Abstract
In freshwater ecosystems, water temperature and discharge are two intrinsically associated triggers of key events in the life cycle of aquatic organisms such as the migration of diadromous fishes. However, global changes have already profoundly altered the thermal and hydrological regimes of rivers, affecting the timing of fish migration as well as the environmental conditions under which it occurs. In this study, we focused on Atlantic salmon (Salmo salar), an iconic diadromous species whose individuals migrate between marine nursery areas and continental spawning grounds. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and both salmon juvenile downstream and adult upstream migrations in three French rivers (the Bresle, Oir and Nivelle rivers). While all three rivers have gradually warmed over the last 35 years, changes in discharge have been very heterogeneous. Juveniles more frequently used warmer temperatures to migrate. Adults migrating a few weeks before spawning more frequently used warm temperatures associated with high discharges. This has already led to modifications in preferential niches of both life stages and suggests a potential mismatch between these populations' ecological preference and changes in their local environment due to global change.
Highlights
Most life cycle events of species are synchronized by abiotic factors, most of which are affected by global change [1,2,3]
The interaction between global climate change and local anthropogenic pressures has altered thermal and hydrological regimes of many rivers throughout the world. This may have altered the occurrence of water temperature and discharge associations favourable to the migration of aquatic organisms and resulted in changes in migration timing, adaptation to new environmental conditions or species extirpation [5,14,15]
To detect changes in the environmental conditions used for migration, i.e. the effective niche, we carried out a second Choc analysis in which daily water temperature × discharge associations were weighted by their corresponding daily proportions of migrating salmon during the kernel estimation stage
Summary
Most life cycle events of species are synchronized by abiotic factors, most of which are affected by global change [1,2,3]. Urbanization intensive agriculture low-impact agriculture natural areas time period number of years excluded analysed water temperature (°C) yearly winter spring summer autumn discharge (m3 s−1) yearly winter spring summer autumn smolts migrating downstream total number annual average spawners migrating upstream total number annual average. To detect changes in the environmental conditions used for migration, i.e. the effective niche, we carried out a second Choc analysis in which daily water temperature × discharge associations were weighted by their corresponding daily proportions of migrating salmon during the kernel estimation stage. To characterize the preferential niche for smolts and spawners (i.e. water temperature and discharge associations preferentially selected by salmon to migrate regardless of the frequency of conditions observed), we built an electivity index based on the Ivlev feeding electivity index [43]. Percentages of significant changes in the occurrence of water temperature × discharge associations in the three niches are provided in the electronic supplementary material, table S1
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
