Effects of on-deck holding conditions and air exposure on post-release behaviours of sharks revealed by a remote operated vehicle

While post-release mortality estimates have been conducted on a range of shark species, the short-term sub-lethal effects of capture, handling and release are poorly known and have been mostly investigated in controlled conditions. In addition, the widely accepted notion that immediate post-release active swimming is an indicator of shark condition has never been tested. This study assessed the effects of deck exposure by analysing post-release behaviour of two species of shark, the draughtboard (Cephaloscyllium laticeps) and the piked spurdog (Squalus megalops) in-situ using a remote-operated-vehicle and in a replicated experiment in controlled conditions. In total, 67 sharks were caught with demersal fish trawls and commercial longlines and subjected to different deck exposures and release environments. Tailbeat rates of deck-exposed sharks were significantly lower than the control sharks, but this effect differed between in-situ and experimental environments. Results indicate that capture has species-specific effects, that post-release effects may last longer than 5 min, and that controlled experiments may not be reliable indicators of post-release effects. Immediate post-release swimming was not a good predictor of post-release behaviour, suggesting capture and release fisheries may have significant sub-lethal effects on some species of shark, and that limiting capture or handling time may reduce post-release effects.