344 



Fishery Bulletin 101(2) 



The size range of striped mullet at the time of estuarine 

 recruitment is well documented; however, the ages of these 

 juveniles are not well known (Chang, et al., 2000). These 

 data are most easily obtained by examining the sagittal 

 otoliths for daily growth increments. The importance of 

 daily growth increments to estimate age in juvenile and 

 lar\'al fish for life history characteristics such as growth, 

 recruitment, and determining spawning season is well 

 documented (Brothers et al., 1976; Ralston and Miyamoto, 

 1983; Jones, 1986; Ralston and Williams, 1988; Campana 

 and Moksness, 1991; Campana et al., 1994; Sponaugle and 

 Cowen, 1997; Gillanders and Kingsford, 1996). 



Several authors (Jones, 1986; Campana and Moksness, 

 1991; Campana et al., 1994) have concluded that daily 

 growth increments should be validated for each species 

 individually because of the variability in the time of deposi- 

 tion of the first daily increment, consistency of their forma- 

 tion, and their legibility with time. Radtke ( 1984 ) validated 

 daily growth increments in cultured larval striped mullet 

 in Hawaii and showed that daily growth increments were 

 deposited well into the juvenile stage. Daily growth incre- 

 ments in juvenile striped mullet caught in the wild from 

 Taiwan have also been validated (Chang et al., 2000). 



The periodicity of increment deposition can be determined 

 by either of two methods. Rings on otoliths from fish of known 

 age (hatched in the laboratory) can be counted back to the 

 core at specified times to check the increments. This method 

 is accurate because factors affecting the fish (environmental, 

 behavioral, and biological) can be more tightly controlled or 

 manipulated in the laboratory (Jones, 1986). However, cul- 

 tured conditions, at best, can only closely approximate condi- 

 tions in the wild. In the second method, the otoliths of wild 

 fishes are chemically marked. Fishes are sampled at various 

 times and the increments deposited after this reference point 

 are counted to estimate daily age (Jones, 1986). 



The purposes of our study were 1 ) to demonstrate that 

 juvenile striped mullet in South Carolina deposit daily 

 growth increments on the sagittae; 2) to validate these 

 marks as daily; 3) to use these validated ages and length 

 data to model growth during the first year. Accurate age de- 

 terminations through daily aging would provide important 

 life history information, such as age at recruitment and the 

 length of the spawning season through back-calculation of 

 the spawning date. In addition, comparisons of the growth, 

 spawning season, and occurrence of juvenile striped mullet 

 from our study were made with historical data of abundance 

 and length frequencies in South Carolina (1986 to 1991). 



Materials and methods 



Data collection and processing 



The juvenile aging study was conducted from April of 1998 

 to December of 2000. A minimum of 20 juvenile striped 

 mullet were collected monthly with a 7-m seine with 6-mm 

 stretch mesh from the mudflats located in Grice Cove near 

 Fort Johnson, South Carolina (Fig. 1). Additional samples 

 came from the South Carolina Department of Health and 

 Environmental C^onlrol (DHEC-) and were caught from an 



electroshock boat in the freshwater portions of the Santee 

 River, Cooper River, Ashley River, and Combahee River Also, 

 the Tidal Creek Project workgroup of the South Carolina 

 Department of Natural Resources (SCDNR) used cast nets 

 ( 1.87-m diameter with 6-mm mesh) in the tidal creeks of the 

 lower Ashley River to catch juvenile striped mullet. A total 

 of 2800 juvenile striped mullet were collected during the 

 study; the number of these fish that were aged was 335. 



Hydrographic data (water temperature and salinity! 

 were recorded after sampling. Each specimen was mea- 

 sured in millimeters (mm) for total length (TL), fork length 

 (FL), and standard length (SL). Sagittal otoliths were re- 

 moved from the fish, and the right otolith was mounted on a 

 microscope slide with Cytoseal mounting medium. Otoliths 

 were polished with a felt polishing wheel loaded with tin 

 oxide polishing compound ( 10 micron grit) by using a Crys- 

 talite lapidary polisher along the sagittal axis until the core 

 was clearly discernible with increments visible to the outer 

 edge. Age was determined at lOOOx magnification under 

 visible light as the mean of two independent readings of 

 the counts of increments from the core to the outer edge. A 

 third reading was made if the counts of the two previous 

 readers differed by more than 10%. 



Age validation 



For the age validation experiment, 275 juvenile striped 

 mullet were collected on 29 February 2000 from Fort John- 

 son Creek, a tributary of Parrot Point Creek in the Charles- 

 ton Harbor estuary (Fig. 1), by seining. Specimens were 

 transported to the laboratory and placed in a 1.3-m diame- 

 ter tank, and acclimated overnight in well water at 15 parts 

 per thousand (ppt) and 20°C. After transferral to a 38.7-L 

 aquarium (15 ppt; 20°C) for marking, fish were immersed 

 in oxytetracycline hydrochloride (OTC) at a concentration 

 of 10 parts per million for five hours (Hettler, 1984). There 

 were no mortalities during the treatment process. Marked 

 specimens were placed in an outdoor 1.8-m diameter tank 

 filled with (15 ppt, 20''C) well water Water in the tank 

 was replaced over a twenty-four hour period with ambient 

 water from Charleston Harbor These juveniles remained 

 in the outside tank for the duration of the experiment to 

 approximate as closely as possible the natural conditions of 

 the local creek habitats. The fish were fed a daily ration of 

 Tetramin floating commercial flakes (Tetramin, Blacksburg, 

 VAl and were observed feeding one day after the OTC treat- 

 ment. Salinity and water temperature, as well as dissolved 

 oxygen concentration, were monitored daily. 



Ten specimens were collected weekly for the next three 

 weeks. Sagittal otoliths were removed and subjected to the 

 same process as that given the otoliths used in the juvenile 

 aging study. Each otolith was checked for the OTC mark 

 under ultraviolet (UV) light at lOOOx magnification by 

 using a Nikon labophot compound microscope. The OTC 

 mark appeared as a fluorescent mark when exposed to UV 

 light (Fig. 2). Once the OTC mark was identified and its 

 position marked, counts of increments after the OTC mark 

 were made under visible light and recorded. This count was 

 compared to the known number of days that had passed 

 since marking with OT(L 



