Age and Growth Estimates for the Sand Tiger in the Northwestern Atlantic Ocean

Abstract

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.