Ontogenetic and spatial variation in the trophic ecology of the shortfin mako shark (Isurus oxyrinchus) in temperate eastern Australia.

The shortfin mako (Isurus oxyrinchus) is a fast-swimming pelagic shark of global ecological and socioeconomic importance, yet it is relatively understudied in temperate eastern Australian waters. This study investigated the trophic ecology of mako sharks using stable isotope analysis (SIA) of nitrogen (δ15N), carbon (δ13C) and sulphur (δ34S). White muscle tissue samples were obtained from captured during game fishing tournaments along the New South Wales coast between 2005 and 2025. Ontogenetic shifts in isotopic composition were observed across individuals. δ15N values decreased with increasing body size, while δ13C values increased with size, suggesting that larger sharks may feed more frequently on prey of lower trophic positions. In contrast, δ34S values indicated mainly pelagic foraging across all size classes. Isotopic niche models revealed partial niche overlaps between size groups, with convergence in larger individuals, indicating shared resource use at maturity. Seasonal effects were weak, potentially reflecting sampling bias and the slow isotopic turnover rate of shark muscle. At the population level, their ecological role appears as a generalist, although individual specialization may occur. These findings demonstrate that ontogenetic variation and spatially variable foraging behaviour should be considered in fisheries management. In particular, the apparent reliance of larger individuals on coastal prey suggests that they may be more vulnerable to recreational fisheries than previously assumed. This study provides the first sulphur-inclusive stable isotope analysis of I. oxyrinchus and offers a species-specific trophic perspective for temperate eastern Australia. It highlights the importance of integrating stable isotope data with tagging studies, prey sampling and stomach content analyses to better resolve trophic pathways. Such integrative approaches will support ecosystem-based fisheries management, help reduce bycatch risks, and strengthen conservation strategies for this endangered species.