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Fishery Bulletin 101(3) 



Ecologically, the blue shark is an apex predator of im- 

 portant teleosts and cephalopods (Stevens, 1973; Tricas, 

 1978; Kohler, 1987). Historical fisheries have shown that 

 sharks are intrinsically sensitive to sustained exploitation 

 (see review by Castro et al., 1999). Slow growth, late ages at 

 maturity, and low fecundities reflect the life history strate- 

 gies of /^-selected species; stock size is closely linked to re- 

 cruitment (Hoenig and Gruber, 1990). Although the current 

 Fishery Management Plan for Atlantic Tunas, Swordfish, 

 and Sharks has established limits on the U.S. commercial 

 and recreational fisheries that impact blue sharks (NMFS, 

 1999), no international management is currently in place. 

 Given a single North Atlantic stock for this species, any 

 fisheries exploitation, regardless of its coastal origin, may 

 impact the population. Accurate age determinations are 

 necessary for both the assessment and management of the 

 blue shark because they form the basis for calculations of 

 growth and mortality rates, age at maturity, age at recruit- 

 ment, and estimates of longevity. 



Age and growth of the blue shark have been described by 

 a number of studies to varying degrees. In the North Pa- 

 cific, Cailliet et al. ( 1983 ) and Tanaka et al. ( 1990) used ver- 

 tebral growth rings and Nakano ( 1994) used both vertebrae 

 and length-frequency modes to establish growth curves for 

 the blue shark. In the North Atlantic, Aasen (1966) aged 

 the species by assigning ages to length-frequency modes. 

 Later, Stevens (1975), Silva et al. (1996), and Henderson et 

 al. (2001) established growth curves from vertebral growth 

 rings of juvenile blue sharks sampled in the eastern North 

 Atlantic. Low sample sizes, inadequate size ranges, and the 

 lack of age validation limit the utility of these studies for 

 the North Atlantic blue shark population. Skomal (1990) 

 generated growth curves for the blue shark from vertebral 

 growth rings, tag-recaptures, and length-frequency data. 

 In that study, vertebrae from oxytetracycline (OTC) in- 

 jected recaptured blue sharks were used to validate age 

 estimates. The purpose of the current study is to augment 

 the work of Skomal (1990) with additional tag-recapture 

 data, with corroborative vertebral readings of a different 

 vertebral processing technique, and with more rigorous 

 growth analyses. 



Materials and methods 



Interpretation of vertebrae 



Vertebrae were obtained from blue sharks caught on 

 research cruises, commercial, and recreational fishing ves- 

 sels, and at sport fishing tournaments between 1966 and 

 2001. Primary sampling took place between Cape Hatteras, 

 NC, and the Gulf of Maine (NE coast of the United States). 

 To adequately represent the entire size range of the species, 

 small sharks were obtained from the eastern Atlantic from 

 cooperative fishermen and research scientists. When pos- 

 sible, the 15'^ through 20''' vertebrae were excised for the 

 study. When such precision was not possible, this section 

 of backbone was approximated by cutting at the branchial 

 region adjacent to the fifth gill arch. Excess muscle and 

 connective tissue were removed from the vertebrae with a 



knife. Vertebrae were stored either frozen or preserved in 

 10% buffered formalin or 70% ethanol. 



Only samples that had measured fork length (FL — tip 

 of the snout to the fork in the tail, over the body curva- 

 ture), total length (TL — tip of the snout to a point on the 

 horizontal axis intersecting a perpendicular line extending 

 downward from the tip of the upper caudal lobe to form a 

 right angle), or precaudal length (PCL — tip of the snout 

 to the precaudal pit, over the body curvature) were used 

 (Kohler et al., 1995). All lengths reported are in FL unless 

 otherwise noted. TL can be converted to FL by using the 

 regression (Kohler et al., 1995): 



FL = 0.8313 (TL) + 1.39 



[«=572 r-=0.991. 



PCL can be converted to FL using the regression (NMFS'^) 

 PCL = 0.9075 (FL) - 0.3956 [n = 106 r2=0.99]. 



One vertebra from each sample was removed for pro- 

 cessing. The centrum was sectioned by using a Ray Tech 

 Gem Saw with two diamond blades separated by a 0.6-mm 

 spacer Each centrum was cut through the middle along the 

 sagittal plane; the resulting bow-tie sections were stored 

 in individual capsules in 70% ethanol. Each section was 

 digitally photographed with a MTI CCD 72 video camera 

 attached to a SZX9 Olympus stereo microscope by using 

 reflected light. All samples were photographed at a mag- 

 nification of 4x. Band pairs (consisting of one opaque and 

 one translucent band) were counted and measured from 

 the images by using Image Pro 4 software (Media Cyber- 

 netics, Silver Spring, MD). Measurements were made from 

 the midpoint of the notochordal remnant of the full bow-tie 

 to the opaque growth bands at points along the internal 

 corpus calcareum. The radius of each vertebral centrum 

 ( VR) was measured from the midpoint of the notochordal 

 remnant to the distal margin of the intermedialia along 

 the same diagonal as the band measurements. Specimens 

 previously processed histologically (Skomal, 1990) were 

 used for counts when whole samples for those specimens 

 were not available for reprocessing. Because of the differ- 

 ent processing method, histological sections were not used 

 for measurements. 



The criterion for what constitutes a band pair (annulus) 

 was based on the contouring of the corpus calcareum in 

 relation to the strength of the band. A clear indentation 

 of the corpus calcareum at the position of an opaque band 

 constituted the consummation of a growth layer within the 

 vertebra and was considered the annulus (Fig. 1). Each lay- 

 er was considered a temporal growth zone. The first opaque 

 band distal to the focus was defined as the birth mark (BR) 

 and a slight angle change in the corpus calcareum coincided 

 with this mark. In addition, identification of the birth band 

 was confirmed with back-calculation and by comparison of 

 the radius of this band with the radius of vertebrae from 

 young of the year (YOY) and full-term embryos. 



•^ NMFS (National Marine Fisheries Service). 2001. Unpubl. 

 data. Apex Predators Program, 28 Tarzwell Dr, Narragansett, 

 RI 02882. 



