Diel migration pattern of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) smolts: an assessment of environmental cues

The timing of smolt migration is a key phenological trait with profound implications for individual survival during both river descent and the subsequent sea sojourn of anadromous fish. We studied relationships between the time of smolt migration, water temperature and light intensity for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta). During 2006–2012, migrating smolts descending the southern Norway River Storelva were caught in a rotary screw trap located at the river mouth. The date of 50% cumulative smolt descent correlated significantly with the date when the river temperature exceeded 8°C for both Atlantic salmon and sea trout smolts. In 2010, smolts of both species were passive integrated transponder (PIT)‐tagged, and the diel timing of their migration was precisely documented. The degree of night migration decreased in both species as the river temperature rose, and at temperatures above 12–13°C, more smolts migrated during day than during night. A multinomial model was fitted for estimating temperature and species effects on probabilities of migration during night, daytime, dusk and dawn. Atlantic salmon smolts preferred migrating under lower light intensities than sea trout smolts during early, but not late spring when both species migrated during bright daylight. In accordance with the early‐season tendency to migrate at night, Atlantic salmon smolts migrated more during darker hours of the day than sea trout. In both species, smaller smolts migrated under dark conditions than during light conditions. Most of the findings on thermal, light and temporal effects on the observed smolt migration pattern can be explained as adaptations to predation avoidance.     

Failure of predator conditioning: an experimental study of predator avoidance in brown trout (Salmo trutta)

Many studies have documented that hatchery‐reared salmonids generally have inferior survival after being stocked compared with wild conspecifics, hatchery and wild salmonids have been observed to differ in their antipredator responses. The response of brown trout (Salmo trutta) juveniles (0+) of differing backgrounds to a live predator was compared in two experiments. First, the antipredator behaviour of predator‐naïve hatchery‐reared brown trout and wild‐exposed brown trout were assessed in behavioural trials which lasted for eight days. Second, predator‐naïve and predator‐conditioned hatchery‐reared brown trout were assessed in identical behavioural trials. Brown trout were ‘predator‐conditioned’ by being held in a stream‐water aquarium with adult Atlantic salmon (Salmo salar) and adult brown trout for two days prior to behavioural trials. Predator‐conditioned hatchery‐reared brown trout spent more time in shelters in the trial aquaria than predator‐naïve hatchery‐reared fish, but did not differ in time spent in the predator‐free area. Predator conditioning may account for the increased time spent in the shelter, but does not appear to have affected time spent in the predator‐free area. However, even if significant alteration in behaviour can be noted in the laboratory, the response might not be appropriate in the wild.     

Survival of migrating sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) smolts negotiating weirs in small Danish rivers

Abstract – The survival of brown trout and Atlantic salmon smolts during passage over small weirs was estimated in two small Danish rivers during the spring of 1998. Parallel groups of smolts were released upstream and downstream of the weirs and recaptured in traps further downstream. The results showed a smolt loss varying from 18 to 71% for trout and 53% for salmon. Furthermore, the surviving smolts from the upstream groups were delayed for up to 9 days compared to downstream groups. The study demonstrated that an increased proportion of total river discharge allocated to fish passage increased the smolt survival. Losses may be because of fish penetrating grids erected at fish farm inlets, predation and delays, which may lead to desmoltification. The low survival may seriously threat both the long-term viability of wild populations of anadromous salmonids and the outcome of the intensive stocking programme in Denmark.     

The effect of physiological and environmental conditions on smolt migration in Atlantic salmon Salmo salar

