362 Abstract.–The lack of age and growth Age distribution and growth of greater estimates and population parameters for the amberjack family prompted our amberjack, Seriola dumerili, from the study to describe otolith structure and determine size-at-age and growth rates north-central Gulf of Mexico* of greater amberjack (Seriola dumerili) from the north-central Gulf of Mexico. Greater amberjack age and growth Bruce A. Thompson was described from a combination of external ridges and internal annuli in Marty Beasley sectioned sagittal otoliths. A mark-re- capture study using tetracycline was Charles A. Wilson consistent with a single annulus being Coastal Fisheries Institute formed each year in two- and three- Center for Coastal, Energy, and Environmental Resources year-old amberjack. Tag-recapture data Louisiana State University from the Gulf of Mexico and South At- Baton Rouge, Louisiana 70803-7503 lantic Cooperative Gamefish Tagging E-mail address (for B. A. Thompson): [email protected] Program also confirmed that a single annulus increment was formed each year. Ages for 597 specimens ranged from young-of-the-year to 15 years, and all fish over 9 years were female. Sag- ittal weight provided a reliable estima- The greater amberjack, Seriola western Atlantic to 10 years for fe- tion of fork length with a relationship dumerili, is a pelagic reef species males and to eight years for males. of FL = 151 (1 – e (–0.04(SW+1.6))): a nega- ranging along the American coasts Females were significantly longer tive exponential equation in the form of from Nova Scotia to Brazil and oc- than males beyond age four. Hum- the von Bertalanffy equation. The fit of the curring throughout the Atlantic, phreys (1986) reported von Berta- negative exponential in this application pro- vides evidence of the disassociation be- Pacific, and Indian oceans, as well lanffy growth parameters for Ha- tween otolith growth and fish growth as as the Mediterranean Sea (Mather, waiian greater amberjack but did fish age. The von Bertalanffy growth 1958; Burch, 1979; Shipp, 1988). It not elaborate on how the fish were curve equation, L = 138.9 (1 – e–0.25(t+0.79)), t is the largest member of the family aged or how many fish were con- compared favorably with previous pub- Carangidae (Hoese and Moore, tained in the data set. lished values. Monthly changes in size and availability indicate a Gulf-wide mi- 1977). The purpose of this study was to gration of this moderately long-lived pe- Commercial landings of amber- collect and describe greater amber- lagic predator. jack (all species) have increased jack otoliths, assess their utility in markedly over the past twenty age estimation, and determine the years. Landings in the Gulf of size-at-age and growth rates of Mexico peaked in 1988 reaching 2.7 greater amberjack from the north- million pounds, with a dockside central Gulf of Mexico. value over 1.6 million dollars, but have since declined to less than one million pounds in 1995–96. Recre- Materials and methods ational catch statistics are incom- plete but are suspected to have Sampling equalled or surpassed the commer- cial catch, raising concern over the Greater amberjack (n=840) were status of the Atlantic stock in the collected off the Louisiana coast late 1970s (Berry and Burch, 1979). from April 1989 to June 1992. Despite their popularity as a recre- Sources of samples included a com- ational and commercial quarry, mercial processor plant (n=18), little is known about the life histo- charterboats (n=352), saltwater ries of amberjacks (Shipp, 1988). fishing tournaments (n=215), spear- Of particular concern to stock- fishing tournaments (n=159), recre- assessment scientists is the lack of ational catches (n=48), and hook- age and growth estimates and popu- lation parameters for the amberjack family (Burch, 1979; Humphreys, * Contribution LSU-CFI-94-9 of the Coastal Fisheries Institute, CCEER, Louisiana Manuscript accepted 12 June 1998. 1986). Burch (1979), using scales, State University, Baton Rouge, LA 70803- Fish. Bull. 97:362–371 (1999). aged greater amberjack from the 7503. Thompson et al.: Age distribution and growth of Seriola dumerili 363 and-line catches from an offshore gas production plat- above, tagged at the base of the dorsal fin with a Hall- form (n=48). In general sampling was not system- print dart tag, injected with oxytetracycline hydro- atic; we examined all fish available. chloride (Agrimycin-100) at a rate of 20–40 mg/kg fish Morphometric measurements and otoliths were weight, and released. Tagged greater amberjack (6) collected. Fish were measured by using fork and to- were recaptured with hook and line or by spear fish- tal lengths (FL and TL) in mm and weighed by using ermen at the release site and sampled as described total and gutted weights in g. Both sagittae were above. Sectioned sagittae were viewed under ultra- removed and stored in 95% ethanol for later exami- violet