“The most surprising result from our study was that sharks had very high carbon 13 in their tissue, which suggests that these species of reef sharks have a dependency on inshore coastal resources.”—Dr Speed. Image: greylochA NEW study has found that the key to protecting vulnerable shark species lies in coastal marine refuges.
By studying the diet and movements of reef sharks using a multidisciplinary approach, a team, led by researchers from the Australian Institute of Marine Science (AIMS) at the UWA Oceans Institute, have shed light on their role in the food web.
Dr Conrad Speed from AIMS who led the study says “We combined the use of acoustic telemetry and stable isotope analysis to determine the feeding ecology of reef sharks. It provided us with information on movements and allowed us to make inferences about their diet.”
Traditionally, diet studies often require animals be euthanised in order to study their stomach contents, and it only provides information of their last meal. Stable isotope analysis is non-lethal and provides a bigger picture on what has been incorporated into the animals system.
The team studied four species of reef sharks at the southern end of Ningaloo Reef over a two-year period.
“The most surprising result from our study was that sharks had very high carbon 13 in their tissue, which suggests that these species of reef sharks have a dependency on inshore coastal resources,” Dr Speed says.
High carbon 13 values are generally associated with coastal habitats such as seagrass and seaweed.
That sharks are closely linked to the inshore food web is surprising given that they are large predators capable of travelling hundreds of kilometres.
“We would have expected foraging within the lagoon, but I would have assumed some of these species would forage out on the reef slope, which would have lead to a lower carbon 13 signature within their tissue.”
The study also found that sharks had a long-term residency to sites. This, coupled with a high dependency on coastal food chains, has important implications for the effectiveness of current protection measures for reef sharks in WA.
“Any impacts that occur inshore, whether the direct removal of their prey by fishing, or other indirect pressures such as development, potentially affect their survival,” says Dr Speed.
Sharks are predators causing top down influences on marine communities. With widespread declines in their populations, they have become the focus of conservation efforts and research. But the role that individual species play has remained unclear, making predictions of consequences of their removal tenuous.
Dr Speed is now looking at movements of reef sharks in sanctuary areas at Ningaloo to determine the effectiveness of existing protected areas on shark populations.
In support, DEC has begun a volunteer observation program to assess if visitors are having any impacts on shark populations.
