The biology and ecology of anadromous brown trout (Salmo trutta) at sea is poorly understood. This study provided information on spatial and temporal distribution of sea trout in the ocean. The behaviour of 115 individuals (veteran migrants, 270–700 mm) was tracked by using acoustic telemetry in a fjord system during April–September in 2012–2013. Overall, fish spent 68% of their marine residence time close to river mouths (<4 km). Most fish registrations (75%) were in nearshore habitats, but pelagic areas were also used. The maximum migration distance of tagged fish was categorized as short (<4 km from river mouth, 40% of fish), medium (4 – ∼13 km, 18% of fish), or long (>∼13 km, 42% of fish). Long-distance migrants had poorer body condition in spring prior to migration, used pelagic areas more often, and returned earlier to fresh water than short- and medium-distance migrants. Marine residence time was 7–183 days and was positively correlated to body length and smolt age, but negatively correlated to the date of sea entry.

The brown trout (Salmo trutta) is an iteroparous, anadromous salmonid that exhibits a complex continuum of feeding migration tactics, ranging from freshwater residency, to potamodromy, to estuarine migration, as well as short- to long-distance coastal migrations. While anadromous migrants are believed to play an important role in the species’ population dynamics, little is known about the factors driving differences in the extent of individual marine habitat use. In this study, 32 brown trout veteran migrants were acoustically tagged prior to their seaward migration and sampled for indices of their nutritional state. Our findings suggest that (i) body condition factor differed among fish adopting different migratory tactics, with outer fjord migrant being in poorer condition; and (ii) within migratory groups, plasma triglyceride concentration was negatively correlated with the duration of marine residency. Results support the idea of condition-dependent migration in veteran migrants, with individual variation in nutritional state influencing the spatiotemporal aspects of marine habitat use. Furthermore, overall marine minimum survival during the summer feeding migration was 86%, the highest reported estimate for this life stage.

The early migration and habitat use of brown trout Salmo trutta post-smolts tagged with acoustic transmitters (n = 50) were investigated in a fjord system in central Norway from 30 April to 26 November 2014. The main aims were to investigate return rate, marine residence time and spatial use of the fjord system. Median seaward migration and return to fresh water dates were 22 May and 4 July, respectively. Of the 40 seaward migrating smolts, 26 returned to fresh water, giving a minimum return rate to fresh water of 65%. Entrance to the fjord from the river occurred mainly at night (80% of the S. trutta), however, no such diurnal pattern was observed during the return migration. Mean marine residence time was 38 days, but with large individual variation (22-99 days). The innermost parts of the study area were more utilized than the outer part of the fjord system during the sea residency, and with more use of the near shore habitat than the open, pelagic areas. Many post-smolts also utilized the outer part of the fjord system, however, and 94% of the post-smolts were recorded at least 14 km from the home river mouth. Marine survival and distribution in the fjord were size dependent with the largest individuals utilizing outer fjord areas and having higher return rates to fresh water. As far as is known, this is the first published study on temporal and spatial behaviour in the marine environment of first-time S. trutta migrants during the full course of their first trip to sea.

The vertical behaviour of 44 veteran sea trout Salmo trutta (275-580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May-February by acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day v. night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May-September was 1·7 m (individual means ranged from 0·4 to 6·4 m). Hence, S. trutta were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May-September was deeper in the near-shore habitats with or without steep cliffs (2·0 m and 2·5 m, respectively) than in the pelagic areas (1·2 m). May-September mean swimming depth in all habitats was slightly deeper during day (1·9 m) than at night (1·2 m), confirming that S. trutta conducted small-scale diel vertical movements. During summer, S. trutta residing in near-shore habitat progressively moved deeper over the period May (mean 1·1 m) to August (mean 4·0 m) and then reoccupied shallower areas (mean 2·3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean 1·3 m). The swimming depths of S. trutta coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of S. trutta, this study illustrates how S. trutta utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of S. trutta and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes.

The aim of this study was to test the hypothesis that hatchery brown trout Salmo trutta smolts, with 50% reduced or no feeding over the last 5 months before release, were more likely to migrate to the sea than individuals with standard feeding ratios. The juvenile fish were divided into three groups 176 days before release: (A) with no feeding, (B) with 50% and (C) with 100% feeding. To study their seaward migration, 40 fish from each feeding group were tagged with acoustic transmitters and tracked by automatic listening stations in the River Nidelva, Trondheim, Norway, its estuary and in the nearest marine environment. At the time of release, mean condition factor was significantly lower in group A and the fish from groups A and B had higher levels of Na+, K+-ATPase. Significantly more fish from group A migrated to the sea, but the rate of downstream progression from release to the estuary did not differ between the three groups. In conclusion, the S. trutta smolts with no access to food in the last 176 day before release were more likely to migrate to the sea. Fish from all three feeding groups, however, appeared to smoltify and had the same rate of downstream progression to the estuary. This indicates that differences in migratory behaviour between individuals from the three feeding groups begin from the time when the fish reach saline waters. It is suggested that feeding in hatcheries has to be greatly reduced (by 50% or more) over several months to have a pronounced effect on the migratory behaviour in S. trutta.