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.

摘要

尽管南半球的地方性水生鱼类比例很高，但上游洄游饲料鱼在淡水生态系统中的营养作用仍未量化。我们将食物网 δ ¹³ C 和 δ ¹⁵ N 分析与鱼类生物量估计相结合，以量化河口资源（来自具有重要生态意义的 Waituna 泻湖）对顶端捕食者、长鳍鳗鱼 ( Anguilla dieffenbachii ) 和褐鳟 ( Salmo ) 的饮食和生长的贡献trutta ) 在 2019 年和 2021 年期间在新西兰奥特阿罗阿 (新西兰) 南部的 Waituna Creek。我们的结果表明，大型捕食性鱼类 (610–830 kg km ^{-1 ) 的生物量异常高}) 通过与近乎原始的河口泻湖相连，维持在退化的农业集水区。在两年的采样期间，从泻湖向上游迁移的Īnanga ( Galaxias maculatus ) 支持了 60%–80% 的长鳍鳗鱼和 40%–90% 的褐鳟生物量。质量平衡模型表明，怀图纳河中的大型掠食性鱼类每年消耗大约两吨洄游银鱼。这些发现突出了河口/海洋补贴在构建新西兰淡水食物网中的更广泛作用，并挑战了“从源头向下恢复溪流”的正统观念。当食物有限时，河口生态系统可能比源头区域更重要，因为它们可以维持大型捕食性鱼类，例如长鳍鳗鱼。