Hydropower development has negatively influenced Atlantic salmon (Salmo salar) populations. Compensatory hatchery‐rearing programmes exist, but released fish suffer from high mortality that may be related to the lack of experience from natural environments in hatchery‐reared smolts and their large body size and high energetic state. Here, we used acoustic telemetry to test how body size, energetic state, and the environmental conditions of the river affect migration in hatchery‐reared smolts. The study was conducted in three consecutive years between 2011 and 2013 in the lower part of the River Umeälven, Sweden. For individual fish, there was no effect of body size but the energetic state of the fish had a negative effect on sea entry. The most important factor affecting sea entry rate was the water discharge in the old river bed that differed among years. Smolts were more likely to enter the sea in years when the discharge was high or when the discharge increased substantially shortly after release. Hatchery‐reared fish had higher migration speed at a slower flowing section compared with a faster flowing section, which was likely a result of large hesitation to enter the rapid section. The increase in water discharge led to an increase in fish migration speed disproportional to the increase in water velocity. Our results highlight the importance of water discharge for the smolts during smolt migration, and we argue that concern should be given to migrating fish when managing regulated rivers.     

Riverine and fjord migration of wild and hatchery‐reared Atlantic salmon smolts

Migration timing, speed, survival and effects of environmental parameters on migration, between wild and hatchery produced Atlantic salmon, Salmo salar L., smolts in the River Lærdalselva were studied. Hatchery‐reared (n = 40) and wild pre‐smolts (n = 40) were tagged with acoustic tags, and an array of receivers along the migration route was deployed. In all, 77 and 85% of the fish from the two groups, respectively, were recorded as migrating smolts, that is, predation rate and/or numbers of fish opting to remain in the river were low. Hatchery‐reared smolts showed a migration pattern, speed and migration route similar to wild smolts, even though the time period between river release and onset of migration was relatively short. Both groups of smolt showed high migration speed through both the river and the fjord compared with other studies.     

Conservation of endemic landlocked salmonids in regulated rivers: a case‐study from Lake Vänern,Sweden

Conservation of migratory salmonids requires understanding their ecology at multiple scales, combined with assessing anthropogenic impacts. We present a case‐study from over 100 years of data for the endemic landlocked Atlantic salmon (Salmo salar, Salmonidae) and brown trout (Salmo trutta, Salmonidae) in Lake Vänern, Sweden. We use this case‐study to develop life history‐based research and monitoring priorities for migratory salmonids. In Vänern, small wild populations of salmon and trout remain only in the heavily regulated Rivers Klar (Klarälven) and Gullspång (Gullspångsälven), and commercial and sport fisheries are maintained by hatchery stocking. These populations represent some of the last remaining large‐bodied (up to 20 kg) landlocked salmon stocks worldwide. We found that one of four stocks of wild fish has increased since 1996; the other three remain critically low. Hatchery return rates for three of four stocks appear stable at roughly 1% and annual fisheries catch is roughly 75 metric tons, with an estimated 7.5% of hatchery smolts being recruited to the fishery; this also appears relatively stable since 1990. Our analysis reveals much uncertainty in key data requirements, including both river return and fisheries catch rates, estimates of wild smolt production and survival, and hatchery breeding and genetics protocols. These uncertainties, coupled with a lack of information on their riverine and lacustrine ecology, preclude effective management of these unique populations. We conclude with a framework for a life history‐based approach to research and monitoring for Vänern salmon and trout, which should be applicable for all endemic, migratory salmonid populations.     

Swimming performance of wild and F1-hatchery-reared Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts

Abstract –  The swimming performance of wild and hatchery-reared smolts of two salmonid species was investigated. Wild Atlantic salmon smolts (WS) and brown trout smolts (WT) of equal size were caught in fish traps during migration. Hatchery-reared smolts of both species (HS and HT for salmon and trout respectively) were first generation offspring from wild broodstock. The swimming performance of individual smolts from the four groups (WS, HS, WT, HT) was tested three consecutive times using a swimming flume with water flowing at a start rate of 0.16 m·s⁻¹ and a constant acceleration rate of 0.167 cm·s⁻² (10 cm·s⁻¹·min⁻¹). Wild caught smolts of both species performed significantly better than those reared in hatchery conditions. The WS group were observed to maintain an average swimming speed ( U _burst) that was 30% faster than the HS group, whereas the wild trout smolts were superior to HT by approximately 25%. Repeated measures revealed species-specific exhaustion patterns. Brown trout smolts maintained consecutive U _burst indicating significant stamina compared with Atlantic salmon smolts that were found to be exhausted by the initial trial.     

