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SCRS/2001/066 Col.Vol.Sci.Pap. ICCAT, 54 (4): 1280-1293. (2002)





Lisa J. Natanson 1


Preliminary analysis of the vertebrae of these three species indicate that the current processing method is adequate for counting bands. Sample collection is ongoing and it is likely that more will be obtained for the shortfin mako and thresher shark, though not for the white shark.

Complete studies on the age of these species using the entire vertebral samples, tag/recapture data and length frequency analysis for verification are ongoing. Validation of band pair periodicity, which is critical for obtaining accurate ages, will be included in these studies.

Information resulting from these studies will include species and sex specific growth rates, size at age, age at maturity, and longevity. The results of this preliminary analysis indicate that the vertebral centra are appropriate structures to use for aging these species. Because of the exploratory nature of these analyses and inadequate samples sizes these data should not be used to estimate growth function parameters for analytic results.

RÉSUMÉ L=analyse préliminaire des vertèbres de ces trois espèces indique que la méthode actuelle de traitement est adéquate pour le comptage des anneaux. La collecte d=échantillons est en cours, et il est probable que plus d=échantillons soient obtenus pour le requin-taupe bleu et le requin- renard, mais pas pour le grand requin blanc. Des études complètes sur l=âge de ces espèces d=après des échantillons de vertèbre entière, les données de marquage/recapture et l=analyse des fréquences de taille pour la vérification sont en cours. Ces études comprendront la validation de la périodicité des paires d=anneaux, qui est critique pour obtenir l=âge précis.

L=information issue de ces études comprendra le taux de croissance de l=espèce et spécifique du sexe, la taille à l=âge, l=âge de maturité et la longévité. Les résultats de cette analyse préliminaire montrent que les vertèbres constituent une structure appropriée pour la détermination de l=âge de ces espèces. Vu le caractère expérimental de ces analyses et la taille inadéquate des échantillons, ces données ne devraient pas servir à estimer des paramètres de la fonction croissance en vue de résultats analytiques.

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El análisis preliminar de las vértebras de estas tres especies indica que el método actual de procesamiento resulta adecuado para el recuento de bandas. Continúa la recopilación de muestras y es probable que se obtengan más muestras de marrajo dientuso y zorro, aunque no de tiburón blanco. Se están realizando estudios completos sobre la edad de estas especies utilizando muestras de vértebras enteras, datos de marcado y recaptura y análisis de frecuencias de talla para fines de verificación. En este estudio se incluirá la validación de la periodicidad de los pares de bandas, la cual resulta crítica para la correcta determinación de edades. La información proveniente de estos estudios incluirá tasas de crecimiento específicas por sexos y por especies, tallas por clase de edad, edad de maduración y longevidad. Los resultados de estos análisis preliminares indican que los centros de las vértebras son estructuras apropiadas para determinar la edad de estas especies. Dado al carácter experimental de estos análisis y las inadecuadas tallas de las muestras, estos datos no deben 1 NOAA/NMFS, 28 Tarzwell Drive, Narragansett, RI 02882, Lisa.Natanson@noaa.gov utilizarse para estimar los parámetros de la función de crecimiento para obtener resultados analíticos.

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Age determination, Growth curves, Life history, Longevity, Shark fisheries

1. INTRODUCTION The shortfin mako, Isurus oxyrinchus, is distributed throughout the temperate and tropical regions of the world’s oceans. Detailed migratory studies on the shortfin mako in the western North Atlantic show that members of this species prefer a temperature range of 17-22oC (Casey and Kohler 1992).

Makos move north from Cape Hatteras as the inshore waters warm beginning in April and May moving into southern New Jersey waters by early June and the New York waters by mid to late June.

