Sea to the mountains: quantifying freshwater eel and trout diet reliance on marine subsidies from upstream migrating fish

ABSTRACT Despite the Southern Hemisphere’s high proportion of endemic diadromous fish, the trophic role of upstream migrating forage fish in freshwater ecosystems remains unquantified. We combined food web δ13C and δ15N analyses with fish biomass estimates to quantify estuarine resource contribution (from the ecologically significant Waituna lagoon) to the diet and growth of the apex predators, longfin eels (Anguilla dieffenbachii) and brown trout (Salmo trutta) in Waituna Creek, Southland, Aotearoa (New Zealand) during 2019 and 2021. Our results demonstrated that exceptionally high biomasses of large predatory fishes (610–830 kg km−1) were maintained in a degraded agricultural catchment through connectivity to a near-pristine estuarine lagoon. Īnanga (Galaxias maculatus) migrating upstream from the lagoon supported 60%–80% of longfin eel and 40%–90% of brown trout biomass over the two years sampled. Mass-balance modelling suggested that large predatory fish in the Waituna Creek consume about two tonnes of migrating whitebait annually. These findings highlight the wider role of estuarine/marine subsidies in structuring New Zealand freshwater food webs and challenge the orthodoxy of ‘restoring streams from the headwaters down’. Estuarine ecosystems may be more important than headwater areas for sustaining large predatory fish, such as longfin eels, when they are food limited.