Early marine survival and movements of escaped Atlantic salmon Salmo salar L. juveniles from a land‐based smolt farm during autumn

The aim of this study was to examine early marine survival and movements of simulated escaped Atlantic salmon Salmo salar L. pre‐smolt and smolt from a commercial smolt farm during autumn. One‐third of the pre‐smolt most likely died in the immediate vicinity of the release location, whereas the corresponding mortality for smolts was lower (8.5%) during the 5‐week study period. The surviving pre‐smolt left the farm area after 2–3 days, predominantly along the shore. In contrast, most of the surviving smolts left the farm area during the first day and 54% seemed to move away from the shore and adopt a more pelagic movement pattern than pre‐smolt. The number of surviving fish recorded in the fjord decreased throughout the study period, possibly due to a combination of fish migrating out of the fjord or undetected mortality. Compared with existing knowledge on migration of released farmed smolts during spring, our results indicate less directional and slower movement rates during autumn. Only two of the tagged fish were detected upstream in the rivers following release. A rapid dispersion of escapees indicates that the potential for recapturing escapees is limited unless recapture efforts are initiated immediately after escape. Hence, there is a need for development of technology that detects and prevents escapees to enter the sea.     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

Effects of intra‐ and interspecific competition and hydropeaking on growth of juvenile Atlantic salmon (Salmo salar)

The effects of hydropeaking and intra‐ and interspecific competition on the growth performance (growth in length, mass and lipid content) of juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta were studied in six experimental channels (three experiencing hydropeaking and three controls with a stable discharge of water). Changes in the water‐covered area in the hydropeaking channels were small to avoid fish stranding. Each channel was divided into three similar‐sized sections and stocked with either low or high density of Atlantic salmon, or a mix of Atlantic salmon and brown trout, with the density of the latter equalling the high‐density treatment of Atlantic salmon. A marked effect of competition was visible as salmon in the low‐density treatment were significantly larger (27–33%) and had a higher mass (30–38%) than salmon in both the high‐density salmon treatment and the high‐density salmon and trout treatment. Hydropeaking had only minor and insignificant effects on the growth performance: overall final length, mass and body lipid content in the salmon experiencing hydropeaking differed by ?9%, ?7% and +2% compared with controls. Furthermore, there was no indication that the competitive regime influenced hydropeaking effects. The increase in both intra‐ and interspecific competition among the juvenile salmon had a pronounced and significant effect on growth. Our study adds to the growing evidence that energetic consequences of hydropeaking are likely to be small for Atlantic salmon and that stress and mortality associated with stranding represent the main source of population impact.     

Laboratory and field investigations of salmon lice [Lepeophtheirus salmonis (Krøyer)] infestation on Atlantic salmon (Salmo salar L.) post-smolts

Hatchery‐reared 1‐year‐old Atlantic salmon post‐smolts (Salmo salar L.), artificially infected with salmon lice [Lepeophtheirus salmonis (Krøyer)] copepodids, were found to suffer from primary alterations (increased cortisol levels) at early lice stages. Secondary alterations, such as osmotic stress (increased chloride levels), first occurred after the preadult stages of the lice appeared. Fish with the highest salmon lice infections died throughout the experiment. Seven years of field investigation of Trondheimsfjorden showed that Atlantic salmon post‐smolts descending coastal waters can become heavily infected with salmon lice. The migrating post‐smolts were only infected with the chalimus stages, showing that the fish had only recently left the rivers. The infection level, however, varied considerably between the years, and, in 1998, the infection was higher than previous years. The experimental results have been combined with the field data to appraise the consequences of the infection.     

