282 



Fishery Bulletin 101(2) 



Carlson''^), with estimated commercial landings peaking 

 at about 129.4 t dressed weight in 1999 (Cortes, unpubl. 

 data). However, life-history information for this species is 

 mostly restricted to some aspects of its reproduction and 

 general biology (Branstetter, 1981; Castro, 1993). The pur- 

 pose of the present study is to investigate the life history 

 and population dynamics of the finetooth shark from the 

 northeastern Gulf of Mexico. Specifically, we wish 1) to es- 

 timate age, growth, and natural mortality, 2) to determine 

 size-at-maturity, and 3) to assess productivity and identify 

 the vital rates to which population growth rates are most 

 sensitive. 



Materials and methods 



Collection and laboratory processing 



Finetooth sharks were collected from fishery-indepen- 

 dent surveys in the northeastern Gulf of Mexico, from St. 

 Andrew Bay to Apalachicola Bay, FL, during April-October 

 from 1995 to 1999 (Carlson and Brusher, 1999). A 186-m 

 long gill net consisting of panels of six different mesh sizes 

 was used for sampling. Stretched mesh sizes ranged from 

 8.9 cm to 14.0 cm in steps of 1.27 cm, and an additional 

 mesh size of 20.3 cm was used. Panel depths when fishing 

 were 3.1 m. Webbing for all panels, except for that with 

 20.3-cm mesh size, was clear monofilament, double-knot- 

 ted and double-selvaged. The 20.3-cm webbing was made 

 of no. 28 multifilament nylon, single-knotted, and double- 

 selvaged. When set, the nets were anchored at both ends. 

 Generally, soak time ranged from 1.0 to 2.0 hours. 



Precaudal (PC), fork (FL), total (TL), and stretched total 

 (STL) length (mm), sex, and maturity state were deter- 

 mined for each shark. We developed several morphometric 

 relationships to convert length measurements. Linear re- 

 gression formulae were determined as FL=1.10(PC)-i-0.60; 

 TL=l.23{FL)+20.34: and SrL=1.10(TL)-i-11.25. All equa- 

 tions were highly significant (P<0.0001) and had coeffi- 

 cients of determination (r^) between 0.97 and 0.99. Because 

 most previous studies on small coastal species have sum- 

 marized information in total length, i.e. the straight line 

 from the tip of the snout to the tip of the tail in a natural 

 position, our results are reported in natural TL to provide 

 a direct comparison. 



Maturity was assessed according to the guidelines of 

 Castro (1993). Males were deemed mature if they pos- 

 sessed hardened claspers and the rhipidion opened freely. 

 Females were considered mature if they were gravid, had 

 oocytes larger than 26 mm in diameter, or if nidamental 

 gland width was greater than 20 mm. 



Vertebrae for age determination were collected from the 

 column between the origin and termination of the first dor- 

 sal fin. Vertebral sections were placed on ice after collection 



^ Carlson, J. K. 2000. Progress report on the directed shark 

 gillncl fishcry:riRht whale .season, 2000. Sustainable Fisheries 

 Division contribution no. SFD-99/00-90. 12 p. Southeast Fish- 

 eries Center, National Marine Fisheries .Sei-vice. 3500 Dclwood 

 Beach Rd.. Panama Citv. FL :i2.108. 



and were frozen upon arrival at the laboratory. Thawed 

 vertebrae were cleaned of excess tissue and soaked in a 5% 

 sodium hypochlorite solution for 5-30 min to remove re- 

 maining tissue. After cleaning, the vertebrae were soaked 

 in distilled water for 30 min and stored in 95% isopropanol 

 alcohol. Prior to examination, vertebrae were removed from 

 alcohol, dried, and measured (length and width in mm). 



Visual enhancement of growth bands 



Various methods were tested to enhance visibility of 

 growth bands. Sagittal sections were cut from the vertebral 

 centrum at different thicknesses and stained with 0.01% 

 crystal violet (Johnson, 1979; Schwartz, 1983), alizarin red 

 (Gruber and Stout, 1983), or left unstained. Each section 

 was mounted on a glass microscope slide with clear resin 

 and examined under a dissecting microscope with trans- 

 mitted light. Growth bands were found to be most appar- 

 ent with the crystal violet stain on sagittal sections with a 

 thickness of 0.35 mm (Fig. 1). 



The distances from the centrum origin to the distal edge 

 of each growth band and from the centrum origin to the 

 centrum edge were measured by using the Image Tools, 

 version 2 software package (UTHSCSA Image Tool, 1997). 

 Each growth band included a broad light mark represent- 

 ing summer growth and a thin dark mark representing 

 winter growth (Branstetter and Stiles, 1987). All three 

 authors aged each specimen without knowledge of its 

 length or sex. Two sets of independent age readings were 

 made, the second set after consultation among the authors. 

 The index of average percent error (APE; Beamish and 

 Fournier, 1981) and the percentage of disagreements by 

 ± ; rings among authors were computed for the first set of 

 age readings. 



Determination of growth curves 



Sex-specific relationships between total length (TL) and 

 vertebral radius (VR) were calculated to determine the 

 most appropriate method for back-calculation. Because 

 no difference was found between sexes (ANCOVA: F=0.40, 

 df=l, P>0.05), they were combined to generate a linear 

 relationship: TL= 153.75(VR\ + 305.68 (P<0.0001; r-=0.90; 

 ;;=239). Because the intercept of the relationship did not 

 pass through the origin, we applied a method proposed by 

 Campana (1990), which is a modified Fraser-Lee equation 

 that uses a biologically derived intercept: 



L,^ L +((0^,-0, )x{L -Z„)/(C? -0„)). 



where L^, = length at age a; 



0^_ = otolith distance from focus to annulus a; 



O^ = otolith radius at capture; 



Lj = length at capture; 



L(, = length at birth; and 



0,1 = otolith radius at birth. 



The biologically derived intercept corresponds to the size 

 of the otolith (or analogous aging structure, i.e. vertebra) 



