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



MATERIALS AND METHODS 



Two hundred and eighteen blacktip sharks 

 were collected in Tampa Bay and adjacent off- 

 shore areas during May 1985-February 1987. 

 One hundred and forty were utilized for age and 

 growth analyses. Sharks were caught with gill 

 nets, longhnes, and rod and reel. Once captured, 

 total length (TL), fork length (FL), and pre- 

 caudal length (PCL) were measured. Total 

 length of embryos was measured with caudal fin 

 extended horizontally. All lengths reported in 

 this paper refer to total lengths. Sharks were 

 then weighed and sexed. Maturity of males and 

 females was determined using morphological and 

 gonadal characteristics described by Clark and 

 von Schmidt (1965) and Wass (1973). 



A section of the vertebral column was re- 

 moved just anterior to the first dorsal fin, al- 

 though in a few instances only caudal vertebrae 

 were obtainable. The vertebrae were stored 

 frozen, then cleaned of connective tissue by 

 soaking the individual centra in a solution of 

 5.25% sodium hypochlorite. Centra were then 

 rinsed and stored in a solution of 70% ethanol. 

 Two techniques were tested to determine opti- 

 mum enhancement of translucent vertebral 

 rings: the silver nitrate technique (Stevens 1975) 

 was compared with a method described by Par- 

 sons (1983) in which the vertebral centrum face 

 is shaded with a No. 1 pencil. The latter method 

 detected differences in microtopography of the 

 centrum face and enhanced the translucent 

 rings. This method proved effective and was 

 used because it took only a fraction of the time of 

 the silver nitrate method. Vertebral centra were 

 read under a dissecting microscope at 10 x power 

 using transmitted light. Centrum radius was 

 measured from the focus to the dorsal margin 

 using vernier calipers. Radii measurements 

 were used to determine a relationship with shark 

 TL. Ring radii were measured from the focus to 

 each translucent ring, along the angle of the cen- 

 trum (Fig. 1). Vertebrae collected from the 

 caudal region were excluded from radius and 

 marginal increment measurements. However, 

 caudal vertebrae were utilized in age determina- 

 tions since ring counts made on centra from dif- 

 ferent areas of the vertebral column resulted in 

 similar age estimations. Translucent rings en- 

 hanced by the pencil method were counted if 

 they extended continuously around the centrum 

 face. Centra were read independently by the two 

 authors. When discrepancies occurred, the ver- 

 tebrae were reread until an agreement was 



reached. Ten centra were unreadable and were 

 discarded from the analysis. Twenty centra of 

 varying size were sectioned with a low speed 

 saw through the focus, along the dorsal-ventral 

 plane, for comparative ring counts with whole 

 centra. Since blacktip sharks are born during 

 late April through early June (Killam 1987), ages 

 and growth rates were estimated based on an 

 arbitrary 1 May birth date. 



Back-calculated size at age was determined by 

 a direct proportion method (Everhart and 

 Youngs 1981): 



iTL)„ = (TL), X (VR)„/(VR), 



where TL„ = calculated length at ring ?;, TL^ = 

 total length at capture, VRc = the centrum ra- 

 dius at capture, and VRn = centrum radius to 

 ring H. Back-calculations were made on blacktip 

 sharks of all sizes. 



Marginal increment analysis examines the dis- 

 tance from the most recently deposited ring to 

 the centrum margin and was utilized to deter- 

 mine the time of year that rings are deposited. 

 The centrum margin becomes difficult to resolve 

 in older fish and, therefore, only sharks with 

 one, two, or three rings were utilized in this 

 analysis. Marginal increments were measured 

 from the distal most translucent ring to the 

 edge of the centrum (Fig. 1). Measurements 

 were made with vernier calipers along the angle 

 of the centrum face. 



Age and weight relationships were deter- 

 mined by fitting a logistic growth curve to ob- 

 served data, as described by Kappenman (1981). 



Ml 



Figure 1. — Diagram of typical vertebral centrum of 

 Carcharhinus limbatus. Measurements taken include: VR = 

 vertebral radius; MI = marginal increment: a, b, and c = 

 ring radii of translucent rings 1, 2, and 3. Measurements 

 were taken along the dorsal-ventral plane. 



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