Predation by cormorants, Phalacrocorax carbo (L.), on the salmonid populations of an Irish river

Abstract. Counts were made of cormorants, Phalacrocorax carbo (L.), feeding on the River Bush. County Antrim. Northern Ireland during the post-dawn period on three occasions. Two of the counts during May 1986 indicated that up to 264 birds may have been feeding at least once per day throughout the catchment during the salmon, Salmo salar L., smolt run. The number of feeding birds had dropped to an estimated 61 by the time of the third count on 1 July 1986. Stomach samples from shot birds showed that upstream feeding was concentrated on wild smolts and brown trout. Salmo trutta L. However, cormorant predalion downstream from the salmon hatchery at Bushmills was restricted solely to hatchery smolts. Estimates of the total daily predation rates were calculated at 653–1214 wild smolts. 107–231 hatchery smolts and 422–785 brown trout. The possible impact of this level of predation on the salmonid stocks of the river was assessed.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

Retrospective analysis of Atlantic salmon (Salmo salar) marine growth and condition in the northwest Atlantic based on tag‐recovery data

The life history of North American Atlantic salmon (Salmo salar) is characterized by extensive round‐trip migrations between freshwater rearing habitats and marine feeding grounds off the coasts of Canada and Greenland. Growth is rapid during the marine migration, and growth rate and condition factor may be indicators of salmon health during this period. Growth data were evaluated from a tag‐recovery program conducted from 1969 to 1991 using hatchery‐reared Atlantic salmon smolts released in the Penobscot River, Maine, U.S.A. Information from recaptures of 3167 salmon that were at large in the marine environment for 1 month to 3 yr was analyzed. Length–weight measurements coupled with time‐at‐large data were used to estimate von Bertalanffy and allometric growth parameters specific to the marine phase. Variations in growth and condition factor in relation to smolt age, release date, and temperature conditions in the northwest Atlantic were also examined. The von Bertalanffy k parameter declined with ordinal release date, indicating faster growth rates during the first year of smolts released earlier in the spring. The 2‐yr‐old smolts had a larger k than 1‐yr‐old smolts, although 1‐yr‐old smolts grew to a larger asymptotic size. Sea surface temperature had variable effects on growth parameters and condition factor, with temperature at the beginning of the migration and in overwintering habitat during the first year at sea having the greatest influence on length–weight relationships. Determining the mechanisms that influence growth of individuals during the marine phase will help elucidate the factors responsible for historic growth trends, establishing a baseline for current research.     

Predation of Atlantic salmon, Salmo salar L., smolts and parr by red-breasted mergansers, Mergus serrator L., on two Scottish rivers

Abstract Current models estimating the impact of red-breasted mergansers, Mergus serrator L., on salmon, Salmo salar L., fisheries in Scotland fail to take account of any annual variation in the proportion of the diet that is smolts. During the 1987–1990 smolt runs, the annual variation in the diet of mergansers was estimated from the stomach contents of birds shot on two Scottish rivers. The proportion of salmon in the diet was greatest early in the smolt run (76–91% by weight), and contained proportionately more smolts than later in the run, when coarse fish were more prominent. There was little annual variation in the proportion of the diet that was juvenile salmon. However, the proportion of these fish that were smolts, was twice as great in some years than in others and this appeared to be independent of estimated annual smolt production in the rivers.     

Growth and osmoregulation in Atlantic salmon (Salmo salar ) smolts in response to different feeding frequencies and salinities

Groups of Atlantic salmon (Salmo salar) yearling smolts were reared in duplicate tanks supplied with freshwater or seawater, and subjected to different feeding frequencies, 100% (fed every day), 50% (fed every other day), 25% (fed every forth day) and 0% (starved), from 26 May to 26 July. After 8 weeks, all the groups were re‐fed in excess for 6 weeks. Fish were maintained on their respective a priori salinity treatments during the 6‐week follow‐up period. Starvation for a period of 8 weeks in freshwater resulted in a loss of hypo‐osmoregulatory ability when smolts were challenged with seawater and unfed smolts maintained in freshwater were unable to adapt to seawater in mid‐July. Ration levels influenced the growth rate and body size significantly. The overall growth rate was higher in freshwater than at corresponding rations in seawater. Partial compensatory growth was observed in the 0 and 25% groups following re‐feeding. Branchial Na⁺,K⁺‐ATPase (NKA) activity decreased rapidly in unfed smolts in freshwater and was the lowest in the starved group, whereas an initial increase was observed in those groups reared in seawater. After re‐feeding NKA activity differences decreased between the former feeding groups. Our results suggest that nutritional factors and/or energy levels are critical for the maintenance of hydro‐mineral balance of salmon smolts.     

