The downstream migration success of Atlantic salmon (Salmo salar) smolts through natural and impounded standing waters

Atlantic salmon (Salmo salar) smolt migration through standing waters is generally known to be associated with slow migration speed and low success. Most previous studies have however been conducted on impounded waters. The bathymetry and specifically the flow dynamics around the outlets differ markedly between impounded lakes and naturally draining lakes. To date no study has attempted to disentangle the effects of the impoundment from that of standing water itself. This question was tested in a single naturally draining standing water and two impounded standing waters within a single catchment using fish tagged with acoustic and PIT tags (in three and two lakes, respectively). Overall migration success (river to estuary) was very low (10%); migration success through the lakes was also very low (total loss rates range: 31 to 55%; 16 to 53%.km(-1)) compared with migration in the river downstream of the lakes (range: 3.9 to 10.8%.km(-1)). Migration success was no lower in impounded lakes compared with the naturally draining lake. 49% of directional movements were in a direction opposite to the migration pathway, indicating that a lack of navigational cues may be in-part responsible for low migration success. We tentatively conclude that successful migration through standing waters without strong directional cues may be more stochastic than previously thought. We suggest that at least for some impoundments (as shown here), the impoundment per se may not reduce migration success, its greater effect being the creation of the standing water through which smolts are required to migrate. However, any impoundment effect is likely to be dependent upon a combination of lake basin shape, the exit route from the lake and the navigational cues available to smolts.