10 Taylor FISH 98(3)

612 Abstract.–During 1986–91, we exam- Age, growth, maturation, and ined 2088 common snook, Centropomus undecimalis, captured in Jupiter and protandric sex reversal in common snook, Lake Worth inlets and adjacent waters on the east coast of Florida and 1784 Centropomus undecimalis, from the common snook captured in Tampa Bay on the west coast of Florida. Of fi sh that east and west coasts of South Florida were sexed, females ranged in length from 397 to 1105 mm FL, and males Ronald G. Taylor ranged from 124 to 925 mm FL. East coast fi sh were larger overall than west Florida Fish and Wildlife Conservation Commission coast fi sh. Age of common snook was Florida Marine Research Institute determined from sectioned otoliths. 100 Eighth Avenue SE Results from the return of 80 oxytetra- St. Petersburg, Florida 33701-5095 cycline-marked otoliths combined with E-mail address: [email protected].fl .us analyses of monthly patterns in mar- ginal increments and the percentage of James A. Whittington otoliths with an annulus on the edge, demonstrated that a single annulus is Florida Fish and Wildlife Conservation Commission formed each year. Common snook can Tequesta Field Laboratory live to 21 years, but most of the fi sh Florida Marine Research Institute in our sample were from 1 to 7 years 19100 SE Federal Highway old. The von Bertalanffy growth models Tequesta, Florida 33469 were signifi cantly different (P<0.001) for each coast and suggested that east Harry J. Grier coast snook grow faster than west coast snook. Common snook are protandric Florida Fish and Wildlife Conservation Commission hermaphrodites. The gonads of 27 Stock Enhancement Research Facility transitional specimens contained both Florida Marine Research Institute degenerating spermatogenic and devel- 14495 Harllee Road oping ovarian tissue, and sex reversal Port Manatee, Florida 34221 was observed in captive common snook. Common snook sex ratios and length- frequency distributions were also con- Roy E. Crabtree sistent with a diagnosis of protandric National Marine Fisheries Service, F/SER23 hermaphroditism. Females smaller 9721 Executive Center Drive North than 500 mm FL were uncommon, and St. Petersburg, Florida 33702 only one female less than 400 mm long was captured. The predicted lengths and ages at which 50% of the fi sh in the population would be females were 767 mm FL and 7.4 years for the east coast and 608 mm FL and 5.1 years for the west coast. Some males on both coasts were sexually mature at lengths less Common snook, Centropomus undec- tribution of mangroves which is their than 200 mm FL and at age 0; most imalis, (Perciformes: Centropomidae) principal habitat (Marshall, 1958; Gilm- age-1 males were mature on both coasts. are valuable euryhaline fi shes that ore et al., 1983). Genetic studies of the All females were considered mature inhabit tropical and subtropical estua- stock structure suggest that in Florida, because they were derived from post- rine systems of the western Atlantic. east coast and west coast populations spawning males. They are abundant off the east coast are separate stocks (Tringali and Bert, of Florida from Cape Canaveral south- 1996). ward around the peninsula to Cedar Anglers target common snook because Key off the west coast. They also inhabit of their fi ghting ability and culinary waters off Galveston, Texas, south to value (Tucker et al., 1985; Matlock and Rio de Janeiro, Brazil (Gilmore et al., Osburn, 1987), and concerns about over- 1983; Rivas, 1986). They are commer- fi shing have resulted in a long history cially exploited throughout most of their of regulation of the fi shery off Florida range, except in Texas and Florida (Bruger and Haddad, 1986). Size limits where they traditionally have supported were fi rst imposed on the fi shery in large recreational fi sheries (Matlock 1953, and the sale of common snook in and Osburn, 1987). Snook are steno- Florida has been prohibited since 1956. thermic: their northern range is limited In 1994, a management goal was estab- Manuscript accepted 6 January 2000. by the winter 15oC isotherm (Shafl and lished to maintain a minimum spawn- Fish. Bull. 98:612–624 (2000). and Foote, 1983), similar to the dis- ing potential ratio (SPR) of 40% for Taylor et al.: Age, growth, maturation, and sex reversal in Centropomus undecimalis 613 common snook stocks (Muller and Murphy1). In 1998, the a microscope slide (Chilton and Beamish, 1982). Annuli Florida Marine Fisheries Commission enacted the current were counted on the section through the core by using restrictive regulations to reduce the harvest of snook from compound microscopes and transmitted light. Two inde- Florida waters to maintain a 40% SPR. pendent readers counted annuli on each otolith without Our study examined age, growth, maturation, and sex knowledge of fi