Insights from field chemical immobilization of northern elephant seals over two decades

Chemical immobilization is essential for ensuring the safety and welfare of wildlife and researchers during animal handling. We analyzed observation data from 1,464 immobilization procedures on 625 northern elephant seals (Mirounga angustirostris) conducted between 2004 and 2025 to describe our practices and insights. Inductions used tiletamine-zolazepam (Telazol®), with supplemental doses of ketamine, diazepam, and/or Telazol for augmentation (required to achieve satisfactory induction) and maintenance (required to maintain chemical immobilization for the procedure duration). Procedures spanned a wide range of body masses (57-668 kg) across the annual cycle (i.e., seasons) and life stages, including juveniles of both sexes and adult females up to 18 years old. For adult females, 68% received Telazol induction doses within ± 0.1 mg/kg of the 1.0 mg/kg target, indicating accurate mass estimation and a broad safety margin when paired with vigilant monitoring and respiratory support as needed. Lower induction doses were associated with a higher likelihood of augmentation drug administration, whereas higher doses were more often associated with faster-than-typical inductions and the administration of a respiratory stimulant. In procedures without augmentation or respiratory stimulant administration, smaller juveniles received higher mass-specific induction doses than adults across the observed body size range, regardless of sex. These findings show that age- and season-based mass estimates can refine dosing precision within a safe range. These findings describe recorded field decisions and procedural events (e.g., timing and sequence of drug administration) rather than direct physiological measurements of anesthetic depth or safety. Our long-term dataset provides empirically derived guidance to improve immobilization efficacy, safety, and consistency for elephant seals and related species.