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.

KEYWORDS:

• Marine subsidies
• trophic interactions
• longfin eels
• brown trout
• īnanga
• coastal lagoon
• stable isotope

Acknowledgements

We thank John Hayes (Cawthron Institute) for constructive feedback and assistance with bioenergetics modelling to inform growth rates and Department of Conservation staff for assistance with data collection. Anna Sintenie (Environmental Law Initiative) provided constructive discussion and feedback on policy aspects. Lisa Floerl developed the map for Figure 1. Thank you to Julie Brown, Josette Delgado and Alexia Saint-Macary for sample preparation and stable isotope analysis. Gretchen Rasch gave editorial support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

SDS and RH were supported by the ‘Fish Futures’ MBIE Endeavour Program [grant number CAWX2101] and YV was supported by the US Fulbright Foundation, Fulbright New Zealand. Data collection and analysis used in this study was supported by the Living Waters Initiative.