Effects of a dredging operation on the movement and dive behaviour of marine turtles during breeding
Dredging presents a risk of injury or mortality to marine turtles via entrainment, vessel strike or the effects of noise and vibration. Dredging operations have increased in recent years, yet there remains an absence of supporting information for development proponents and regulators to accurately determine dredge-related impact predictions for marine turtles within an Environmental Impact Assessment. This absence therefore has implications for their conservation, as certain dredging impacts may be overlooked or the implementation of control measures and response strategies may be ineffective.
This study presents a case study involving a major resource development whose proponent undertook satellite tracking of 48 reproductively-active flatback turtles (Natator depressus) at different stages of its dredging operation (before: n = 20, during: n = 10 and after: n = 18). In addition, trained Marine Fauna Observers were on-board dredge vessels to mitigate interaction and detect injury and mortality events. We compared the dredge-related impact predictions stated in the Environmental Impact Assessment with the quantified dredge-related impact to flatback turtles as represented by changes to their movement and dive behaviour between each stage of the dredging operation and any injury and mortality events. Additionally, we considered the effectiveness of implemented dredge-related control measures in preventing their injury or mortality.
We found that during the active dredging operation, flatback turtles increased their use of the dredging areas. Dive behaviour results showed they undertook longer and deeper resting dives during dredging, utilising the now deeper waters of the dredging areas. Possible reasons for this preferential use of areas being dredged are discussed. Despite their increased use and the presence of active dredge vessels, no events of injury or mortality were recorded. The implemented control measures may have been effective in preventing injury or mortality, though the spatial scale of their effectiveness may have been smaller than first anticipated.
Our results are of great value for stakeholders involved in marine turtle conservation and managing dredging operations worldwide as no comparison of quantified marine turtle survivorship, movement and dive behaviour data at different stages of a dredging program has been recorded or reported previously.