Blood parameters of wild-caught harbor seals (Phoca vitulina) in the German North Sea: reference intervals and potential influencing factors

Abstract

Reference ranges for blood parameters are essential for diagnostic assessment and treatment decisions for harbor seals (Phoca vitulina) in rehabilitation or under permanent human care. Moreover, they are also important for evaluating the health status of free-ranging animals, thereby allowing conclusions about population health. To date, robust datasets of free-ranging presumably healthy individuals are rare. Here, we evaluated reference values for 14 hematologic and 21 serum biochemical parameters of 576 harbor seals that were wild-caught in the German North Sea over almost three decades. After the removal of outliers, nonparametric or bootstrapping methods were applied, depending on the sample size for each parameter, to estimate 95% reference intervals and their 90% confidence intervals. Additionally, we investigated the influence of three factors, as well as potential time trends, on each blood parameter using the Mann–Whitney-U test. Here, the factors which showed a moderate effect on only a few of the tested blood parameters were age class, sex, and season. LDH values were higher in juveniles than in multi-year animals, while creatine showed higher values in multi-year animals. Females showed higher values for iron, BUN, and triglycerides compared to males. Red cell distribution width (RDW), total-protein and cholesterol values were higher in spring than in autumn. No specific changes over time were determined for any of the blood parameters. This study enhances the knowledge on blood reference values of free-ranging harbor seals, delivering important baseline data especially for further health assessments of the Wadden Sea harbor seal population. Continuous health examinations of free-ranging marine mammal populations represent an important early warning tool for identifying potential threats affecting these animals. Therefore, they can provide valuable information to support effective management and conservation measures.