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Ibbotson AT, Beaumont WRC, Pinder A, Welton S, Ladle M. Diel migration patterns of Atlantic salmon smolts with particular reference to the absence of crepuscular migration.Abstract -The real-time diel pattern of Atlantic salmon smolt migration was observed for 8 years using automatic resistivity counters verified by video surveillance. A clear dominant nocturnal migration was demonstrated 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, such that when mean daily temperatures were below 12°C, hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12°C, 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.

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Microthermal gradients, involving signi®cant variations in temperature over distances of a few centimetres to a few metres, were investigated in the water column and substratum of the River Frome and Bere Stream in Dorset, UK, which are groundwater dominated streams fed by chalk aquifers. In many of the sections surveyed, strong lateral contrasts of up to c. 7 8C were evident as a consequence of solar heating of shallow channel margin zones and thin surface layers isolated by¯oating vegetation from the main body of¯ow. Shading by instream, emergent and riparian vegetation, and by river banks also caused signi®cant microthermal gradients in the water column. Detailed logging of temperatures in the substratum at selected sites revealed damping of variation with increasing depth below the bed surface, seasonal reversal in bed temperature gradients and considerable local variation in the substratum temperature pro®les of a pool-rie sequence. The latter did not conform to the pattern expected from advective heat transfer associated with downwelling of water at the rie head and upwelling at the tail, and measurements of interstitial¯ow velocities and particle size suggested more complex¯ow circulation and heat transfer. There was some evidence that the microthermal gradients identi®ed were of ecological signi®cance.