SynopsisThe silky shark, Carcharhinus falciformis, and scalloped hammerhead, Sphyrrza lewini, represent >80% of the shark by-catch of the winter swordfish/tuna longline fishery of the northwestern Gulf of Mexico. This catch represents a potential supplemental fishery. yet little is known of the life histories of the two species. This report relates reproductive biology data to age and growth estimates for 135 C. ,falciformis and 78 S. lewini. Unlike other regional populations. C. fafciformis in the Gulf of Mexico may have a seasonal 12 month gestation period. Males mature at 210-220 cm TL (6-7 yr); females at >225 cm TL (7-9 yr). Application of age at length data for combined sexes produced von Bertalanffy growth model parameter estimates of L, = 291 cm TL. K = 0.153, t,, = -2.2 yr. Adult male S. lewini outnumbered adult females in catches because of differences in the distributions of the sexually segregated population. Males mature at 18Ocm TL (10 yr); females at 2.50 cm TL (1.5 yr). von Bertalanffy parameter estimates for combined sexes of this species were L, = 329cm TL, K = 0.073, t,, = -2.2yr.

SynopsisLength at age and growth rates for 59 bull sharks, Carcharhinus leucas, collected from the northern Gulf of Mexico were estimated from the band patterns formed seasonally in the vertebral centra. The combined age at length data for both sexes were applied to a von Bertalanffy growth model producing paramfter estimates OfL, = 285 cm TL, K = .076, t, = -3.0 yr. Lengths at age for males and females were similar except that males did not attain as great a length as females. Growth was apparently slow and varied among individuals, but in general, was estimated to be 15-20 cm yr-l for the first five years, 10 cm yr-' for years 6-10,5-7 cm yr-I for years 11-16, and less than 4-5 cm yr-l thereafter. Males mature at 210-220 cm TL or 14-1.5 yr of age; females mature at >225 cm TL or 18+ yr of age. The largest male (245 cm TL) was 21.3 yr old; the largest female (268 cm TL) was 24.2 yr old.

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Lengths at age and growth rates for 55 sand tigers Odontaspis taurus from the northwest Atlantic Ocean were estimated from banding patterns in vertebral centra and compared with growth data on 15 specimens raised in public aquaria. This shark appears to form two pairs of growth bands annually in the vertebral centra; each band pair is composed of a wide opaque zone and a narrow translucent zone. One band pair represents summer growth (March–September), the other winter growth (September–March). Male and female sand tigers grow at similar rates, but females attain a much larger size. Growth rates are similar to those of other lamnoid sharks; growth for ages 0–1 is 25–30 cm/year, declining approximately 5 cm every 2 years to a minimum of 5–10 cm/year. Males reach maturity at 190–195 cm total length (TL), or 4–5 years; females mature at more than 220 cm TL, or 6 years. The largest (oldest) male examined (248 cm TL) was 7.5 years old; the largest (oldest) female (272 cm TL) was 10.5 years old. A von Bertalanffy growth equation that best fit the data points for both sexes combined had the following parameter estimates: Maximum (asymptotic) length, Lmax = 303 cm TL; growth coefficient, K = 0.18; and initial time, t0 = –2.09. Analysis of back‐calculated lengths at age produced the smallest standard errors around the parameter estimates for each sex: for males, Lmax = 301 cm TL, K = 0.17, and t0 = –2.25 years; for females, Lmax = 323 cm TL, K = 0.14, and t0 = –2.56 years. Contrary to previous studies, which indicated that sand tiger females taken south of Cape Hatteras are pregnant each year, all females collected in the Virginia region were postpartum and in a resting stage. We propose that sand tigers are only pregnant in alternate years, that the reproductive cycle is at least 2 years long, and that only two young are produced in each litter. This low fecundity, in combination with other K‐selected life history characteristics, makes this species extremely susceptible to overfishing. A conservative management strategy will be required to sustain population sizes of this species.

Age and growth estimates for sand tiger sharks, Carcharias taurus, in the western North Atlantic were derived from 96 vertebral centra collected from sharks ranging from 94 to 277 cm total length (TL), and compared to previously published age and growth data. The oldest female and male sand tiger sharks aged in this study were 17 and 15 years of age, respectively. von Bertalanffy growth parameters derived from vertebral length-at-age data are L ¥ = 295.8 cm TL, k = 0.11 year -1 , and t 0 = -4.2 years for females, and L ¥ = 249.5 cm TL, k = 0.16 year -1 , and t 0 = -3.4 years for males. Sexual maturity is estimated to be 9-10 years for females and 6-7 years for males. Weight-to-length relationships determined for female and male sand tiger sharks in the western North Atlantic are; W = 1.3 · 10 -4 · L 2.4 (r 2 = 0.84, n = 55) and W = 9.0 · 10 -5 · L 2.5 (r 2 = 0.84, n = 47), respectively, and 7.9 · 10 -5 · L 2.5 (r 2 = 0.84) for the sexes combined. Our results show sand tigers possess a slower rate of growth than previously thought. This information is crucial for accurately assessing this population's ability to recover, and further justifies the need for this species to be fully protected.