The Use of Animal-Borne Imaging to Assess Post-Release Behavior as it Relates to Capture Stress in Grey Reef Sharks, Carcharhinus amblyrhynchos

Abstract

Sharks are subjected to extensive commercial and recreational fisheries worldwide. Current management, which imposes bag limits, minimum sizes, and quotas, mandates the release of large numbers of sharks each year, but little is known of post-release behavior and survivorship. Using animal-borne imaging technology, we examined the effects of handline capture on post-release behavior of six grey reef sharks, Carcharhinus amblyrhynchos, at Johnston Atoll (Central Pacific) as it relates to physical trauma and physiological stress induced by capture. To quantify the extent of physical trauma, 25 grey reef sharks (including these six), ranging from 56–135 cm fork length, were examined for evidence of external tissue damage after 2.0–12.8 minutes of handline capture. In addition, these fish were blood sampled to quantify relative changes in acid-base biochemistry. Although blood lactate increased and blood bicarbonate decreased significantly relative to the duration of the capture event, blood pH did not drop significantly and there was no evidence of respiratory or metabolic acidoses. Post-release behavior, as evidenced by animal-borne imaging, included group (n=3) and solitary (n=2) activities that had been previously described in this species. A single shark exhibited aberrant behavior, which included a two-minute period of disorientation, lack of movement, and loss of equilibrium; this behavior was attributed to extensive physical trauma associated with hook damage. When coupled with quantified information relative to the capture event, we found that animal-borne imaging is a useful tool for collecting direct observations of post-release behavior in sharks so that fishery managers and researchers can better assess the impacts of various capture techniques.