Sentinels of change: divergence in trophic niche of New Zealand sea lions and fur seals from first human contact to today

New Zealand sea lions Phocarctos hookeri were once widespread on New Zealand’s coasts but were hunted to remnant populations in the Subantarctic islands during expansion of European settlements. In contrast, New Zealand fur seals Arctocephalus forsteri maintained their widespread distribution, persisting at remote colonies. The resource niche positions of these species provide insights on structure of underlying food webs and contribution of alternate sources of organic matter to ecosystem function. We measured changes in their resource niche positions with stable isotope analysis (δ¹³C, δ¹⁵N and δ¹⁵N_AA) of bone collagen from first Māori settlement (1250-1450 CE), middle European expansion (1650-1850 CE) and modern (2016 CE) time periods. Samples from each time period were collected from the Subantarctic islands, central region and from reestablishing sea lion populations on the mainland South Island. In the Subantarctic islands, high trophic positions were maintained through time but divergence in the resource niche of the 2 species between pelagic and benthic food webs occurred in the modern time period, consistent with resource limitation. In the central and mainland regions, both species declined in trophic position during the modern time period. In the central region, the 2 species maintained distinct resource niche discrimination while in the mainland region, they converged in the modern time period, consistent with high overlap on a reduced diversity of low trophic level pelagic prey. These patterns provide a unique view of the long-term changes in trophic relationships between marine top predators during the full history of human occupation in New Zealand.