Partial migration in diadromous fishes drives the allocation of subsidies across the freshwater-marine ecotone

Grégoire Saboret,Michael M Douglas,Alison J King,David A Crook,Duncan J Buckle

doi:10.1515/ami-2020-0108

Grégoire Saboret, Michael M Douglas + Show 3 more

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https://doi.org/10.1515/ami-2020-0108

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Abstract

Abstract Migratory animals can act as cross-boundary subsidies sustaining ecosystem functioning, such as diadromous fishes that migrate between fresh water and seawater and carry nutrients and energy across the freshwater-marine ecotone. Frequency and timing of migration are however highly variable within and among populations. We hypothesized that in catadromous fishes (i.e., diadromous fishes that grow in freshwater and spawn in the sea, such as eels), the import of subsidies by migratory juveniles could outweigh the export of subsidies by adults due to skipped spawning migration. We used the diamond mullet Planiliza ordensis, as a model species, and determined life-history traits using a combination of length-to-age data, acoustic telemetry and otolith (fish ear stone) microchemistry. We used a mass balance approach to model individual mass acquisition and allocation, and extended our model to other life-history strategies. Our results showed high intra-population variation of migratory behaviour in P. ordensis, with few individuals migrating every year to spawn. We estimated that an individual P. ordensis acted as a net 42.6g biomass subsidy in fresh water, representing a retention of more than 50% of the juvenile mass at freshwater entry. Our model predicts that skipped spawning is likely to alter the allocation of subsidies in diadromous species, highlighting the important effects of individual variation in migratory behaviour on fluxes of energy and nutrient at ecosystem scales. We encourage future studies to consider how variation in migratory behaviour is likely to affect the direction and magnitude of biomass fluxes across ecotone boundaries.

Highlights

IntroductionMost ecosystems are recipients of allochthonous resources (i.e. not originating in the region where they are found), such as nutrient, organic matter and prey, that enhance in situ productivity [1,2]
Most ecosystems are recipients of allochthonous resources, such as nutrient, organic matter and prey, that enhance in situ productivity [1,2]
In the Daly River (DR), the seasonal oscillations in 87Sr/86Sr ratio between 0.72 and 0.73 matched with annual increments and 87Sr/86Sr in between the primordium and the first increment were very close to the marine value of 0.70907 [67], confirming marine residence in the juvenile phase and catadromy (e.g. Fig. 2)

Summary

Introduction

Most ecosystems are recipients of allochthonous resources (i.e. not originating in the region where they are found), such as nutrient, organic matter and prey, that enhance in situ productivity [1,2]. Recent theoretical and empirical studies suggest that low to moderate allochthonous inputs (or ‘subsidies’) can stabilize trophic dynamics, defined as the movement of carbon, nutrients, and energy among organisms in an ecosystem [3,4]. The transfer of large amounts of organic matter from agricultural activity into natural ecosystems can fundamentally alter trophic dynamics: trophic cascades can be initiated and rare or uncommon species can become invasive while functionally important common species decline [9]. Such examples demonstrate the sensitivity of ecosystems to

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