New Australian research highlights the delicate nature of the relationship between humans and sharks.
A study of the world’s Marine Protected Areas (MPAs) has found that they are three to 15 times too small to support the long-term conservation of coral reef sharks, which are victims of overfishing.
And in a separate project, researchers have linked an increase in whale shark injuries in Western Australia’s Ningaloo Reef Marine Park to boat encounters.
Reef-associated shark species such as whitetip reef, blacktip reef, grey reef, Caribbean reef, and nurse sharks are considered good candidates for conservation using MPAs because they are highly dependent upon their reef habitat.
However, until now it has been unclear how large these areas need to be to protect species and achieve conservation goals.
A team from the University of Queensland (UQ), James Cook University and the University of Tasmania studied the movements of individual sharks tracked in the Atlantic, Pacific and Indian Oceans and 8000 hours of underwater surveys.
In all, 459 sharks from five species were followed using a global network of acoustic receivers, and local shark abundances were monitored in 36 countries as part of Global FinPrint – a global effort to monitor shark abundances in coral reef habitats.
Writing in the journal Current Biology, the researchers suggest that no-take reserves, where fishing is banned, need to extend between 10 and 50 kilometres along coral reefs to significantly improve population numbers.
“Existing protected areas on coral reefs would need to be enforced as strict no-take reserves and be up to five times larger to effectively conserve reef sharks,” says UQ’s Ross Dwyer.
“Those in the Atlantic, where reef sharks are generally less abundant, would need to be on average 2.6 times larger than those in the Indian and Pacific Oceans.”
In the second project, a team from the Australian Institute of Marine Science (AIMS), discovered that almost one-fifth of whale sharks (Rhincodon typus) in the Ningaloo Reef Marine Park have major scarring or fin amputations, an increase on previous years.
“Some of the major scars were probably bite marks from predators, but most were the marks of blunt trauma, lacerations or amputations arising from encounters with ships, particularly propellers,” says whale shark scientist Emily Lester.
Of the 913 whale sharks imaged between 2008-2013, 146 (16%) displayed some form of serious injury, almost doubling previous records from 2011.
“One possible explanation is that there is an increase in shipping activity throughout the whale sharks’ range – inside Ningaloo and out – and collisions are becoming more frequent,” says Lester.
The research, published in the Marine Ecology Progress Series, couldn’t reveal the number of fatalities from boat encounters, because whale sharks are negatively buoyant, meaning they sink to the ocean floor when they die.
“A collision between a large ocean-going vessel and a whale shark wouldn’t be felt by the ship, as a result, it’s likely that we’re underestimating the number of mortalities from ship strike since our study could only document sharks that survived their injuries,” Lester says.
As whale sharks swim for thousands of kilometres beyond the Ningaloo park boundary, exactly where the injuries were sustained is unknown.
The researchers suggest the first step to reducing the number of injuries is to identify collision hotspots through spatial modelling.
Amelia Nichele is a Cosmos journalist.
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