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.     

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.     

High variability in spawning migration of sea trout,Salmo trutta,in two northern Swedish rivers

Abstract Telemetry was used to examine spawning migration of sea trout, Salmo trutta L. (n = 126), in two rivers in northern Sweden. The spawning areas defined by radio‐tagged fish differed between the river systems. In the River Vindelälven, sea trout spawned in the main stem and 80% of tagged individuals returned to areas where hatchery‐reared juveniles had been previously stocked. In the River Piteälven, 74% of tagged sea trout ascended tributaries for spawning. Tagged fish were categorised into three groups of migratory pattern. cart (classification and regression tree) analysis indicated that distance from tagging location to spawning site (S_dist) explained the migratory patterns. Large S_dist separated fish with stepwise upstream migration from those with up‐ and downstream migrations and one‐directional direct migration. Fish tagged early in the season migrated the longest distance to spawning areas. Stocking locations and sex explained the large search behaviour up‐ and downstream in the rivers. The findings are important for the sustainable management of sea trout in the Gulf of Bothnia.     

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.     

Population decline in the endemic Atlantic salmon (Salmo salar) in Kapisillit River,Greenland

Atlantic salmon, Salmo salar L., is found throughout the North Atlantic, with thousands of rivers having spawning populations. In Greenland, spawning is limited to one river in West Greenland, the Kapisillit River (64?N), and the salmon are limited to the lower few kilometres of the system. Using mark–recapture, it was estimated the parr population was 5,953 individuals, and that the population size has declined by 52% since 1959. In spite of this decline, parr density remains high, being between 0.26 and 0.62 parr/m². Using a historical age‐length key, an estimated minimum of 635 smolt will have descended to the sea in 2017. These will be caught in a fishery currently subject to no regulatory measures and fishing remains the most likely driver of the population decline. The genetically distinct population is endemic to Greenland, and managers should implement measures to conserve this genetic integrity and local biodiversity.     

Inter-stage survival of wild juvenile Atlantic salmon, Salmo salar L.

A biological model was developed to calculate annual survival between life stages of juvenile Atlantic salmon, Salmo salar L., in Catamaran Brook, a small stream basin (52 km²) in the Miramichi River catchment in New Brunswick, Canada. Seven years’ data (1990–1996) were used in the model. Input variables included: daily fish counts and measurements of parr (3–4 age classes), smolts, and adult salmon at a fish-counting fence near the stream mouth; biennial quantification of all habitat types along the watercourse; fish density estimated by electric fishing at 30 sites; and estimates of young-of-the-year emigration via stream drift. Continuous recording of stream discharge provided data to assist in interpretation of survival estimates. Annual survival for juvenile salmon in their first 3 years of life in the stream averaged between 31% and 34%. The greatest annual variation (CV = 0.699) occurred at the egg to 0+ (summer) stage with a low of 9.2% survival recorded for a winter with an atypical midwinter flood event; parr and pre-smolt survival were similarly affected. Survival from egg deposition (after correction for losses caused by predation and retention/non-fertilization) to smolt emigration was between 0.16% and 0.52%, which is low relative to estimates from many other studies. Survival of smolts to returning 1-sea-winter adults (grilse) averaged 8.5%. Potential errors in the computation of the model are discussed, e.g. inaccurate counts of spawning adults during high autumn stream flow. A possible explanation for the low egg to smolt survival was the environmental conditions experienced during various winters. Mean egg survival was 1.3 times higher (39.3%) and egg to smolt survival increased to 1.03% when the two winters characterized by extremely low discharge or midwinter freshets were excluded from the calculation. Density-dependent factors related to a beaver dam, which limited spawning distribution, may also have contributed to poor survival and increased fry emigration in one year. Environmental factors, particularly winter conditions, in streams such as Catamaran Brook may act as bottlenecks to natural production of Atlantic salmon.     

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.     

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.     

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.     

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.     

Density of Atlantic salmon, Salmo salar L., smolts in the river Orkla, a large river in central Norway

Abstract. Estimates of Atlantic salmon, Salmo salar L., smolt density were conducted in the river Orkla, central Norway, in spring 1983 by tagging 2671 smolts. The fish were captured in three sections of the river and marked by cutting the fins. Smolts larger than 110 mm were separated from fish between 100 and 110 mm, resulting in four series of markings. During the smolt run, 1285 smolts were captured in two traps lowered from a bridge, situated downstream of the area of the river where the fish were tagged. The recapture of tagged smolts was 27 specimens (1%). varying between 0·8% and 1·2% for the different series of markings. The estimated smolt densities varied between 2·8 and 4·4 smolts per 100 m2 from the four markings, with 4·1 smolts per 100 m2 (95% confidence interval 2·8–6·1) based on the total number of recaptures.     

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.     

Marine mortality in the river? Atlantic salmon smolts under high predation pressure in the last kilometres of a river monitored for stock assessment

The River Bush (Northern Ireland) is an index river for the estimation of Atlantic salmon, Salmo salar L., stock size, population dynamics and marine survival rates. Marine survival estimates are based on the number of smolts counted at a trap 3.5 km upstream of the river outlet. The survival from release to coastal inshore waters for acoustic‐tagged smolts released at the Bushmills trap varied between 32% and 68%, with both year and brightness during river exit playing a significant role in explaining the variations in survival. This constitutes an important survival bottleneck. Contrary to true marine mortality, this significant loss of smolts in the river and nearshore environments could be reduced by focused management actions. More studies on other rivers, where smolts are enumerated above the head of tide, could further partition smolt and post‐smolt mortality, help differentiate true marine survival and help understand fluctuations in adult returns.     

