614 



Fishery Bulletin 98(3) 



ery-independent sampling techniques to recapture tagged 

 fish. Otoliths from recaptured OTC-injected common snook 

 were processed, read, and measured according to Chilton 

 and Beamish (1982) and Beamish and McFarlane (1983). 

 The number of annuli formed after the OTC mark was 

 then compared with the number of days since injection. 



Measurements from the core to each assumed annulus 

 and to the margin of the otolith were made with a digital 

 image-processing system on an axis extending along the 

 sulcal ridge to the proximal margin of each section. We 

 expressed the distance from the final annulus to the edge 

 of the otolith (the marginal increment) as a percentage of 

 the distance between the last two annuli formed on the 

 otolith, or for fish with only a single annulus, as a percent- 

 age of the distance between the otolith core and the first 

 annulus. The monthly percentage of otoliths with an annu- 

 lus on their margin was plotted by capture month to show 

 when the annuli were formed and to reveal their repeti- 

 tive seasonal trend in formation. The annularity of the 

 assumed age mark was demonstrated for each age class by 

 plotting monthly mean measurements for all ages pooled 

 into three inclusive groups. 



Reproduction 



Sections of each gonad were prepared for histological anal- 

 ysis and scored according to the level of reproductive 

 activity, or class. Gonad samples were processed with a 

 modification of the periodic acid Schiffs (PAS) stain for 

 glycol-methacrylate sections, with Weigert's iron-hemato- 

 xylin as a nuclear stain and metanil yellow as a counter- 

 stain (Quintero-Hunter et al., 1991). Seasonal spawning 

 patterns and spawning frequency of the common snook 

 that we examined were reported by Taylor et al. (1998). 

 We considered common snook in Taylor et al.'s (1998) 

 and Grier and Taylor's (1998) classes 2-5 to be sexually 

 mature. For a more detailed description of common snook 

 testicular maturation see Grier and Taylor (1998). These 

 classes included snook that had testes with evidence of 

 active spermatogenesis and regressed testes with evidence 

 of previous gonadal development. 



We diagnosed hermaphroditism in common snook by 

 observing histological sections of transitional-sex-stage 

 individuals and sex-specific age- and length-frequency dis- 

 tributions, according to the criteria of Sadovy and Shap- 

 iro (1987). We also conducted an experiment with captive 

 snook to document sex reversal in individual fish. We 

 raised common snook from eggs at the Florida Fish and 

 Wildlife Commission's Stock Enhancement Research Facil- 

 ity. On 6 July 1995, 137 age-4 snook ranging from 445 

 to 608 mm (mean=534 mm) were identified as males on 

 the basis of presence of flowing milt at the vent and were 

 tagged with passive-integrated transponders (PIT tags). 

 These fish were held for 13 months in a one-quarter acre 

 outdoor pond that had constant exchange of ambient sea- 

 water. Water temperature and salinity ranged from 17 to 

 37°C and from 15 to 34'^f, respectively. During December 

 1995 and January 1996, additional well water (17°C and 

 3%f ) was added to maintain 17°C water temperature and 

 to reduce the salinity to IB'lc. Rations consisted of squid. 



sardines, and dried trout pellets. Fish were fed at the rate 

 of 2% body weight per day. 



These fish were examined and sexed on two occasions: 

 after 2 months and after 13 months. Fish that could not be 

 positively identified as males from the presence of flowing 

 milt at the vent were sacrificed and examined by histologi- 

 cal analysis. 



A logistic function was fitted to the percentage of females 

 in our samples by length and age to estimate the length 

 and age at which SC/r of the males in the population trans- 

 formed into females. Regressions were performed with 

 coast as a categorical effect. If coast was a significant 

 effect, the equations were calculated for each coast. The 

 inflection point of the logistic curves was used as an esti- 

 mate of the length or age at which 507c of the population 

 had undergone transition from male to female. 



Results 



The 3872 common snook that were examined ranged in 

 length from 124 to 1105 mm (Fig. 1). Of the fish that we 

 sexed, females ranged in length from 397 to 1105 mm 

 (7i=1448) and males ranged from 124 to 925 mm (n=2276; 

 Fig. 2). East coast females were usually larger than west 

 coast females (Fig. 2). East coast females ranged in length 

 from 448 to 1 105 mm (7!=683 ), and west coast females ranged 

 from 397 to 1032 mm in=765; Fig. 2). East coast males 

 ranged in length from 124 to 908 mm (7!=1258) and west 

 coast males ranged from 129 to 925 mm (/! = 1018; Fig. 2). The 

 sex ratio (male to female) of the sample from the east coast 

 (EC) was 1.8:1 and from the west coast (WO was 1.3:1, sig- 

 nificantly skewed towards males on both coasts (EC: /^=85, 

 df=l, P<0.01; WC: ^-^=36, df=l, P<0.01i. The length-length 

 and length-weight regressions for common snook on each 

 coast were significantly different (P<0.05); the relationship 

 between SL, FL, and TL are presented in Table 1. 



Age and growth 



Common snook annuli are formed once each year, usually 

 during late winter or spring. We recaptured 80 common 

 snook that had been previously injected with OTC. At recap- 

 ture, these fish ranged in length from 360 to 960 mm and 

 ranged in age from 2 to 14 years. OTC-injected individuals 

 were at large fi-om 4 to 2505 days. All of the recaptured 

 common snook that were at large long enough to have formed 

 an annulus («=51) showed the expected pattern of annulus 

 formation: one per year The fish at large for the longest 

 period was 2 years old when it was injected and was recap- 

 tured 2505 days later (Fig. 3). This fish had formed seven 

 annuli after the OTC mark and had been at large for 6 years 

 and 11 months; thus the rate of annulus formation was con- 

 sistent with our predicted rate of one annulus per year The 

 oldest fish recaptured was 8 years old when injected and 

 was recaptured 5 years and 10 months later at age 14. This 

 fish had formed six annuli after the OTC mark. 



Monthly patterns in marginal increments were also con- 

 sistent with the formation of a single annulus each year 

 We plotted the monthly mean percentage offish whose oto- 