sh size or capture date. If the two readings reversal of common snook which previously had not been disagreed, both readers read the otolith again, for a total diagnosed as being protandrous hermaphrodites. Earlier of four readings. If three of the four readings agreed, then studies derived ages from either scales or whole otoliths, this reading was accepted as the annulus count; otherwise and ages were poorly validated (Volpe, 1959; Thue et al.2). the otolith was excluded from further analysis. Since these reports, studies on a variety of species have An annual growth zone in a common snook otolith com- shown that scales are not reliable for aging long-lived prises a narrow, concentric opaque band, formed each fi shes and that scale-derived and whole-otolith–derived winter as growth slows, and a wider, translucent band, age estimates are often lower than the validated estimates formed each summer as growth increases. Opaque winter derived from sectioned otoliths (Beamish and McFarlane, bands were enumerated as assumed annuli because they 1983; Casselman, 1983; Lowerre-Barbieri et al., 1994; were the salient feature in each section. Ages were assigned Crabtree et al., 1996). A validated method for aging snook to each fi sh on the basis of a 1 June birth date because is necessary to assess previous estimates of growth rates, most snook have completed annulus formation by 1 June mortality, and longevity. Our objectives were to describe and because June corresponds to the approximate begin- validated aging techniques, length and age composition, ning of the snook spawning season (Taylor et al., 1998). and length and age at maturity of common snook popula- Annulus counts were adjusted on the basis of an assumed tions from the east and west coasts of Florida. We also diag- birth date of 1 June. All otoliths that had an annulus on nosed common snook as protandrous hermaphrodites. the edge from fi sh captured between 1 December and 1 June were assigned an age of one less than the annulus count. Fish captured after 1 June and before 1 December Methods were assigned ages equal to the annulus count. In earlier studies (Volpe, 1959; Thue et al. 1982), whole On the east coast of Florida, common snook were collected otoliths or scales were used to estimate ages. To evaluate with hook-and-line gear during June–August 1987–91 in the validity of using these structures to estimate snook Sebastian, Jupiter, and Lake Worth Inlets. Young-of-the-year age, we examined whole and sectioned otoliths from 199 and juvenile common snook were collected on both coasts fi sh that included at least fi ve individuals from each abun- during April and May in 1989 and 1990 with a 3-m cast dant age class and the oldest individuals. Whole otoliths net (19-mm stretched mesh) from protected backwaters of were read three times by a single reader. Whole otoliths coastal rivers. In December 1989, dead or moribund snook were submerged in glycerin and read over a dark back- that had succumbed to low water temperatures were col- ground with refl ected light. Scale impressions from 48 lected on both coasts. A detailed description of additional col- common snook of different ages and lengths were made lections made with various nets on both coasts may be found on acetate slides and read under compound microscopes in Taylor et al., 1998. Data, otoliths, and gonads collected equipped with transmitted light. Ages from these scales and used in that study were used in the present study. were compared with ages derived from sectioned otoliths In the laboratory, total length (TL), fork length (FL), and of the same fi sh. standard length (SL) were measured to the nearest mm; The von Bertalanffy (1957) growth equation FLt = all measurements reported in our study are fork lengths. L(1 – e(–K(t–t0))) was fi tted to observed age-length data Weights were measured to the nearest gram (g), and gonads with nonlinear regressions. These predicted lengths at age were weighed to the nearest 0.01 g. Gonad samples were were compared with the average observed lengths at age fi xed in 10% buffered formalin for histological processing. that included some seasonal growth that occurred after Sagittal otoliths were removed and stored dry. the formation of the fi nal annulus. Likelihood-ratio tests were used to compare parameter estimates for males and Age and growth females (Kimura, 1980). If coast had a signifi cant effect, the equations were calculated for each coast. Length- A Buehler Isomet low-speed saw was used to cut four sec- weight regressions were calculated by linear regression of tions approximately 0.5 mm thick that were mounted on log10-transformed data. Length-length and length-weight regressions were calculated for common snook of both sexes with pooled data. 1 Muller, R. G., and M.

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