Trawl capture of Port Jackson sharks, Heterodontus portusjacksoni, and gummy sharks, Mustelus antarcticus, in a controlled setting: Effects of tow duration, air exposure and crowding

Abstract

Delayed effects of fisheries capture on the physiology and condition of sharks are poorly understood, but information on the post-release fate of sharks that have been incidentally captured, handled, and released is important to elaborate effective fisheries management measures for by-catch shark species. By-catch is often substantial during commercial trawling operations, and fish are exposed to a multitude of different stressful stimuli during trawl capture. We subjected Port Jackson sharks, Heterodontus portusjacksoni, and gummy sharks, Mustelus antarcticus, to trawl capture in a controlled setting to investigate effects of tow duration, exposure to air, and crowding in the codend, and monitored their condition via repeated blood sampling during a 72-h recovery period subsequent to the capture event. Port Jackson sharks experienced a low degree of physiological disturbance in response to our capture treatments and no mortality was observed during or after any experiments. Conversely, homeostatic balance of gummy sharks was severely disrupted by trawl capture, and immediate and delayed mortality was substantial (up to 87%) during some experiments. Moribund gummy sharks showed significantly increased blood lactate (>15 mmol/L) and potassium levels (>8 mmol/L) compared with surviving sharks, but these differences did not become evident until 6–12 h after the capture event. There was no strong evidence for an increase in physiological disturbance with increasing tow duration in either species. Extended periods of air exposure (>10 min) following a capture event may lead to additional physiological stress, but simulated crowding as performed in the present study did not result in increased physiological stress compared with trawl capture of individual animals. The results of this study suggest that trawl capture may lead to significant immediate and delayed mortality in gummy sharks, and that extended air exposure on deck may further exacerbate the deleterious effects of capture stress.