Project Title
Capture-related stress physiology and post-release fate of small Australian sharks
Abstract
Recent declines in shark populations worldwide call for intensified and applied shark research, effective management plans are urgently needed. Only a few shark species are targeted by commercial fisheries in Australia, while many more are caught as bycatch, and are subsequently discarded dead or alive. The fate of those sharks released alive is an essential factor for the assessment of the impact of fisheries on shark populations, but is so far completely unknown. Some will probably survive, while others will succumb to injuries suffered during capture or to infections resulting from such injuries, or may die as an indirect consequence of the deleterious effects of capture, such as fatal physiological stress or impaired predator evasion or foraging behaviour. Furthermore, a single minor stressful event, such as handling, is sufficient to significantly depress circulating levels of reproductive steroid hormones in elasmobranchs for many days.
Only a few studies have addressed the effect of capture on elasmobranchs, all of them used sharks caught in the wild. Because many factors, including water temperature, dissolved oxygen, and most likely also the nutritional state of an animal, affect exercise-related physiological processes, an unambiguous interpretation of results obtained in the wild can be difficult. The duration of stress exposure has a major influence on the magnitude of the stress response. Even though it is a key parameter to be considered, it is often impossible to be determined in the field.
To avoid these uncertainties, I've developed a technique to simulate capture with different types of fishing gear in aquaculture tanks using captive sharks. This approach allows me to control the above mentioned parameters, and monitored the recovery phase of the individual sharks.
The primary aim of this study is to establish reference curves of physiological parameters relevant to the sharks' stress reaction in the lab, demonstrating the change of these parameters over time. Sharks are subjected to various durations of capture stress exposure, after which several blood samples are taken during a recovery period of 72 hours. Simulated capture gear types include hook-and-line, gill-nets and trawl-nets, and experiments are conducted in circular 5000 litre aquaculture tanks. Hook-and-line and gillnets are suspended from a tripod frame sitting on top of the tank, and are connected to a load cell measuring tension caused by struggling sharks. These struggling activity profiles are used to quantify struggling effort. Trawling is simulated by placing sharks into trawl codends that are being pulled around the experimental tank by two rotating arms powered by an electric motor.
Aside from providing insight into a so far poorly understood aspect of elasmobranch physiology, the results of this study will help to increase the accuracy and power of large scale tagging studies by providing information on a so far unknown, but important factor influencing the probability of recapture of an animal. Data on the post-capture fate of sharks will refine mathematical fisheries management models, and being able to account for the effect of initial capture will benefit other studies that interact with animals in the wild.
Supervisor
See also
Oceania Chondrichthyan Society
Shark Foundation Switzerland
