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.

Reproductive periodicity, localised movements and behavioural segregation of pregnant Carcharias taurus at Wolf Rock, southeast Queensland, Australia

“…The present study and studies on Carcharias taurus populations in the SW Atlantic (Lucifora et al 2002), South Africa (Dicken et al 2006a(Dicken et al , 2007, and the NW Atlantic (Branstetter & Musick 1994) indicate that very recent post-partum females may behave differently to female sharks that are about to mate. These postpartum females undertake a resting period in cooler waters and do not migrate to warmer waters with the other females to mate, they migrate for the mating season in the following year.…”

“…Although a migration of pregnant sharks to warmer waters for pregnancy has been suggested for the South African and SW and NW Atlantic Carcharias taurus populations (Bass et al 1975, Branstetter & Musick 1994, Lucifora et al 2002, Dicken et al 2006a, 2007, this has not been suggested in relation to the Australian east coast population. Previous analyses of the movement of sexually mature female sharks suggest that they move southwards in spring and early summer and the return movement to northern sites occurs in the autumn and winter months (Otway & Parker 2000, Otway et al 2003, Otway & Burke 2004.…”

“…While a biennial reproductive cycle (1 rest-year between pregnancy events) has been reported for C. taurus in the southwest Atlantic (Lucifora et al 2002) and off the coast of southern Africa (Dicken et al 2006a(Dicken et al , 2007, both annual (i.e. pregnant each year) and biennial reproductive cycles have been suggested for C. taurus populations in the northwest Atlantic (Gilmore 1993, Branstetter & Musick 1994 and off the southeast coast of Australia (Gordon 1993, Compagno 2001, Otway et al 2003.…”

“…In the NW Atlantic, SW Atlantic and South Africa pregnant Carcharias taurus are thought to aggregate in warmer waters (Bass et al 1975, Branstetter & Musick 1994, Lucifora et al 2002 which may enhance embryo development (Bass et al 1975). Near-term pregnant female sharks in South African waters migrate to cooler waters in July and August prior to parturition which may occur in August or September (Bass et al 1975) or between November and February (Dicken et al 2006a(Dicken et al , 2007.…”

“…Mature female sharks may remain in warmer southern waters (Gilmore et al 1983, Gilmore 1993) and reproduce annually. Alternatively, they may migrate north to cooler waters after parturition to rest for a year (biennial reproductive cycle) (Branstetter & Musick 1994) or they may give birth anywhere in their range and reproduce biennially (Compagno 2001).…”