By August they are found in Maine and Nova Scotian waters (Casey and Kohler 1992). Though not targeted commercially in U.S. waters, makos are caught as by-catch and as well as being subjected to an intensive recreational fishery, primarily off the New England states, New York and New Jersey (Casey and Kohler 1992). Age estimates were obtained for the shortfin mako in the western North Atlantic by Pratt and Casey (1983). They could not validate their age interpretations, however, and concluded that band pairs were deposited biannually (two band pairs deposited per year). The biannual theory of band periodicity in lamnoids has been under continued debate. Parker and Stott (1965) first suggested biannual band pair deposition in their study of the basking shark, Cetorhinus maximus. Pratt and Casey (1983) followed this with their study on the shortfin mako (Pratt and Casey

1983) and Branstetter and Musick (1994) suggested biannual band pair deposition for the sand tiger shark, Carcharias taurus, based on marginal increment analysis (MIA). However, studies on mako sharks from other oceans and current lamnoid age research does not support the contention of biannual band pair deposition (Wintner and Cliff 1999, Natanson et al. in press). Cailliet et al. (1983,

1985) assumed annual band pair deposition for Pacific coast shortfin mako and white sharks. Wintner and Cliff (1999) stated that they could not determine band periodicity using MIA in the white shark off the coast of South Africa, though one tetracycline injected (OTC) recapture suggested annual deposition. More recently, Natanson et al. (in press) validated annual band pair deposition in the porbeagle, Lamna nasus, with known age individuals and direct OTC methods up to an age of 11 years. With the exception of Natanson et al. (in press) and Winter and Cliff (1999), direct validation of band periodicity, such as OTC injection or known age tag/recaptures, has not previously been reported in lamnoids. In view of these two recent studies, which contradict the biannual band pair deposition hypothesis, a revision of the age estimates for the shortfin mako using updated techniques and increased sample sizes with an emphasis on obtaining validation is being undertaken.

The white shark, Carcharodon carcharias, occurs in coastal and offshore waters and is most common in cold and warm temperate seas (Compagno 1984). In the western North Atlantic, it is found from Newfoundland to the Gulf of Mexico (Casey and Pratt, 1985). White shark sightings are common off New England during the summer (Casey and Pratt, 1985). Prior to being listed as a prohibited species in 1998 in US waters (NMFS 1999), white sharks were sought after by recreational fishermen. It was not unusual for them to be weighed in at sportfishing tournaments or for large specimens to be landed on Long Island, NY, during August. White sharks have been aged from the eastern Pacific using vertebrae without validation (Cailliet et al. 1985). Wintner and Cliff (1999) aged white sharks in South Africa using vertebrae with marginal increment analysis and one OTC recapture for validation. To date, no age study has been conducted on white sharks from the North Atlantic.

The thresher shark, Alopias vulpinus, is a pelagic species with a nearly circumglobal distribution in warm waters (Compagno 1984). It is found throughout the western North Atlantic Ocean from Newfoundland to Cuba and into the Gulf of Mexico (Compagno 1984). Little is known about the thresher’s migratory patterns, however, data collected at shark fishing tournaments along the northeastern U.S. coast from New Jersey to Massachusetts, show that this species is in the region by mid-June. They move north into the waters off Massachusetts and Maine by July where they remain.

These sharks can be found in this region until October (NMFS unpub. data 2). All size ranges of males and females including pregnant and recently post-partum females are seen in June (NMFS unpub.

Data2). Young of the year threshers are found off the coast of North Carolina in the fall, indicating the possible presence of a nursery area (C. Jensen pers. comm. 3). The thresher shark is not targeted by a commercial fishery but is taken as incidental catch and as a sport fish. Early observations from the Apex Predators Program indicate that thresher sharks may go through long cycles of abundance and decline (Casey et al. 1981). Shark tournament observations prior to 1975 show few, if any, threshers landed. Between the mid- 70s and 80s threshers were sporadically landed. Tournament landings have gradually increased since the mid-1980’s and remained fairly consistent, though at a low level, with notable exceptions; for example 1999, a total of 37 threshers were caught at two tournaments held on the same weekend in June; more than is usually reported from tournaments in an entire season (NMFS unpub. Data2). Cailliet et al. (1983) generated von Bertalanffy growth parameters for thresher sharks in California waters using vertebrae and length-frequency methods. Welden et al. (1987) attempted to validate the vertebral counts using radiometric dating, but the results were inconclusive. The thresher shark has not been aged in the western North Atlantic.

