Migration behaviour and loss rate of trout smolts in the transitional zone between freshwater and saltwater

Abstract

Migration between freshwater nursery grounds and saltwater feeding areas is a critical event in sea trout life history. During their seaward migration, smolts encounter both natural and man-made obstructions as well as increased exposure to predators. This is particularly true in the transition zone between freshwater and saltwater where smolts enter a new environment, change their behaviour and encounter new predators. Over two years (2013-14), 81 trout smolts were trapped and acoustically tagged 17 km upstream of the tidal limit in the River Frome in Dorset, UK. Smolt migrations were then tracked by acoustic receivers deployed throughout the lower river and its estuary and the detected movements were used to estimate loss rate and migration behaviour. A Bayesian State Space model was applied to separate detection and transition probabilities. More than 90% of the in-river detections occurred at night whereas detections at the saline limit and throughout the estuary were spread evenly between day and night. Median migration speed in the river was 65-70 km day^-1 in both years, whereas the migration speed was slower through the estuary with median speeds of less than 10 km day^-1. The loss rate was similar across the study zones (range 0.5-1.1 % km^-1); hence there was no pronounced increase in mortality in any particular part of the transition zone as reported by a number of similar studies from other systems. Even though no individual study zone displayed a particularly elevated loss rate, the cumulative loss of tracked trout smolts through the 33 km section was 24%, demonstrating a significant cost in terms of loss associated with smolt migration.