Longtail tuna played a similar ecological role in northern and
eastern Australia by consuming a range of prey categories (e.g.
pelagic and demersal fishes) in similar proportions. However,
the diversity and composition of the diet varied significantly
among regions, seasons and fish sizes. The diversity of prey in
the northern region was nearly twice that of the eastern region,
which probably reflects the higher diversity of fishes generally
found in the tropical northern region (Blaber 2002). Also, the
overall prey diversity, variation in diet composition and feeding
intensity was highest during autumn and winter and decreased
markedly in spring and summer. This pattern was consistent for
both regions, despite fish in each region having significantly
different diet composition. In the northern region, longtail tuna
primarily consumed small schooling pelagic species such as
Sardinella spp. and Stolephorous spp. during spring and summer,
while Stolephorous spp., Clupeidae sp. and Engraulidae sp.
were primarily consumed in the eastern region during this time.
There was a consistent pattern of seasonal variation in the
diversity and composition of the diet of fish from the two regions,
considering the vastly different environmental regimes of the
two regions. In contrast to the subtle seasonal variation in water
temperature in the subtropical-temperate climate of the eastern
region (Ridgway and Godfrey 1997), the tropical northern region
experiences a dynamic monsoonal climate with a ‘wet’ season
between October and February and a ‘dry’season between March
and September. The numerous large estuaries in the region flood
during the wet season and discharge large volumes of turbid
freshwater into the Gulf of Carpentaria, significantly changing
the salinity, temperature and turbidity regime of coastal waters
(Blaber et al. 1995).