Ecological and biochemical considerations based on stable isotope analysis of blood: A case study from two juvenile lamnid sharks

Abstract

Shortfin mako (Isurus oxyrinchus) and white shark (Carcharodon carcharias) are migratory top predators, worldwide distributed, and keystone species considered Vulnerable by International Union for Conservation of Nature (IUCN). Their ecology is generally well described for adult stages, however habitat preferences and trophic ecology of juvenile phases (i.e., neonates, young of the year [YOY], juveniles) are still poorly described. This knowledge is important to strengthen management and conservation strategies. Additionally, there is also a scarceness of data concerning the impact of tissue biochemical composition on isotopic ratios. This study investigates the influence of the tissue biochemistry on the blood isotopic composition of young shortfin mako and white sharks. Our results provide biological information to understand the ecology and behavior of young shortfin mako and white sharks in a shared nursery area in Baja California (Mexico) using a multi-tissue approach. We compared stable isotopes (SIA) of carbon (δ13C) and nitrogen (δ15N) from different blood fractions (i.e. whole blood, RBCs and plasma) among juvenile shortfin mako and white sharks. The results indicated consistent increase in δ15N values from newborn to larger juvenile stages and includes a first estimation of the incorporation rates in different blood components. Based on our isotopic results, we proposed that the blood biochemistry between life stages influences the isotopic composition of tissues, more than dietary changes in consumed prey. The similar isotopic results of both species by size suggest a resource sharing and common habitat use throughout extended periods, reinforcing the importance of Sebastian Vizcaino Bay as nursery area, and especially as parturition ground for shortfin mako sharks. Finally, this study provides important information for blood sample preservation and represents a first characterization of blood isotopic composition, highlighting the need for further research to better explore how isotopic values can be influenced by different factors (i.e., biochemistry and ecology) and constitutes a baseline for further research.