Whilst the adult salmon counter on the River Frome at East Stoke provides vital information regarding the spawning stock of adult Atlantic salmon ascending the river, the ability to count smolts descending the river facilitates an early indication of the future state of the salmon stocks and allows the partitioning of mortality between the freshwater and marine life-phases. Whilst this information is vital for the conservation management of the species relatively few rivers have the facilities to carry out this monitoring. Where facilities do exist they are usually based on mechanical trapping.
This may disrupt the natural migration pattern and requires handling the fish (with consequential danger of increasing mortality). For this reason, we have developed the first known ‘hands-off’ counting system for smolts. This involves using acoustic bubble screens to deflect them from the main river into a millstream and through tubes containing resistivity counters. The smolts then pass by a window through which time-lapse video recording is used to distinguish between salmon smolts, sea trout and other coarse fish (dace, roach etc of comparable size). The video recordings also act as a check on the accuracy of the automatic counters.
The real time diel pattern of Atlantic salmon smolt migration shows a clear dominant nocturnal migration early in the migration period, later becoming increasingly diurnal, until rates became approximately equal at day and night. Migration patterns were related to water temperature, and when mean daily temperatures were below 12ºC hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12ºC there was no significant difference between diurnal and nocturnal migration rates.
Migration patterns showed a distinct suppression of migration at dawn and dusk throughout the migration period. It is hypothesised that this behaviour is an active decision and/or an adaptive strategy either to take advantage of increased food in the form of invertebrate drift or to reduce predation risk from actively feeding piscivores or both.