Influence of flow on migratory behaviour of salmon smolts

A study on the effect of flow velocity on the downstream migration of smolts, i.e. juvenile salmon and trout, in a major northern Swedish river, Ume älv, is presented. Few studies have been done on Swedish smolt migratory behaviour. This study comprises telemetric studies of downstream migrating smolts and flow velocity measurements in field. To achieve more comprehensive information about the flow field numerical simulations are carried out. This is done by solving the three dimensional Reynolds-averaged Navier-Stokes equations using the Fluent computational fluid dynamics (CFD) package. The field measurements are also used to validate the numerical flow simulation. The river Ume älv, where the study is preformed, is fully used for power production. However, its largest tributary river, Vindelälven, is unregulated and holds important spawning grounds for salmon (Salmo salar) and trout (Salmo trutta), and downstream of the confluence of the two rivers there is only one hydropower plant, the Stornorrfors power plant. This constitutes a major hindrance for fish migrating between the Bothnian Sea and the spawning grounds of the river Vindelälven. The juvenile fish leaving the Vindelälven downstream towards the ocean can migrate either through the turbines and deep rock tunnels of the power plant or through the surface weirs at the regulating dam. Because of the flow regime during the migration period of smolts most smolts are believed to follow the main stream through the turbines leading to a mortality of approximately 25 %. The objective of the study is to determine the migration paths of salmon and trout smolts and relate their behaviour to hydraulic factors. The results from the study can be used to improve smolt survival by taking appropriate measures to divert the smolts from the turbines, e.g. by changing spill rules or installing diversion systems. In spring 2002 22 salmon and 22 trout smolts were tagged with internal, individually coded radio transmitters (ATS). After the release approximately five kilometres upstream of the dam at Stornorrfors power plant their passages were registered at three plantary loggers and their movements tracked manually along their swimming paths. Simultaneously, flow velocities were measured in three dimensions using an Acoustic Doppler Profiler (ADP). Results from the tracking, flow measurements and flow simulation indicate that the smolts move along the main stream of the river. Since the tracking data only yield information on the smolt positions in the horizontal plane it cannot directly be concluded whether the smolt migration is passive or active. However, results from American studies on local salmon have shown that smolt migration occurs in the upper part of the flowing water. Combined with the fact that the water velocities in the upper part of the water column along the smolts’ migration paths are close to the mean transport velocity of the smolts it indicates that the downstream migration is passive. This study is part of a more extensive project that comprises studies of both upstream and downstream migration of fish. The objective of the project is to study the effect of flow hydraulics on fish migratory behaviour