Modelling surfacing behaviour of southern bluefin tuna in the Great Australian Bight

Abstract

Large numbers of juvenile southern bluefin tuna (SBT; Thunnus maccoyii) migrate into the warm shelf waters of the Great Australian Bight (GAB) each austral summer. Whilst in the GAB, they aggregate in schools that spend substantial periods in the surface layer of the water column. In this study we investigate biological, temporal and environmental factors influencing this surfacing phenomena using an extensive archival tagging dataset collected between 1998 and 2011. High frequency data on the vertical movement of SBT collected by these tags were used to calculate the proportion of time fish spent in the shallowest 20 m during each day and night period. Estimates of fish location derived from light sensor data on the tags allowed us to investigate the influence that local environmental conditions had on a fish's surfacing behaviour. Although there is high variability in surfacing behaviour within and between individuals, some general patterns emerge. There are clear diel differences in surfacing, with the proportion of time fish spend at the surface tending to be high during most days and either very high (>90%) or very low (<10%) during most nights. Complex relationships were found between surfacing behaviour and the environmental variables considered (sea surface temperature, chlorophyll, salinity and wind speed). The results from this study have relevance to the commercial purse-seine fishery targeting surface schools of SBT in the GAB during the austral summer, as well as the scientific aerial survey conducted each summer that collects data on sightings of surface schools of SBT in order to derive a relative abundance index used directly in management.