Evolution of artificial spawning sites for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta): field studies and numerical modelling in Aurland, Norway

C Hauer,P Flödl,U Pulg,F Reisinger

doi:10.1007/s10750-019-04173-1

Abstract

The presented study investigates the evolution of artificial gravel placements for Atlantic salmon and sea trout in Aurlandselva in Western Norway. Various monitoring methods have been applied including (i) quantifying the spatial extent and dynamics of spawning sites over the monitoring period, (ii) grain size distributions as well as (iii) applying numerical hydraulic and sediment transport modelling with the aim to test the predictability of such numerical tools. The spawning sites were not clogged by fine sediments, but were reshaped due to scouring and sediment transport. The scouring resulted in a volume loss of the gravel banks between 32 and 95% in the monitoring period of 5 years. The application of hydrodynamic-numerical modelling, however, showed that the modelling methods were not sufficient to predict erosion of the gravel or the site. The study showed that the areas are sensitive especially to local scale micro-topographical roughness elements. The complex three-dimensional hydraulic processes and the coarse substrate in the non-fluvial river environment makes it impracticable for multi-dimensional modelling to predict dynamics of gravel. A novel sediment criterion was introduced to estimate the near-bottom turbulence by relating the dm of introduced gravel compared to the d90 of the bed surface substrate composition.

Highlights

Spawning sites for salmonids are one of the key habitats in the life cycle of the fish (Rosenfeld, 2003; Jonsson & Jonsson, 2011) and may function as a bottleneck for the entire population in terms of (a) quantity (e.g. Armstrong et al, 2003; Bjorn & Reiser, 1991; Barlaup et al 2008) or (b) quality (e.g. Gregory & Bisson, 1997, Pulg et al, 2013)
The results are presented under the line of three topics: (i) the mapping of spawning sites including quantification of erosion rates as well as prediction of life time characteristics, (ii) the analysis of the grain size distribution at spawning sites and (iii) the prediction of morphodynamics based on the twodimensional depth-averaged numerical model
The restoration of spawning habitats by artificial gravel placement is an important measure to overcome the bottleneck of salmonid fish reproduction in regulated rivers of Western Norway (Barlaup et al, 2008; Fjeldstad et al, 2012; Pulg et al, 2018)

Summary

Introduction

Spawning sites for salmonids are one of the key habitats in the life cycle of the fish (Rosenfeld, 2003; Jonsson & Jonsson, 2011) and may function as a bottleneck for the entire population in terms of (a) quantity (e.g. Armstrong et al, 2003; Bjorn & Reiser, 1991; Barlaup et al 2008) or (b) quality (e.g. Gregory & Bisson, 1997, Pulg et al, 2013). One driver beside gravel movement/armoring by floods for the degradation of spawning habitats are often fines infiltrating the gravel matrix and clogging pore space (Soulsby et al, 2001; Pulg et al, 2013). Fine sediment infiltration yields reduced oxygen supply at spawning sites (Lisle & Lewis, 1992), resulting in a range of physiological impacts including reduction in weight and length, morphological adaptations (extended yolk sac), and hypoxia (Kemp et al, 2011). The challenging components for the occurrence of salmonid spawning sites in Western Norway are rather high-gradient rivers and erosion of gravel under supply-limited geological conditions (Hauer et al, 2013, 2015). River regulation has partly reduced gravel supply by reducing flow dynamics, riverbed and bank stabilization and dams (Barlaup et al, 2008; Forseth et al, 2017)

Methods

Results

Discussion

Conclusion