Sadovy et al,: Age and growth of Epinephelus guttatus in Puerto Rico and St Thomas 



517 



Methods 



Samples of Epinephelus guttatus were obtained from 

 local fishermen and from Fisheries Research Labora- 

 tory (FRL) research programs using hook-and-iine and 

 arrowhead fish traps (3.2cm (1.25 in.) galvanized mesh). 

 Monthly collections were made between September 

 1987 and January 1989 with a minimum of 80 fish for 

 most months from Puerto Rico. Smaller monthly 

 samples from February 1988-January 1989 were 

 received from St. Thomas, which lies on the same 

 geological platform. Fish from St. Thomas were taken 

 by hook-and-line and by fish trap (3.81cm (1.5 in.) 

 galvanized mesh). 



For each fish, the weight (whole weight to nearest 

 gm) and length (fork length (FL) and standard length 

 (SL) to nearest mm) were measured. Otoliths (sagit- 

 tae) were extracted, washed, and stored dry prior to 

 processing. Preliminary work determined these cal- 

 careous structures to be more suitable than other 

 calcareous structures for ageing purposes: dorsal 

 spines exhibited growth lines but the central portion 

 was often eroded resulting in an incomplete growth 

 history, and scale markings were irregular and thus 

 considered unreliable for ageing. Gonads in good con- 

 dition were removed whenever possible, fixed in David- 

 son's fixative (Yevich and Barszcz 1981), embedded in 

 paraffin, sectioned at S^m, and stained with hematoxy- 

 lin and eosin. 



Examination of whole otoliths under transmitted 

 light revealed alternating opaque and translucent zones 

 (terminology follows that of Wilson et al. 1983). To 

 count the zones, however, otoliths had to be section- 

 ed. Preliminary sectioning in two planes (frontal and 

 transverse; N=20 otoliths in each plane) established 

 that transverse sections most clearly revealed growth 

 zones. For sectioning, otoliths were mounted with glue, 

 using a hot glue gun, on small cards, and sectioned 

 through the focus with a single 7.2cm (3 in.) diameter, 

 low-concentration diamond blade on a Buehler Isomet 

 low-speed saw. From each otolith, three sections of 

 0.36-0.43 mm were mounted on glass slides using Flo- 

 Texx mounting medium. 



Otolith width (OW) of a sample of unsectioned oto- 

 liths from a wide size-range of fish was measured to 

 describe the OW/FL relationship. Measurements of 

 otolith sections were made from the point where the 

 sulcus meets the focus to the dorsal margin of the 

 otolith (the region of most rapid growth) and to the 

 proximal edge of each opaque zone. Measurements 

 were also made from the distal edge of the outermost 

 (opaque) zone to the dorsal margin for marginal incre- 

 ment analysis. Total number of opaque zones was 

 noted. Measurements were made with an ocular 

 micrometer, to the nearest micrometer unit (where 



1mm = 32 micrometer units). Each otolith was read 

 twice. When readings disagreed by more than one 

 opaque zone, the otolith was eliminated. Readings of 

 a subsample of otoliths from Puerto Rico, of a wide 

 range of ages and size-groups, were also made by an 

 independent researcher. 



To validate the temporal significance of opaque 

 zones, a field study was undertaken. Individuals were 

 captured by hook-and-line baited with squid from a 

 30 X 30 m area on a shallow (7 m) relatively-unfished reef 

 known as "El Negro," 6km off western Puerto Rico. 

 The study site was visited over a 15-month period be- 

 tween April 1988 and June 1989. Individuals were 

 tagged with a numbered FLOY anchor tag inserted 

 into the dorsal musculature, and/or with FLOY ab- 

 dominal tags for identification. Each fish was measured 

 (FL) and injected with a dosage of lOOmg/kg body 

 weight of Terramycin 100 (Pfizer) (ImL contains 

 100 mg oxytetracycline-OTC-hydrochloride), and 

 released. The dosage necessary to produce a visible 

 mark under longwave ultraviolet light was established 

 by preliminary tests (50 and lOOmg/kg body weight 

 were tested; 50mg/kg body weight did not consistent- 

 ly leave OTC marks) and the correct dosage determined 

 on-site from a weight/FL relationship. Fish recaptured 

 were measured and the otoliths examined for opaque 

 zone formation following deposition of the OTC time 

 marker. 



Data were analyzed separately for Puerto Rico and 

 St. Thomas using Lotus 1-2-3 and Basic Fishery 

 Science Programs (Saila et al. 1988). The Kolmogorov- 

 Smirnov two-sample test and the t -test were used to 

 compare size-frequency distributions and mean size, 

 respectively (Sokal and Rohlf 1981). Weight (W) on FL 

 regressions were calculated using the relationship 

 W = aFL''. The SL:FL and OW:FL regressions were 

 determined. The Lee method (Carlander 1981) of back- 

 calculating body length from prior annuli was used: 



Li = a -1- [(Le - a) (0,/OR)], 



where: L; = length at time of ith annulus formation 

 a = intercept 



Lc = length at time of capture (FL) 

 0; = otolith radius at time of ith annulus 



formation 

 OR = otolith radius at time of capture. 



This method requires knowledge of the relationship 

 between OR along the line of measurement and FL. 

 The constant a is obtained from this relationship and 

 used in Lee's formula. 



Growth was assumed to conform to the von Ber- 

 talanffy growth function (VBGF) (Ricker 1975). This 