The Apex Predators Program (APP) of the National Marine Fisheries Service (NMFS) is currently conducting age and growth studies on these three species for the western North Atlantic.

Methodology, sample sizes and preliminary findings will be discussed.


2.1 Vertebral Aging Vertebrae from all species were obtained from sharks caught on commercial and research vessels and at sportfishing tournaments. Sampling took place along the western North Atlantic coast between Prince Edward Island, Canada and the east coast of Florida, U.S.. Multiple vertebrae were removed from the area just above the shark’s branchial chamber whenever possible, with the exception of commercially valuable specimens, where samples were obtained closer to the head. On rare occasions, caudal vertebrae were collected and in some cases the entire vertebral column was removed. Vertebrae were then dried, stored frozen, in 70% ETOH or 10% Formalin until processed.

With rare exceptions, only samples that had measured fork length (FL - tip of the snout to the fork in the tail, over the body) or 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, over the body; Kohler et al. 1995) were used. All lengths reported in this document are over-the-body FL unless otherwise noted. All conversions used in the study are from Kohler et al.

(1995) with the exception of the TL to FL relationship for the thresher shark calculated in this study:

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In shortfin mako and white sharks, weight was occasionally the only actual measured parameter;

in which case, it was converted to FL using the regressions in Kohler et al. (1995).

One vertebra from each sample was removed for processing. The centrum was sectioned using a Ray Tech Gem Saw4 with two diamond blades separated by a 0.6 mm spacer. Each centrum was cut through the middle along the sagittal plane and the resulting “bow-tie” sections were stored individually in capsules in 70% ETOH. Each section was digitally photographed with a MTI CCD 72 video camera attached to a SZX9 Olympus4 stereo microscope using reflected light. All samples were 2 NMFS Apex Predators Program, 28 Tarzwell Drive, Narragansett, RI 02882 3 C. Jensen, North Carolina Department of Marine Fisheries, Beaufort, NC 4 Reference to trade names does not imply endorsement by the National Marine Fisheries Service, NOAA.

photographed at a magnification of 4X. Band pairs (consisting of one opaque and one translucent band) were counted and measured from the images using Image Pro 44 software. Measurements were made from the midpoint of the isthmus of the full bow-tie to the opaque growth bands at points along the internal edge of the corpus calcareum (Fig. 1a). The radius of each centrum (VR) was measured from the midpoint of the isthmus to the distal margin of the intermedialia along the same diagonal as the band measurements.

The birth band, as indicated by a change in the angle of the centra, is often the most pronounced first band (Fig.1a-c). To confirm the position of the birth band, the size at birth was calculated by multiplying the birth band radius (BR) by the ratio of the vertebral radius (VR) to the FL at capture.

The calculated birth size was then compared to known sizes at birth from the literature. Average BR was also compared to VR measurements of young of the year (YOY) sharks. Since this study is preliminary and no validation has been accomplished, subsequent bands were counted using the criteria for the porbeagle, which is a closely related species and has validated annual bands (Natanson et al. in press).

Bands were counted two times by one reader (shortfin mako and white shark) and once each by two readers (thresher shark). Each specimen was read without prior knowledge of either the length or previous count. Initial band counts were compared and specimens were reread if counts did not coincide.

The relationship between VR and FL was calculated in order to confirm the interpretation of the birth band and to determine the relationship between these parameters. Preliminary results indicate that this relationship is linear for the shortfin mako and white sharks (Figs 2,3) a slight curvature is evident at the upper end of the thresher shark relationship, based on two large individuals (Fig.4)

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The VBGF was calculated by using the non-linear regression function in Statgraphics3.

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