Temporal shifts in stable isotopes tracks climate variability in a benthic foraging marine predator

Understanding how top-order predators respond to environmental variability is essential for predicting the resilience of marine ecosystems under climate change. We investigated long-term trophic niche dynamics in 115 adult female Australian fur seals (Arctocephalus pusillus doriferus) by analysing δ¹³C and δ¹⁵N values from 4187 whiskers subsamples collected between 2010 and 2023. Using wavelet analysis derived growth rates, we temporally aligned isotopic data over a 22-yer period and applied trophic niche metrics to quantify individual specialisation and population-level variation. Isotopic trends revealed a consistent increase in δ¹³C and decrease in δ¹⁵N values, with significant associations to lagged values of broad-scale climate indices (SOI, IOD, SAM). However, these environmental drivers did not explain niche width and individual specialisation. A marked shift was observed after-2015 in the niche dynamics coinciding with the strongest El Nino event for the area. These changes suggest that individuals broadened their diets within a narrowing resource base, potentially in response to heightened intra-specific competition, altered prey availability or changes in the environmental baseline. Our findings highlight the importance of integrating long-term isotopic data with niche metrics to detect subtle ecological responses to climate variability. While AUFS females display some buffering capacity, continued environmental change may challenge their foraging flexibility and impact population resilience.