Tonic immobility in the zebra shark, Stegostoma fasciatum, and its use for capture methodology

When conducting biological investigations on shark species, capture and brief restraint procedures may be required in order to collect ecological, behavioral and physiological data. Tonic immobility is an innate, reversible, coma-like stasis displayed by a large number of taxa, including sharks, and has been used for brief restraint, but is rarely used during capture itself. Here, we present a novel, non-lethal capture method of zebra sharks, Stegostoma fasciatum, using tonic immobility, in the field. Zebra sharks were caught by free diving, and on SCUBA, inducing a state of tonic immobility by quickly applying tight-fisted manual pressure to the caudal fin, instead of the dorso-ventral inversion method. Divers were then able to collect tissue samples for genetic analysis, and in some cases, maintain the animals in a state of tonic immobility during transport up to the research vessel for additional data collection. Decreased duration of anthropogenic intervention, and minimal physical struggle associated with this capture and restraint method is expected to reduce physiological markers of distress. Tonic immobility induction and other methods of non-lethal capture deserve continued investigation.