Defining the water flow cues for navigation in migrating Atlantic salmon smolts

For migratory species, successful navigation is critical to fitness. In Atlantic salmon, for example, there is evidence that during migration from natal streams to the sea, passage through waters with poorly defined or mixed water velocity patterns may constrain directional navigation, causing individuals to become trapped or delayed in lakes or other bodies with slowly flowing water. In this study, we determined the minimum water velocities needed to elicit a behavioural response, in this case a change in the direction of holding position, in both wild origin and domesticated salmon smolts. Smolts required a directional flow in excess of 8.9 cm s^-1 to exhibit effective directional orientation towards the current. Smolts of a domesticated farm origin exhibited a similar qualitative and quantitative response as wild fish. These results suggest that, in areas where the downstream migrating Atlantic salmon smolts pass through low-directional water flow, it may be possible to manipulate directional flows above this minimum threshold, at least temporarily, as a management tool to increase migration success. This is likely to be particularly true where smolts are passing through dams, reservoirs or other impounded waters.