Movements and mortality of two commercially exploited carcharhinid sharks following longline capture and release off eastern Australia

Carcharhinus plumbeus (sandbar shark) and C. obscurus (dusky shark) occur in many global fisheries as targeted species and/or bycatch. However, little is known about their movement and the possible fate of discards. We redressed this lack of knowledge using pop-up satellite archival tags (PSAT) and acoustic tagging technologies off the eastern coast of Australia. Eight sharks of each species were caught by demersal longline, fitted with both types of tag and then released. PSATs indicated that 2 C. plumbeus and 1 C. obscurus died within 8 h of release, while tracks over periods of 1 to 60 d were obtained for 13 sharks. All surviving sharks first swam in an easterly, offshore direction to outer-shelf waters during the first 24 h. All C. plumbeus then moved approximately south (i.e. increasing latitude) by distances of up to similar to 350 km. In contrast, most C. obscurus moved approximately north by distances between 212 and 606 km. Over an 18 mo period following release, acoustic tag detections occurred for 4 C. plumbeus (mostly within 30 km of release, suggesting some philopatry) and 6 C. obscurus (similar to 515 km south and similar to 310 km north). Both species spent similar to 85% of their time in waters <100 m deep. Diel patterns in vertical movements of C. plumbeus were detected, with a preference for deeper water during daylight hours. Both species mainly utilised water temperatures between 22 and 26 degrees C. Information from this study can be used for the effective management of commercially exploited stocks of both species.