The movement of sharks is written in their blood

Sam Munroe and a volunteer team take physical measurements of a shark

Australian scientists have figured out a way to find out where sharks go by analysing their blood rather than tagging them with satellite trackers.

The Great Barrier Reef study found that the travel history of the sharpnose shark is written in their blood with chemical “fin-prints” showing they tended to stay within smaller areas than previously believed.

“Small-bodied sharks that are both predator and prey, such as the Australian sharpnose, may be particularly important links between food webs,” says lead researcher Sam Munroe, who studied the sharks at James Cook University in Townsville.

“Information on their movements can improve our understanding of how the ecosystems function, while also helping us predict species most at risk from the impacts of a changing environment.”

This is the first time a chemical tracking technique, known as stable isotope analysis, has been used to estimate shark movement.

The results show the sharks remained within the same 100km area for up to one year.

Dr Sam Munroe taking a blood sample from a sharpnose shark at, Cleveland Bay, Queensland

Different levels of elements such as carbon and nitrogen are found in different environments. Munroe compared these levels from seagrass and plankton in different bays along the coast with the levels in blood and muscle samples to map where the sharks had been.

“We didn’t know if it would work for sharks on this scale,” she says. “But now we have shown it can be done for at least some shark species and we can hopefully apply this cost-effective approach in other environments.”

Satellite tagging is a popular tracking method and provides detailed movement information over huge scales but can cost thousands of dollars per animal tagged.

“Using isotopes means we don’t get the same level of detail, but they only cost around $10 to 15 per tissue sample, and we get a broad understanding of where the animal was in the last year,” she says.

Munroe says the small travel distances of the sharks may be due to a trade-off between moving widely to maximise their food and mating opportunities, or staying put to conserve energy.