In the Stable Isotope Laboratory at CICESE (Ensenada Center for Scientific Research and Graduate Education, Mexico) animal tissues (pup fur from Pacific harbor seals, Phoca vitulina richardii) were ground for isotopic determinations. Samples were stored in a dissecator until they were loaded into a Costech\u00ae Carrousel. Samples were combusted at high temperature (1000\u02daC) in a pure oxygen atmosphere in an elemental analyzer coupled to a Finnigan MAT Delta V Advantage continuous flow stable isotope mass spectrometer.Stable carbon and nitrogen ratios are expressed as \u03b413C or \u03b415N according to the following equation:\u03b413C or \u03b415N (\u2030) = [Rsample\/Rstandard)\u20131]* 1000 (1) where R is 13C\/ 12C or 15N\/ 14N. Isotopic values of carbon and nitrogen are reported relative to Pee Dee Belemnite and atmospheric nitrogen standards, respectively. The accuracy of isotopic measurements was verified using secondary standard reference materials (Glutamic acid, a pure carbonate Merck and the laboratory internal references Lanugo for N, and CH94 for C).