Bomb dating and age determination of large pelagic sharks

Abstract

Abstract:Despite their notoriety and role as apex predators, the longevity of large pelagic sharks such as the porbeagle (Lamna nasus) and shortfin mako (Isurus oxyrinchus) is unknown. Vertebral growth bands provide an accurate indicator of age in young porbeagle, but age validation has never been reported for any large shark species past the age of sexual maturity. Here, we report the first application of bomb radiocarbon as an age validation method for longlived sharks based on date-specific incorporation of radiocarbon into vertebral growth bands. Our results indicate that porbeagle vertebrae recorded and preserved a bomb radiocarbon pulse in growth bands formed during the 1960s. Through comparison of radiocarbon assays in young, known-age porbeagle collected in the 1960s with the corresponding growth bands in old porbeagle collected later, we confirm the validity of porbeagle vertebral growth band counts as accurate annual age indicators to an age of at least 26 years. The radiocarbon signatures of porbeagle vertebral growth bands appear to be temporally and metabolically stable and derived mainly from the radiocarbon content of their prey. Preliminary radiocarbon assays of shortfin mako vertebrae suggest that current methods for determining shortfin mako age are incorrect.

Validated age and growth estimates for Carcharhinus obscurus in the northwestern Atlantic Ocean, with pre- and post management growth comparisons

“…The δ 13 C values (mean ± SE=−12.68‰±0.101‰) of the samples were typical of metabolically derived carbon (Kalish and Johnston 2001;Campana et al 2002). Radiocarbon values ranged from −84.2‰ to 99.9‰ Δ 14 C, which correspond to the years 1949.5 and 1984, respectively.…”

“…The porbeagle and Atlantic sharpnose are both faster growing species with shorter life spans (Campana et al 2002;Natanson et al 2002;Carlson and Baremore 2003) and are likely able to more rapidly compensate for density related changes due to shorter generation times (Cortés 2002). The relatively broad periods defined as pre- (1963-1989) and post-depletion (1990-2010) and pre-management for the purposes of this study spanned the intensification, peak, and decline of the U.S. Atlantic coastal shark fishery and may have masked changes that occurred within those years; unfortunately, sample size constraints prevented us from splitting the data into smaller intervals.…”

“…Samples used in Natanson et al (1995) were not reprocessed, and the original age estimates were used; however, the BR and VR were remeasured as described above to provide consistency with the current digital analysis system. The identity of the birth band was confirmed by comparing BR values with the VR from young of the year (YOY) and full term embryos (Natanson et al 2002).…”

“…The line did not pass through the origin so we used the Fraser-Lee equation modified for log-transformed data (Natanson et al 2002) to back-calculate size-atbirth as:…”

Compensatory Growth of the Sandbar Shark in the Western North Atlantic Including the Gulf of Mexico

“…Other studies using the same bomb-radiocarbon dating methods have validated vertebral centra ages for other long-lived sharks up to 42 years of age (Campana et al 2002;Passerotti et al 2010). However, Francis et al (2007) reported that the ages of Porbeagles from New Zealand that were more than 45 years were underestimated using vertebral centra.…”