Cardiac disease has been extensively documented in marine mammals; however, it remains difficult to diagnose antemortem. Assays measuring cardiac troponin I (cTnI) and N-terminal pro-brain natriuretic peptide (NT-proBNP) are used as sensitive and specific biomarkers of cardiac disease in many species, but have not been widely investigated in marine mammals. This study aimed to provide a set of reference values for cTnI and NT-proBNP in belugas (BW) (), sea otters (SO) (), Steller sea lions (SSL) (), and California sea lions (CSL) () with and without cardiac disease, and to determine if these biomarkers are useful indicators of cardiac disease in these species.