Feeding ecology of broadbill swordfish, Xiphias gladius, off eastern Australia in relation to physical and environmental variables

We examined the feeding ecology of broadbill swordfish, Xiphias gladius (Linnaeus, 1758), from eastern Australian waters through the analysis of gut contents of 638 fish (size range 660–2450 cm orbit to fork length) collected between 1997 and 2005, the results of which were compared with a suite of oceanographic and environmental variables from the region. Cephalopods with an index of relative importance (%IRI) of 61.65% dominated the prey followed by fish (38.10 %IRI); crustacea were eaten rarely (< 1% IRI). A total of 56 prey taxa was identified, consisting mainly of cephalopods dominated by Ommastrephes bartramii (Lesueur, 1821). The main fish prey were from the families Nomeidae and paralepididae. Size of fish, year, area, and sea surface fluorescence were significantly correlated with feeding activity. In particular, swordfish taken in nutrient-rich offshore waters had a significantly higher prey mass than those from more oligotrophic inshore waters associated with the East Australia Current. Cephalopods dominated the prey offshore whereas fish were the main group in inshore waters. Stable isotope analysis indicated that δ¹⁵N increased significantly with fish size (r² = 0.65), paralleling a change in diet from fish with low δ¹⁵N values to cephalopods with relatively high values.