Delayed smolt migration of stocked Atlantic salmon parr

Abstract The timing of the smolt run in the Dale River in western Norway was monitored from 2002 to 2007 after annual stocking in late autumn 2000 to 2005 with 5000–10800, 11–16 g, Atlantic salmon, Salmo salar L., parr. The releases yielded an annual smolt production of 1000–2000 individuals, mainly 1+ smolts. Almost 5700 stocked smolts were trapped during smolt migration and 60% of these were genotyped for family identification. The date for 50% descent varied by 14 days from year to year. For the most part, however, the 2+ stocked smolts and the majority of the wild smolts left the river in May, while the 1+ stocked smolts migrated 23–26 days later in June. It was concluded that the strategy of stocking large parr in late autumn may conflict with the natural timing of smolt migration the following spring.     

Migration pathways,speed and mortality of Atlantic salmon (Salmo salar) smolts in a Scottish river and the near‐shore coastal marine environment

Long‐distance migration of Atlantic salmon (Salmo salar) is known to result in high levels of mortality. For a species experiencing global population decline, it is thus vital to better understand migration behaviour, both in the river and marine stages. Atlantic salmon smolts (n = 50) were tracked using acoustic telemetry in the River Deveron, Scotland, and adjacent coastal area. Higher rates of mortality were observed in the river (0.77% per km) than the early marine stage of migration (0.0% per km). Mortality likely resulted from predation. Higher swim speeds were recorded in the early marine stage compared with the river (marine = 7.37 ± 28.20 km/day; river = 5.03 ± 1.73 km/day [mean ± SD]), a potential predator avoidance behaviour. The majority of smolts leaving the river did so in darkness and on a flooding tide. Overall river and marine migration success were linked to nights of lower lunar brightness. Marine migration speed decreased with increasing environmental noise levels, a finding with implications for fisheries management. The migration pathway in the early marine environment did not follow obvious geographical features, such as the coastline. Thus, we suggest that early marine environment pathways are more influenced by complex water currents. These findings highlight factors that influence smolt migration survival and behaviour, areas on which future research should focus.     

The smolt run and postsmolt survival of Atlantic salmon, Salmo salar L., in relation to early summer water temperatures in the northern Baltic Sea

Abstract –  The timing of the smolt migration of Atlantic salmon, Salmo salar L., was investigated during 1972–2002 in the Simojoki, a river flowing into the northern Baltic Sea. The onset of the smolt run was positively correlated with the river water temperature; a rise in water temperature above 10 °C being the main proximate environmental triggering factor. There was also a weaker correlation between the decreasing river discharge in the spring and the onset of the smolt migration. The duration of the main run was shorter in the years when the onset of the smolt run was delayed. No differences were found in the onset timing or in the duration of the smolt run between wild smolts and semi-wild smolts released into the river as parr. A polynomial equation fitted to the annual data on the survival of Carlin-tagged wild smolts and the sea surface temperature (SST) in June off the river mouth appeared to follow a dome-shaped pattern. Survival was lower in cold early summers (SST <9 °C) than in those with an average SST (9–11.9 °C), and lower again, although not significantly, in warm early summers (SST ≥12 °C). Too low and probably also too a high water temperature in early summer could thus be one of the underlying reasons for the fluctuations observed in postsmolt survival in the Baltic Sea.     

Piscine reovirus (PRV) in wild Atlantic salmon,Salmo salar L., and sea‐trout,Salmo trutta L., in Norway

This is the first comprehensive study on the occurrence and distribution of piscine reovirus (PRV) in Atlantic salmon, Salmo salar L., caught in Norwegian rivers. PRV is a newly discovered reovirus associated with heart and skeletal muscle inflammation (HSMI), a serious and commercially important disease affecting farmed Atlantic salmon in Norway. A cross‐sectional survey based on real‐time RT‐PCR screening of head kidney samples from wild, cultivated and escaped farmed Atlantic salmon caught from 2007 to 2009 in Norwegian rivers has been conducted. In addition, anadromous trout (sea‐trout), Salmo trutta L., caught from 2007 to 2010, and anadromous Arctic char, Salvelinus alpinus (L.), caught from 2007 to 2009, were tested. PRV was detected in Atlantic salmon from all counties included in the study and in 31 of 36 examined rivers. PRV was also detected in sea‐trout but not in anadromous Arctic char. In this study, the mean proportion of PRV positives was 13.4% in wild Atlantic salmon, 24.0% in salmon released for stock enhancement purposes and 55.2% in escaped farmed salmon. Histopathological examination of hearts from 21 PRV‐positive wild and one cultivated salmon (Ct values ranging from 17.0 to 39.8) revealed no HSMI‐related lesions. Thus, it seems that PRV is widespread in Atlantic salmon returning to Norwegian rivers, and that the virus can be present in high titres without causing lesions traditionally associated with HSMI.     

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.     

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.     

The performance of sea trout,Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994.

Abstract Since 1975, with partial recoveries in 1979 and 1984–1986, the numbers of finnock and adult sea trout, Salmo trutta L., in the Burrishoole system, western Ireland, have declined. The population decreased sharply from 1987, and collapsed in 1989 and 1990. Finnock were virtually absent in 1989 and one-year maidens equally so in 1990. Sea trout smolt recruitments from 1992 to 1994 were the lowest recorded since 1970. The modal fork length of finnock decreased from 28 cm in 1987 to 26 cm in 1990. Up until 1989, the percentage return of smolts, as finnock, ranged from 11.4 to 32.4%. In 1989 a minimum was reached at 1.5%, with values of 10.0, 3.7, 7.4 and 9.9% from 1990 to 1994. Marine survival of smolts, to total first return to fresh water, historically ranged from 19 to 66%. Survival dropped to 1.8% in 1989. Observed changes in the structure of the sea trout population and the collapse in marine survival suggest that the current west of Ireland sea trout problem is based in the marine habitat.