Temporal and size-related variation in the diet, consumption rate, and daily ration of mackerel tuna (Euthynnus affinis) in neritic waters of eastern Australia

The diet, food consumption, and ration of mackerel tuna (Euthynnus affinis) were studied in Australian neritic waters. Overall, 43 prey taxa were identified from 271 stomachs. The diet was primarily pelagic clupeoids (78% by wet weight, WW; 71% by frequency of occurrence, FO) and demersal fish (19% WW; 32% FO). Multivariate regression tree analysis revealed that temporal differences, followed by fish size, explained most of the variation in the diet composition. Autumn diets differed from those in other seasons because tuna ate virtually only engraulids then. During other seasons, engraulids were still the dominant taxon in the diet, but fish also consumed a greater variety of other prey. Small tuna seemed to target small pelagic crustaceans and teleosts, and medium and large tuna to consume larger pelagic and demersal teleosts. Prey consumption increased with tuna size from 26.42 to 108.03 g d 1 for small and large tuna, respectively. Conversely, daily ration decreased with increasing tuna size from 4.10 to 1.95% body weight per day for medium and large tuna, respectively. Mackerel tuna consumed an estimated 25 036 t year 1 in the study region (170 990 km2). Diet studies are becoming increasingly important in informing ecosystem models, and this study confirmed the need for sampling regimes to capture temporal and size-related variation in diet composition, to maximize the utility of data for use in such models.