Population demographics and seasonal variation of the Burrunan Dolphin (Tursiops australis) in the Gippsland Lakes

Coastal ecosystems are under increasing threat as encroachment from human populations continues to put pressure on top order predators such as marine mammals. The recently described Burrunan dolphin (Tursiops australis),... [ view full abstract ]

Coastal ecosystems are under increasing threat as encroachment from human populations continues to put pressure on top order predators such as marine mammals. The recently described Burrunan dolphin (Tursiops australis), endemic to south-eastern Australia is characterised by small geographically and genetically isolated coastal populations with low genetic diversity. The only two known resident populations of Burrunan occur adjacent to a large urban (Port Phillip Bay) and agricultural (Gippsland Lakes) centre and are reported to be negatively effected by accumulation of heavy metals and tourism. At present, the Burrunan is listed as ‘Threatened’ under Victoria’s Flora and Fauna Guarantee Act 1988, however further classification is prevented due to data deficiency. A series of boat-based surveys were conducted in the Gippsland Lakes to provide the first direct abundance estimate for the Burrunan dolphin worldwide. The most recently developed abundance modelling technique (Mark-resight Robust Design) was used, based on the rigorously refined photographic identification capture-recapture methodology. It was found that a maternally based resident population of ≈ 65 (64; 95%CI 41–97, 66; 95%CI 54–81) individuals occupy the Gippsland Lakes, across multiple seasons and multiple years. A unique winter calving is facilitated by multiple waves of transient animals enter the system in winter (112; 95%CI 101–124) and early spring (128; 95%CI 98–166). Transient animals are thought to migrate to Tasmania possibly facilitating both winter and summer breeding between two populations >300km apart. Confirmation from historical data shows that not only are some transient animals returning to the Lakes annually, others have switched between resident and transient behaviour possibly relating to maturity. The influx of transient animals coincides with a dramatic shift in the spatial habitat use within the Lakes, and may reflect changes in prey availability, sexual competition, and/or the avoidance of boat traffic. Although the Burrunan species are not yet considered for IUCN Red Listing, we recommended that the very small (<250 mature individuals) Gippsland Lakes population be listed as ‘Endangered’.