Field immobilisation of adult Weddell seals using intramuscular butorphanol and midazolam

BACKGROUND: When working with free-ranging phocid seals, methods of chemical immobilisation require ongoing refinement to reduce complications, particularly apnoea, during research procedures. METHODS: Adult Weddell seals (n = 20) at Cape Crozier, Antarctica, were chemically immobilised with intramuscular injection of butorphanol and midazolam in 2024 and 2025. RESULTS: Butorphanol and midazolam were administered intramuscularly at 0.16 ± 0.03 and 0.19 ± 0.03 mg/kg, respectively. No apnoea lasting more than 2 minutes was observed, nor were any other adverse effects. LIMITATIONS: The sample was limited to 20 adults, predominantly mid-lactation females (n = 14). Additional data on other demographic groups, varying the dose combination, more detailed records of seal vital signs, and additional physiological measurements (e.g., blood gases) would provide valuable insight into this drug combination for Weddell seals. CONCLUSION: In this limited study, intramuscular butorphanol and midazolam were a safe and effective combination for field immobilisation of adult Weddell seals.