol and stored in the dark. The position of 

 the tetracycline mark was determined by pre- 

 paring these otoliths as described above and al- 

 ternately viewing them under ultraviolet and 

 visible light. 



Otoliths were examined at 630-1000 x magnifi- 

 cation using a compound microscope equipped 

 with a high resolution closed circuit television to 

 improve contrast. Daily growth increments were 

 independently counted by two readers. Counts 

 were considered in agreement if they differed by 

 two or less increments. When the counts differed 

 by two increments, the median value was used; 

 when they differed by one increment, the 

 gi'eater value was used. If counts differed by 

 three or mure, those otoliths were reexamined 

 by both readers in a joint effort to resolve the 

 differences. About 5% of the otoliths were ex- 

 cluded from the data set because differences in 

 counts could not be resolved. 



Results 



Approximately 2,000 larval and juvenile 

 sheepshead, ranging in size from about 5-10 mm, 

 were collected during the study (Fig. 2). About 

 90% of the individuals were 6.5-8.0 mm (mean = 

 7.0 mm); fish larger than approximately 8 mm 

 represented less than 2% of the sample. Rings in 

 otoHths of larval and juvenile sheepshead were 

 clear (Fig. 1), and results of the vahdation ex- 

 periment confirmed daily increment formation, 

 at least between 7 and 10 mm. Of 26 specimens 

 sacrificed six days after tetracycline treatment, 

 19 showed clear marks and had produced six 

 increments beyond the reference mark. Refer- 

 ence marks could not be located on six of the 

 remaining specimens, and the increments could 

 not be counted on one specimen. The specimens 

 sacrificed after 15 d post-treatment showed clear 

 reference marks and had produced 15 incre- 

 ments. 



A sample of 129 larval and juvenile sheeps- 



Manuscript accepted April 1989. 

 Fishery Bulletin, U.S. 87:985-988, 1989. 



985 



