Taylor et al : Age, growth, maturation, and sex reversal In Centropomus undeamahs 



613 



common snook stocks (MuUer and Murphy'). In 1998, the 

 Florida Marine Fisheries Commission enacted the current 

 restrictive regulations to reduce the harvest of snook from 

 Florida waters to maintain a 409; SPR. 



Our study examined age. growth, maturation, and sex 

 reversal of common snook which previously had not been 

 diagnosed as being protandrous hermaphrodites. Earlier 

 studies derived ages from either scales or whole otoliths, 

 and ages were poorly validated (Volpe. 1959; Thue et al.'-). 

 Since these reports, studies on a variety of species have 

 shown that scales are not reliable for aging long-lived 

 fishes and that scale-derived and whole-otolith-derived 

 age estimates are often lower than the validated estimates 

 derived from sectioned otoliths (Beamish and McFarlane, 

 1983; Casselman, 1983; Lowerre-Barbieri et al., 1994; 

 Crabtree et al., 1996). A validated method for aging snook 

 is necessary to assess previous estimates of growth rates. 

 mortality, and longevity. Our objectives were to describe 

 validated aging techniques, length and age composition, 

 and length and age at maturity of common snook popula- 

 tions from the east and west coasts of Florida. We also diag- 

 nosed common snook as protandrous hermaphrodites. 



Methods 



On the east coast of Florida, common snook were collected 

 with hook-and-line gear during June- August 1987-91 in 

 Sebastian, Jupiter, and Lake Worth Inlets. Young-of-the-year 

 and juvenile common snook were collected on both coasts 

 during April and May in 1989 and 1990 with a 3-m cast 

 net (19-mm stretched mesh) from protected backwaters of 

 coastal rivers. In December 1989, dead or moribund snook 

 that had succumbed to low water temperatures were col- 

 lected on both coasts. A detailed description of additional col- 

 lections made with various nets on both coasts may be found 

 in Taylor et al., 1998. Data, otoliths, and gonads collected 

 and used in that study were used in the present study. 



In the laboratory, total length (TL). fork length (FL). and 

 standard length (SL) were measured to the nearest mm; 

 all measurements reported in our study are fork lengths. 

 Weights were measured to the nearest gram (g). and gonads 

 were weighed to the nearest 0.01 g. Gonad samples were 

 fixed in lO'/r buffered formalin for histological processing. 

 Sagittal otoliths were removed and stored dry. 



Age and growth 



A Buehler Isomet low-speed saw was used to cut four sec- 

 tions approximately 0.5 mm thick that were mounted on 



' Mullen R. G.. and M. D. Murphy. 1998. A stock assessment of 

 common snook. Centropomus undecimaUs. Rep. to the Florida 

 Marine Fisheries Commission, Florida Dep. of" Environ. Protec- 

 tion. Florida Marine Research Institute, 100 Eighth Ave. SE, St. 

 Petersburg, Florida 33701, 53 p. 



- Thue, E. B.. E. S. Rutherford, and D. E. Buker. 1982. Age, 

 growth and mortality of the common snook, Centropomus undec- 

 imalis (Blochi, in the Everglades National Park, Florida. U. S. 

 National Park Service South Florida Research Center Report 

 T-683, 32 p. 



a microscope slide (Chilton and Beamish. 1982). Annuli 

 were counted on the section through the core by using 

 compound microscopes and transmitted light. Two inde- 

 pendent readers counted annuli on each otolith without 

 knowledge offish size or capture date. If the two readings 

 disagreed, both readers read the otolith again, for a total 

 of four readings. If three of the four readings agreed, then 

 this reading was accepted as the annulus count; otherwise 

 the otolith was excluded from further analysis. 



An annual growth zone in a common snook otolith com- 

 prises a narrow, concentric opaque band, formed each 

 winter as growth slows, and a wider, translucent band, 

 formed each summer as growth increases. Opaque winter 

 bands were enumerated as assumed annuli because they 

 were the salient feature in each section. Ages were assigned 

 to each fish on the basis of a 1 June birth date because 

 most snook have completed annulus formation by 1 June 

 and because June corresponds to the approximate begin- 

 ning of the snook spawning season (Taylor et al., 1998). 

 Annulus counts were adjusted on the basis of an assumed 

 birth date of 1 June. All otoliths that had an annulus on 

 the edge from fish 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 

 were assigned ages equal to the annulus count. 



In earlier studies (Volpe, 1959; Thue et al. 1982), whole 

 otoliths or scales were used to estimate ages. To evaluate 

 the validity of using these structures to estimate snook 

 age, we examined whole and sectioned otoliths from 199 

 fish that included at least five individuals from each abun- 

 dant age class and the oldest individuals. Whole otoliths 

 were read three times by a single reader Whole otoliths 

 were submerged in glycerin and read over a dark back- 

 ground with reflected light. Scale impressions from 48 

 common snook of different ages and lengths were made 

 on acetate slides and read under compound microscopes 

 equipped with transmitted light. Ages from these scales 

 were compared with ages derived from sectioned otoliths 

 of the same fish. 



The von Bertalanffy (1957) growth equation FL, = 

 HI - e'-*^"-'o'i) was fitted to observed age-length data 

 with nonlinear regressions. These predicted lengths at age 

 were compared with the average observed lengths at age 

 that included some seasonal growth that occurred after 

 the formation of the final annulus. Likelihood-ratio tests 

 were used to compare parameter estimates for males and 

 females (Kimura, 1980). If coast had a significant effect, 

 the equations were calculated for each coast. Length- 

 weight regressions were calculated by linear regression of 

 logjQ-transformed data. Length-length and length-weight 

 regressions were calculated for common snook of both 

 sexes with pooled data. 



Age validation 



We captured 754 common snook 327-961 mm and injected 

 them with oxytetracycline (OTC) at a dosage of 25-mg 

 OTC/kg offish body weight. Fish were then double-tagged 

 with dart and internal-anchor tags and released at the 

 original capture site. We relied on anglers and our own fish- 