Reduction of brown trout, Salmo trutta L., salmon, S. salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), smolt bycatches in eel pound nets

Two types of bycatch reduction devices (BRDs) were tested in the Randers Fjord, Denmark, aimed at deflecting the surface-oriented smolts of brown trout, Salmo trutta L., salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), before they became trapped, while retaining the catches of eel, Anguilla anguilla (L.). The methods tested were: a) placing a floating guard net between the wings; and b) submerging the pot net (i.e. last enclosure before fyke net) 55–100 cm below mean sea level. After each haul the pound net-setups were changed from the control to one of the types of BRD, and vice versa. Both methods significantly reduced the bycatch of brown trout smolts, while catches of legal-sized eels were not affected. Submerging the pot net reduced smolt catches of all species; mean reductions were 91.1% for brown trout, 74.5% for rainbow trout, and 86.1% for salmon.     

Long-term changes in the smolt size and age of Atlantic salmon, Salmo salar L., in a northern Baltic river related to parr density, growth opportunity and postsmolt survival

Abstract –  The size of wild Atlantic salmon ( Salmo salar L.) smolts in 1972–2004 was studied in relation to parr density, smolt age, growth opportunity and postsmolt survival in the Simojoki River. There was a significant negative regression between the annual mean smolt size and the density of wild >1 year parr in the previous autumn, but not between the annual mean smolt size and age. The density of reared parr released into the river or the growth opportunity, based on the day length and air temperature during the previous summer, did not affect the size of wild smolts. The data on postsmolt survival based on recaptures of Carlin-tagged smolts showed a significant positive relationship ( P  < 0.01) between the survival of postsmolts and the annual size of wild smolts. It is hypothesised that the increased density of wild >1 year parr could have contributed to the decreased smolt size since the 1990s, and the reduced size of wild smolts could be included among the factors resulting in their declined postsmolt survival in the Baltic Sea.     

Timing of spawning in cultured and wild Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in the River Vosso, Norway

Spawning time and size of wild and nonnative cultured female Atlantic salmon (Salmo salar L.) were studied in the River Vosso, Norway. Cultured immigrants were smaller, and peak spawning occurred 21 and 26 days earlier relative to wild salmon in 1991 and 1992, respectively. It was calculated that hatching and initial feeding of offspring of cultured females peaked 12 and 9 days earlier in spring 1992 and 19 and 8 days earlier in spring 1993 relative to offspring of wild females. Spawning time of the cultured salmon is similar to most Norwegian salmon populations and to cultured fish in other streams. In the River Vosso it also overlaps with that of brown trout (Salmo trutta L.), which may increase hybridization. The early time of breeding in nonnative fish indicates local adaptation in this trait in wild salmon. The consequences of early breeding of cultured fish are unpredictable in relation to spring temperatures but probably reduce their relative breeding success. The smaller size may also reduce the success of cultured females.     

Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea

The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 1991–1993 on wild and reared smolts of the Simojoki river salmon stock. The tag recovery data analysed by log-linear models revealed significant differences in both spatial and temporal sea migrations between the wild and reared salmon; the variation was attributed to the year of release and to the origin of the fish. Grilse accounted for the majority of reared returners (76%) but for a smaller proportion (46%) of the wild fish. The effect of smolt size could be studied only in the smolt groups tagged in 1991. Wild fish were more frequently (71%) caught in the Baltic Main Basin than were reared fish (51%) during their second sea year, and the size variation between wild and reared smolts did not explain the recovery site. No such differences in spatial distribution were found during the third sea year. The tagging place (hatchery/trap) of the reared fish did not affect their later sea migration. The differences in sea migration patterns suggest that the wild salmon are more vulnerable to the intensive salmon fishery in the Baltic Main Basin than are reared fish.