Assessing the smallest by one of the biggest: Australian sea lions as investigators of primary production in the Great Australian Bight

The deployment of animal-borne electronic tags is revolutionising our understanding of how marine predators respond to their environment by providing oceanographic information. Most studies have been undertaken on animals... [ view full abstract ]

The deployment of animal-borne electronic tags is revolutionising our understanding of how marine predators respond to their environment by providing oceanographic information. Most studies have been undertaken on animals inhabiting oceanic habitats. In contrast few studies have focused on the neritic zones where the physics is mainly investigated by the conventional oceanographic methods. However, as many predators spend their life in coastal waters, they can potentially provide additional platforms for obtaining oceanographic observations on these ecosystems.
The Great Australian Bight (GAB) represents a complex oceanographic system affected by the circulation of diverse water masses. It is generally considered to be an area of limited biological productivity, however, the eastern GAB is characterized by seasonal coastal upwelling that underpins Australia’s largest volume fisheries and diverse apex predator communities. Although the importance of this upwelling system to the productivity of the region is recognised, lack of in-situ data measurements have resulted in many unresolved questions, especially on its seasonal contribution to the local primary production (PP).
The eastern GAB is home to Australia’s largest population of Australian sea lions, which forage benthically in these waters year-round. As platforms for instrumentation, they provide an unprecedented opportunity to gather data on the oceanographic environment at a high spatio-temporal resolution. Since 2007, sea lions have been equipped with Argos-Conductivity-Temperature-Depth recorders collecting cross-shelf profiles over a 1,000 km of shelf. More recently, individuals have been equipped with a new generation of tags that include both fluorescence and irradiance sensors. These measurements have been used to estimate changes in water column PP (biomass) over 4 (2014) and 5 months (2015) periods. Our results provide evidence that the system tends to be less productive in winter with, for instance, a maximum initially measured around 1600 mg.C.m-2 at 50 m depth in April and reaching 1200 mg.C.m-2 in July in near surface waters south to Kangaroo Island.
As witnesses and in-situ investigators of change in these systems, sea lions are now playing a critical role in better understanding the oceanography of the region, and its importance to fishery production and communities of marine predators that depend on it.