Trophic ecology and stable isotope niche space of tuna, billfishes and sharks in New Zealand waters

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Context Highly migratory species (HMS), such as tuna, billfishes and sharks, are a major component of the top predator guild in oceanic ecosystems, but the trophic relationships of many populations remain poorly understood. Aims This study aimed to characterise the trophic ecology and habitat use of 10 HMS collected from two subtropical oceanic regions around Aotearoa–New Zealand. Methods We examined stable isotope ratios (δ13C and δ15N) in muscle tissue of HMS considering differences in capture location and body size. Key results Three trophic groups were distinguished: (1) swordfish, (2) tuna and Lamna nasus (porbeagle) in northern New Zealand and (3) marlins and other pelagic sharks. Despite stable isotope overlap among taxonomically similar species, subtle differences in foraging strategies likely contribute to variation in ecological roles. Trophic position estimates revealed that HMS occupy high trophic levels, though these estimates may be influenced by variations in δ15N baseline. Stable isotope analysis indicated consistent trophic relationships across the two investigated oceanic regions, with evidence of ontogenetic shifts in resources use in some species. Conclusions Stable isotope ratios varied widely among HMS, suggesting exploitation of diverse foraging habitats and extensive migration. Implications These data highlight the diverse trophic roles of HMS and emphasise their significance in structuring marine food webs and ecosystem functioning.

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