Southern bluefin tuna habitat use and residence patterns in the Great Australia Bight

Abstract

Juvenile southern bluefin tuna (SBT) undertake seasonal migrations to and from the coastal waters of southern Australia until around age 5. In particular, they are present in the waters of the Great Australia Bight (GAB) over the summer months, where they are targeted by a commercial purse seine fishery. Arrival to and departure from the GAB represent important constraints on the valuable fishery, and this information is also now seen as essential for managing other activities within the GAB. As such, understanding the general patterns of the distribution of juvenile SBT and the environmental factors might be driving their distribution is valuable for establishing a baseline against which any future changes to population arrival or departure may be assessed. As part of a large investigation into the ecosystem of the GAB, data from a multi-year archival tagging program focused on juvenile SBT were used to construct statistical models of spatial usage within the residence areas of the GAB. Climatological data describing mixed-layer-depth, sea-surface temperatures, along with distance to the shelf break and coast, spatial position and time of year were used as predictor variables. Generalized additive models were fit which accounted for the seasonal progression of SBT into the GAB throughout their annual cycle and predicted the broad scale environmental features associated with their average distribution. The statistical models predicted habitat use by juvenile SBT in the proximity of the continental shelf break, with a preference for warmer waters in conjunction with deeper mixed layers. The model-generated annual distribution climatology can be used to plan the timing for a range of activities such as fishing, shipping, oil and gas exploration, and also inform development of forecasting applications. The predictions are also useful for determining large scale variation from the typical usage patterns of SBT in the GAB into the future.