Climate Change Effects on Fish Passability across a Rock Weir in a Mediterranean River

Daniel Mameri,José Maria Santos,Maria Teresa Ferreira,Stefan Schmutz,Rui Rivaes

doi:10.3390/w13192758

Daniel Mameri, José Maria Santos + Show 3 more

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https://doi.org/10.3390/w13192758

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Abstract

Climate change represents a major challenge for the management of native fish communities in Mediterranean rivers, as reductions in discharge may lead to a decrease in passability through small barriers such as weirs, both in temporary and perennial rivers. Through hydraulic modelling, we investigated how discharges from a large hydropower plant in the Tagus River are expected to affect the passability of native freshwater fish species through a rock weir (Pego, Portugal), equipped with a nature-like fish ramp. We considered not only mean daily discharge values retrieved from nearby gauging stations (1991–2005) for our flow datasets, but also predicted discharge values based on climatic projections (RCP) until the end of the century (2071–2100) for the Tagus River. Results showed that a minimum flow of 3 m3 s−1 may be required to ensure the passability of all species through the ramp and that passability was significantly lower in the RCP scenarios than in the historical scenario. This study suggests that climate change may reduce the passability of native fish species in weirs, meaning that the construction of small barriers in rivers should consider the decreases in discharge predicted from global change scenarios for the suitable management of fish populations.

Highlights

Rivers have long been among the most endangered ecosystems worldwide, facing multiple threats including the introduction and dispersal of invasive species [1,2], chemical [3] and thermal pollution [4], flow regulation [5,6], longitudinal fragmentation [7] and climate change [8], with the later acting as an enhancer of the previous ones [9,10]
This study suggests that climate change may reduce the passability of native fish species in weirs, meaning that the construction of small barriers in rivers should consider the decreases in discharge predicted from global change scenarios for the suitable management of fish populations
The main goal of this study is to evaluate the passability of the low-head Pego weir, in the Tagus River, Central Portugal, to the different migratory fish species, in relation to historical (1991–2005) and future flow conditions based on two global warming scenarios (RCP 4.5 and RCP 8.5), following the reports of ISI-MIP: Inter-Sectoral Impact Model Intercomparison Project [38]

Summary

Introduction

Rivers have long been among the most endangered ecosystems worldwide, facing multiple threats including the introduction and dispersal of invasive species [1,2], chemical [3] and thermal pollution [4], flow regulation [5,6], longitudinal fragmentation [7] and climate change [8], with the later acting as an enhancer of the previous ones [9,10]. Fish movement and migration may be further limited by the presence of small barriers such as weirs, which are generally far more numerous than large dams and clearly represent significant barriers to fish migration [17,18] These instream structures change the depth and velocity patterns, creating vertical drops that change the hydrodynamics of aquatic systems and may prevent the movement of migratory fish to spawning, feeding and refuge areas [19], and their permeability should be assessed for a proper management of these populations [20,21]. The passability of a given fish species and life history stage through weirs will depend on key hydraulic variables, namely flow velocity and water depth [23], that structure habitat suitability [24]

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