Abstract Research using stable isotopes analysis (SIA) of carbon (δ 13 C) and nitrogen (δ 15 N) in blood components is lacking, because of the challenge of sample collection, processing, and storage in remote areas. There also is a paucity of information regarding the effect of tissue biochemical composition on isotopic ratios with few comparisons among taxa. We collected blood samples from shortfin mako sharks ( n = 70; 2016) and Guadalupe fur seals ( n = 25; 2017). All samples were centrifuged to obtain plasma from sharks and serum from the Guadalupe fur seals, and all the samples were prepared for SIA and analyzed using a Costech 4010 elemental analyzer interfaced with a Delta V Plus isotope ratio mass spectrometer. We found significant differences between plasma δ 13 C values of shortfin mako sharks (−17.6 ± 0.9‰) and serum of Guadalupe fur seals (−20.3 ± 1.2‰), but we did not find any differences for δ 15 N values between the two species. The differences in δ 13 C values between species are probably due to the specific blood composition and to the different biochemical characteristics and different adaptations within taxa. These findings highlight the importance of further research on the influence of biochemistry features on isotopic results, in this way a more accurate assessment will be possible for this factor, separating it from the dietary influences on stable isotopic values.