Marine depth use of sea trout Salmo trutta in fjord areas of central Norway.

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 first months at sea: marine migration and habitat use of sea trout Salmo trutta post-smolts.

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.

Long-term hypo-osmoregulatory capacity in downstream migrating Atlantic salmon Salmo salar L. smolts.

The duration of hypo-osmoregulatory capacity in downstream migrating Atlantic salmon Salmo salar L smolts previously stocked as startfed young-of-the year (YOY) parr was tested in the River Dalåa from mid-May to late-June 1999. Hypo-osmoregulatory capacity, measured as plasma osmolality and chloride, was assessed after seawater (SW) challenge tests (168 h, salinity = 35). All S. salar exhibited sufficient hypo-osmoregulatory capacity at the initiation of downstream migration in mid-May. Migrating S. salar smolts caught in mid-May and retained in fresh water displayed no signs of de-smoltification as they maintained hypo-osmoregulatory capacity through June. This indicates a physiological smolt window that lasts a minimum of 6 weeks (330 degree days; D°) for hatchery-produced S. salar smolts stocked as YOY parr. Based on the observed river migration speeds, it can be assumed that the S. salar smolts entered SW 2-4 weeks after initiation of migration in the upper parts of the River Dalåa. Hence, based on smolt migration and SW tolerance, it is suggested that stocking of YOY parr is a viable enhancement strategy in the River Dalåa.

Does reduced feeding prior to release improve the marine migration of hatchery brown trout Salmo trutta smolts?

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.

Riverine and near coastal migration performance of hatchery brown trout Salmo trutta.

To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, LF , 158-288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome-lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in LF or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 LF s(-1) but decreased significantly as the smolts entered the estuary (0·015 LF s(-1) ). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.

Development of seawater tolerance and subsequent downstream migration in wild and stocked young-of-the-year derived Atlantic salmon Salmo salar smolts.

This study investigated the development of hypo-osmoregulatory capacity and timing of downstream migration in wild Atlantic salmon Salmo salar smolts from the River Stjørdalselva and stocked young-of-the-year (YOY), derived S. salar smolts from the tributary River Dalåa. Both wild and stocked S. salar smolts developed seawater (SW) tolerance in early May, persisting through June, measured as their ability to regulate plasma osmolality and chloride following 24 h SW (salinity = 35) exposure. Although the majority of downstream migration among the stocked S. salar smolts occurred later than observed in their wild counterparts, the development of SW tolerance occurred concurrently. The wild S. salar from Stjørdalselva and stocked YOY smolts from the River Dalåa started to migrate on the same cumulative day-degrees (D°). The study revealed no downstream migration before development of SW tolerance. This emphasizes the importance of incorporating physiological status when studying environmental triggers for downstream migration of S. salar smolts. Overall, these findings suggest that the onset of smolt migration in stocked S. salar smolts was within the smolt window from an osmoregulatory point of view.

Seawater tolerance and post-smolt migration of wild Atlantic salmon Salmo salar × brown trout S. trutta hybrid smolts.

High levels of hybridization between Atlantic salmon Salmo salar and brown trout Salmo trutta have been reported in the River Driva. This study presents the underlying mechanisms of development of seawater (SW) tolerance and marine migration pattern for S. salar×S. trutta hybrids. Migrating S. salar×S. trutta hybrid smolts caught in the River Driva, Norway (a river containing Gyrodactylus salaris), displayed freshwater (FW) gill Na(+), K(+) -ATPase (NKA) activity levels of 11·8 µmol ADP mg protein h(-1), which were equal to or higher than activity levels observed in S. salar and S. trutta smolts. Following 4 days of SW exposure (salinity 32·3), enzyme activity remained high and plasma ion levels were maintained within the normal physiological range observed in S. salar smolts, indicating no signs of ion perturbations in S. salar×S. trutta hybrids. SW exposure induced an increase in NKA α1b-subunit mRNA levels with a concurrent decrease in α1a levels. Salmo salar×S. trutta post-smolts migrated rapidly through the fjord system, with increasing speed with distance from the river, as is often seen in S. salar smolts. The present findings suggest that S. salar×S. trutta smolts, as judged by the activity and transcription of the NKA system, regulation of plasma ion levels and migration speed more closely resemble S. salar than S. trutta.

Seawater tolerance in Atlantic salmon, Salmo salar L., brown trout, Salmo trutta L., and S. salar × S. trutta hybrids smolt.

High levels of hybridization between Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) have been reported in the Gyrodactylus salaris infected Rivers Vefsna and Driva in Norway. The survival and behaviour during the sea phase of such hybrids is unknown. The reported work documents ionoregulatory status after 24 h seawater challenge tests (24hSW) and gill Na+/K+-ATPase (NKA) activity of migrating wild smolts of Atlantic salmon, brown trout and hybrids at two sampling dates during the 2006 smolt run in River Driva. Salmon, trout and hybrids contributed to 27, 52 and 21% of the catches, respectively. The large contribution of hybrids suggests both a high hybridization rate and a high survival rate from fry to smolt. Both salmon and hybrids had a well-developed seawater tolerance at the time of downstream migration, revealed by small ionoregulatory effects and no or low mortality rates during the 24hSW tests. The trout were not fully adapted to seawater, and high mortality rates were observed (71 and 92%) during the 24hSW tests. The NKA activity was not significantly different between salmon and hybrids. Most of the hybrids were physiologically capable of direct entry to full strength seawater. The incomplete seawater tolerance in trout compared to salmon corresponds well with differences in life-history patterns between these two species. The life history strategy of the hybrids during the sea phase is not known, and further investigations on the marine behaviour and survival is needed to evaluate the role of hybrids in the risk of spreading G. salaris to nearby river systems.

Pre-winter lipid stores in young-of-year Atlantic salmon along a north-south gradient.

The pre-winter lipid stores of young-of-the-year (YOY, age 0 year) Atlantic salmon Salmo salar were analysed along a north-south gradient from c. 71 to 58 degrees N, with winter conditions ranging from >200 days of ice cover to no ice. The rivers sampled in Northern Norway represent some of the most northerly S. salar rivers. There was an increase in lipid content with increasing latitude, and mean lipid content (size adjusted to common mass) for YOY in northern rivers were almost three times higher: 0.035 g compared to 0.013 g in southern rivers. The relationship was not sensitive to variation in sampling time or variation in YOY body size. The lipid stores, however, varied markedly between rivers and also between neighbouring rivers, indicating different strategies or opportunities for pre-winter lipid storage both at latitudinal and local scales.