FISHERY BULLETIN: VOL. 87. NO. 4. 1989 



DISCUSSION 



Centrum analysis 



Perhaps the most common Hmiting factor in 

 many studies concerning shark species is the 

 acquisition of sufficient specimens over the en- 

 tire size range of the species. The relatively 

 large sample size reported here made annulus 

 verification and subsequent growth estimation 

 possible. Marginal increment, length-frequency, 

 and length-month analysis suggest that ring 

 periodicity in juvenile C. linibatus is annual. 

 Annual ring deposition in sharks has been veri- 

 fied or validated for several species, including 

 Prionace glauca (Stevens 1975), Rhizopriono- 

 don terraenovae (Parsons 1983), Negaprion 

 brevirostris (Gruber and Stout 1983), C. 

 amblyrhynchos (Radtke and Cailliet 1984), 

 Triakis semifasciata (Smith 1984), C. plumbeus 

 (Casey et al. 1985), C. leucas (Branstetter and 

 Stiles 1987), C. falcifonnis and Sphynia leivini 

 (Branstetter 1987b), Galeocerdo cuvieri Brans- 

 tetter et al. 1987), and S. tiburo (Parsons 1987). 

 Cailhet et al. (1986) provided an extensive re- 

 view of elasmobranch species for which age and 

 growth rates have been estimated. In some 1am- 

 noid species such as /. oxyrinchus (Pratt and 

 Casey 1983) and Cetorhinus tna.rinuts (Parker 

 and Stott 1965), deposition of two rings per year 

 has been suggested. Therefore, it appears that 

 for each species being examined, periodicity of 

 ring deposition must be verified or vaUdated 

 before proper age and growth estimates can be 

 attained. 



Early Growth 



Because Tampa Bay is a nursery area for 

 Carcharhinus limbatKS (Killam 1987) the cap- 

 ture of numerous juvenile specimens was fairly 

 easy. Rapid early growth rates of these young 

 sharks made determination of periodicity of ring 

 deposition possible. Marginal increment analysis 

 suggested that one translucent ring is deposited 

 during the winter months of December-January, 

 and that opaque tissue is deposited distally to 

 these rings, during periods of rapid growth in 

 warmer months. A similar pattern of ring de- 

 position has been identified in C amblyrhynchos 

 (Radtke and Cailliet 1984; Cailliet et al. 1986). 

 The opaque areas have been found to contain 

 higher amounts of calcium and phosphorus than 

 the adjacent translucent rings. 



The rapid early growth of juvenile C. limbatus 



produced distinct separations in length-fre- 

 quency modes for sharks <120 cm TL. Modes 

 are more difficult to resolve in larger C. lim- 

 batus because fish of similar sizes may represent 

 a variety of age classes owing to differences in 

 individual growth rates and the decrease in 

 gi'owth rate as age increases. Ketchen (1975) 

 utilized length-frequency analysis to estimate 

 early age classes oiSqualus acanthias, 44-70 cm 

 TL. It appears that only early age classes of 

 sharks undergoing rapid growth can be esti- 

 mated by analyzing length-frequency distribu- 

 tions. Casey et al. (1985) found that in 3-8 year 

 old C. plumbeus, several age classes may be 

 represented at any one length. 



A length-month distribution subjectively 

 assigned C. limbatus to age classes and pro- 

 vided estimates of growth rates. Modes identi- 

 fied by this method were subjectively assigned 

 to age classes. Three age classes were apparent 

 for blacktips 62-118 cm. As with the length- 

 frequency distributions, the length-month 

 distribution becomes increasingly difficult to 

 resolve in older sharks. Pratt and Casey (1983) 

 utilized this method to estimate three age 

 classes of juvenile /. oxyrinchus, 54-175 cm 

 TL. Parsons (1985) used this procedure to esti- 

 mate age and gi'owth through maturity for the 

 rapidly growing R. terraenovae whose males 

 mature as early as 2.0-2.4 years and females 

 mature at 2.4-2.8 years. 



Early growth rates have been examined in 

 only a few species of sharks. Juvenile C. leucas 

 gi'ew at 18 and 16 cm/yr during the first 2 years, 

 respectively, decreasing to 11 cm/yr in larger 

 sharks (Thorson and Lacy 1982). Rhizopriono- 

 don ten'aenovae growth rates for age classes 

 and I were 30 and 10 cm, respectively, which 

 corresponded to a 100% increase in length for 

 age individuals and a 15% increase at age I 

 (Parsons 1983). Young A^. brevirostris growth 

 rates did not exceed 25 cm/yr and probably aver- 

 aged 10-20 cm/yr (Gruber and Stout 1983). 

 Galeocerdo cuvieri appeared to gi'ow > 20 cm/yr 

 until near maturity (Branstetter et al. 1987). 

 Carcharhinus li)nbafus had growth rates of 21.0 

 and 19.2 cm/yr for age classes and I. A very 

 similar growth rate was determined for juvenile 

 C. li)nbatus in the northern Gulf of Mexico 

 (Branstetter 1987a). 



It appears that early growth in more pelagic 

 species may differ. Young /. oxyrinchus showed 

 rapid first year gi-owth rates of 49.0 cm/yr and 

 second year rates of 32.0 cm/yr (Pratt and Casey 

 1983). They found that gi-owth of /. oxyrinchus 



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