Investigations of Δ 14C, δ 13C, and δ 15N in vertebrae of white shark (Carcharodon carcharias) from the eastern North Pacific Ocean

Abstract

The white shark, Carcharodon carcharias, has a complex life history that is characterized by large scale movements and a highly variable diet. Estimates of age and growth for the white shark from the eastern North Pacific Ocean indicate they have a slow growth rate and a relatively high longevity. Age, growth, and longevity estimates useful for stock assessment and fishery models, however, require some form of validation. By counting vertebral growth band pairs, ages can be estimated, but because not all sharks deposit annual growth bands and many are not easily discernable, it is necessary to validate growth band periodicity with an independent method. Radiocarbon (14C) age validation uses the discrete 14C signal produced from thermonuclear testing in the 1950s and 1960s that is retained in skeletal structures as a time-specific marker. Growth band pairs in vertebrae, estimated as annual and spanning the 1930s to 1990s, were analyzed for Δ14C and stable carbon and nitrogen isotopes (δ 13C and δ 15N). The aim of this study was to evaluate the utility of 14C age validation for a wide-ranging species with a complex life history and to use stable isotope measurements in vertebrae as a means of resolving complexity introduced into the 14C chronology by ontogenetic shifts in diet and habitat. Stable isotopes provided useful trophic position information; however, validation of age estimates was confounded by what may have been some combination of the dietary source of carbon to the vertebrae, large-scale movement patterns, and steep 14C gradients with depth in the eastern North Pacific Ocean.