Sea trout (Salmo trutta) migratory patterns in a large and complex fjord: comparing old mark-recapture and recent acoustic telemetry studies in Sognefjorden, Norway

Sammendrag

Declining sea trout (Salmo trutta) populations in the western part of Norway has been documented in recent years. The most probable reasons for this decline seem to be linked to the marine life stage. The Norwegian fjords constitute the marine habitat of many sea trout populations, and the physical features of large depths and long distances to the coastal habitats may require behavioural adaptations to optimize growth and survival. River Laerdalselva, in the innermost part of the very large Sognefjord, is renowned for its population of large sea trout. Historical data on migratory patterns during the marine phase from mark-recapture studies conducted in River Laerdalselva from 1950 to 1965 were Re-examined and compiled. From a total number of 460 (280 post spawners and 180 smolts) externally ID tagged fish, recaptures in the sea constituted 8 and 46% of smolts and adults, respectively. This illustrates a high fisheries-related mortality. The two main areas of recaptures were the innermost 50 km of the fjord, and outer fjord/coastal areas <150 km distance from the river. This indicates that a large proportion of the population migrated very long distances, leaving the fjord system. New data on the River Laerdalselva sea trout were obtained from 2008-2010 using acoustic transmitter tagged smolts (N=40) and adults (N=31). Due to physical constraints (>1000 m depths), a widely spaced receiver grid was used. The data obtained showed a similar pattern to the mark-recapture studies, with large migration distances and very few fish utilizing the innermost parts of the fjord close to the river for feeding. The major difference between the datasets was a lack of registrations in the fjord mouth/coastal regions in the 2008-2010 data. This may indicate alterations in migratory patterns between the two periods. The acoustic telemetry data also provided information on migratory timing from the river. Adult fish migrated in March-April at low water discharge, whereas smolts seemed to migrate later (May-June) on distinct high water discharge periods.