Can Hematological Biomarkers Indicate Diagnoses and Prognosis in Stranded Harbor Seals ( <scp> <i>Phoca vitulina</i> </scp> )?

ABSTRACT Wildlife health assessments offer critical insights in applied ecological research, but their interpretation is often limited by missing complementary data. There is heightened need for such data for European harbor seals ( Phoca vitulina vitulina ) given their recent population decline. We investigated trends in and hematological markers of health and survival of pre‐ and post‐ weaned harbor seal pups using a 10‐year rehabilitation database ( n = 2428), and established hematological reference ranges from clinically healthy pups at release (age 3–12 months; n = 260). Hematological parameters at admission were related to pup age class and diagnoses. Specifically, values for parasitic pneumonia—the predominant diagnosis at admission for post‐weaned pups—fell outside reference ranges for some parameters. Both body mass and hematological parameters at admission were related to the probability of surviving to release. Parameters associated with survival differed with age class but included those closely related to immune function, parasitic burden, anemia, and dehydration. Trends in admission numbers and body mass were mostly driven by changes in stranding protocols; however, there was an indication of emerging trends associated with population decline. These findings provide hematological markers to assess health and survival of both stranded and free‐living harbor seal pups.