The broadsword and the beasts: large nektonic prey are crucial for the swordfish (Xiphias gladius) but diverse micronektonic species dominate its diet in the Indian Ocean

Context Swordfish are among the most abundant top predators in the Indian Ocean, and play a key role in pelagic food webs. However, their feeding habits and ocean-wide trophic niche remain poorly known. Investigating swordfish foraging behaviour, diet variability and trophic position can provide important information on predator–prey dynamics and improve our understanding of open-ocean ecosystems functioning. Aims This study aims to evaluate the ecological role of swordfish in pelagic food webs across major biogeochemical provinces of the Indian Ocean and adjacent waters of the Atlantic. Methods We investigated the trophic ecology of swordfish using stomach content analysis and stable isotopic value (δ13C and δ15N) measurements from white muscle tissue. On the basis of 872 non-empty stomachs and 759 muscle samples collected between 1998 and 2011 across five biogeochemical provinces, we described swordfish prey composition, prey size structure and size-related prey biomass. We further analysed feeding patterns and swordfish’s relative trophic position within the pelagic ecosystem. Key results Epipelagic and vertically-migrating mesopelagic fishes, together with cephalopods, constituted the principal prey groups by number and reconstituted weight, alternating in dominance among provinces. Ommastrephid squids formed the primary component of the cephalopod prey group, a pattern consistently reported across ocean basins. Stomach content analysis showed an ontogenetic dietary shift, with dominance transitioning from fish to cephalopod prey. Although micronektonic prey dominated numerically, more than 70% of the total consumed prey biomass originated from large nektonic species, highlighting their crucial importance for swordfish populations. Muscle δ15N values increased with body size, indicating a rise in trophic position during growth, which was likely to be driven by the consumption of larger fish prey and an increasing contribution of cephalopods to the diet. Conclusions Swordfish exhibit a flexible and opportunistic feeding strategy, with their diet being largely composed of a limited number (5–7) of dominant prey species that vary across provinces and seasons. Muscle δ13C and δ15N values indicated a distinct isotopic niche within each Indian Ocean province, suggesting mid-term (months) residential behaviour at the provincial scale. Implications Residential behaviour may increase swordfish vulnerability to local depletion under excessive fishing pressure, potentially leading to mesopredator-release effects and broader trophic cascades. Continuous long-term monitoring of the Indian Ocean pelagic ecosystem is therefore essential to support sound management and sustainable exploitation of pelagic resources. This should include larger-expanded stomach-content sampling, improved δ13C and δ15N baseline and mid-trophic-level values and modelling of their pathways. In addition, estimates of tissue turnover rates, currently unavailable for swordfish, are needed to improve the evaluation of migratory and residential behaviour and refine interpretation of stable isotope data.