light (405–435 nm wavelength) at 50–250× nation. Lapilli, asterisci, and dorsal spines were re- magnification for detection of the tetracycline mark. moved from several fish to evaluate their usefulness Attempts made to compare age estimates from sag- for age estimation. Otoliths were cleaned of organic ittal otoliths with dorsal and anal spines and verte- tissue by rinsing in a 50% hypochlorite solution brae were not successful. To determine reproducibil- (Clorox), air dried, weighed to the nearest 0.1 mg ity of age estimates, sagittae were aged indepen- with a Sartorius model 1801 microbalance, and dently by two readers. Reproducibility of age esti- stored dry (Wilson et al.,1991). mates was compared by using the coefficient of varia- Age estimates were made by using techniques simi- tion (CV) and index of precision (D) (Chang, 1982). lar to those of Wilson et al. (1991) for billfish. Whole Tag-recapture data for the Gulf of Mexico and otoliths were sputter coated with a mixture of gold South Atlantic (n=711) were obtained from the Co- and palladium and viewed with reflected light un- operative Gamefish Tagging Program (CGTP). Fish der a dissecting microscope for description of exter- that had been at large for at least 365 days after nal morphology. These samples were also viewed with tagging and that showed positive growth were used. a Cambridge Stereoscan 150 scanning electron mi- Twenty-five specimens met these criteria. Following croscope (SEM) to obtain detailed photographs at the method of Labelle et al. (1993), we used the von magnifications ranging from 20 to 650×. Bertalanffy growth curve parameters to predict Sagittae were embedded in epoxy resin (Spurr, changes in length from the tag recapture data to Embed 812, or Araldite GY 502) and sectioned with verify our age estimates. Like Labelle et al. (1993) a Buehler Isomet low-speed saw to yield a thin (about we employed Fabens’ (1965) length increment model 1-mm) transverse section containing the core to predict length at recapture: (Beckman, 1989). Transverse sections were ground =+ − −−kt∆ with various grades (300–2000 grit) of wet and dry lrii l()(), L∞ l i1 e i sandpaper until the core was at the surface, then µ polished with 0.3 m alumina polish. Thin sections where li = length at release of individual i; ∆ were mounted on glass slides with a clear thermo- ti = time of liberty of individual i; plastic cement (Crystalbond 509) and viewed under lri = estimated length-at-recapture of indi- transmitted light with an Olympus BH-2 compound vidual i; and microscope at 50–250× magnification. Age estimates L∞ and k = von Bertalanffy parameters estimated from sectioned sagittae were made by combining from otolith ages. counts of translucent and opaque zones and other growth features viewed under transmitted light and Predicted recapture lengths were then compared with counts of associated ridges on the ventral and observed recapture lengths. As Labelle et al. (1993) medioventral portions of the rostrum as determined pointed out, this procedure is not statistically rigor- in billfish species (Wilson and Dean, 1983; Prince et ous but is useful for comparative purposes. al., 1986; Wilson et al., 1991). An analysis of variance (ANOVA) was used to test Validation and verification of our age estimation for sex-related differences in mean fork length, mean technique were attempted by using several tech- gutted weight, mean sagittal weight, and mean age niques. Validation of annulus formation using mar- among sources and within age class. Analysis of co- ginal increment analysis proved futile owing to our variance (ANCOVA) was used to test for differences inability to determine the condition of the otolith in the relations (fork length [L] versus total weight edge. Therefore we pursued additional techniques [Wt], fork length versus sagittal weight [SW], age that would corroborate our age estimates. A mark- versus sagittal weight) between sexes (Cerrato, 1990; recapture study was carried out from a gas produc- Kimura, 1980). Statistical inferences were made with tion platform (Mobil USA, West Cameron block 352) a significance level of α=0.05. The relationship be- located in the Gulf of Mexico 50 miles south of tween fork length and otolith weight was modeled Cameron, Louisiana. Greater amberjack (n=48) were with a negative exponential (von Bertalanffy) be- captured by hook and line, measured as described cause it provided the best fit (highest r2). 364 Fishery Bulletin 97(2), 1999 –k(t – t0) The von Bertalanffy model Lt = L∞(1 – e ) was also used to describe the relation between length and Table 1 age. Relative ages were assigned to fish by using a Monthly mean fork length (mm) and sex ratio of charterboat- birth date of 1 April based on trends in gonadoso- caught greater amberjack from the northern Gulf of Mexico matic indices and the regression of sagittal weight on ranked by decreasing mean fork length.