THE furore in Western Australia over installation of baited drum lines at two specific beach zones in order to provide WA beachgoers with some protection from increasing rates of shark attack, clearly shows that sharks have become members of the elite “charismatic marine megafauna” club.
This club, which was previously occupied only by whales, dolphins and turtles, has a membership comprising only those species to which certain environmental groups attribute mythical powers. Perhaps the mythical powers arise from their starring roles in such esteemed movies as Finding Nemo. Perhaps the issue is more to do with certain environmental groups attempting to exploit the ignorance of the urban majority regarding predator-prey relationships in Australia’s oceans. Whatever the reason, due to an apparent inability of people today to conduct rational debate in the media (both social and mainstream), it’s evident that any fisherperson who catches a shark in the public eye is, from now on, likely to be in for a rough ride.
This is why a new scientific paper researching the survival of sharks released from fishing gear is timely. Researchers from the University of Miami have published a study in the journal Marine Ecology Progress Series that examined the stress responses and survival of five shark species following experimental capture and release. As motivation for their study, the researchers pointed out that today there are increasing numbers of regulations that require release of threatened or protected shark species, and they also observed that many recreational fishers are now adopting voluntary release procedures for sharks in the name of conservation.
Using a standardised fishing technique (baited drumlines with circle hooks and hook timers to tell how long the shark was on the line), the researchers studied the responses of five shark species, including blacktip sharks (Carcharhinus limbatus), bull sharks (Carcharhinus leucas), great hammerhead sharks (Sphyrna mokarran), lemon sharks (Negaprion brevirostris), and tiger sharks (Galeocerdo cuvier).
Upon capture, each shark was given a health assessment before being tagged with a satellite tag and then released. Subsequent survival rates were determined based on the tag data uploaded by the satellite tags. Based on the information gathered, they found the stress response and mortality rates were highest for hammerhead sharks, which had only 53.6 per cent survival, four weeks after capture. In contrast, bull sharks displayed 74.1 per cent survival, while tiger sharks seemed very robust and exhibited minimal stress response and 100 per cent survival.
Overall, the susceptibility to stress and mortality, ranked from highest to lowest, was established as follows: hammerhead shark > blacktip shark > bull shark > lemon shark > tiger shark.
The study was interesting because it found that hammerhead sharks were quite vulnerable to stress and mortality resulting from capture on drum lines, while other species (e.g. tiger sharks) were basically unaffected by the same process. The researcher attributed this to differences in the swimming activity and flight responses of the different species. Hammerhead sharks were known to be fast and explosive swimmers when hooked. Tiger sharks, on the other hand, were observed as slower endurance swimmers and thus were less likely to be affected by the hooking process.
A short video explaining the findings of the study can be found at: http://rjd.miami.edu/research/projects/stressed-out-fish, while those who are interested in reading the complete paper can find it here: http://rjd.miami.edu/wp-content/uploads/2014/01/Gallagher-et-al.-2014.pdf
