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.     

High marine survival rates of sea‐ranched Atlantic salmon smolts (Salmo salar) that had previously matured as male parr

Abstract The growth and recapture rates of Atlantic salmon, Salmo salar L., smolts that had previously matured as parr were compared with those of immature siblings in a sea‐ranching experiment in western Norway. The smolts were the offspring of three river populations. They were individually tagged, transferred to cages in a small bay where estuarine salinity gradients had been established and released on four dates from mid‐May to early June after either 3 or 7 days of acclimatisation. The previously mature male parr (PMMP) were smaller than their siblings before release, but performed much better during their time in the sea. As grilse, they reached sizes comparable with the formerly immature fish, and their recapture rate was more than four times higher (8.6 vs 2.1%). This difference decreased somewhat during the following years because 0.85% of the immature fish and only 0.1% of PMMP were recaptured as multi‐sea‐winter salmon. The highly successful marine performance of the PMMP demonstrated that this life history strategy may be more flexible than previously believed.     

The effects of flow on Atlantic salmon (Salmo salar) redd distribution in a UK chalk stream between 1980 and 2015

Atlantic salmon are an ecologically and economically important migratory fish in the UK, whose stocks have been declining over the past 30 years. Future climate and water use changes have the potential to alter the reproductive behaviour and distribution of salmon within a river, by restricting times and ability to access suitable spawning areas. As the survival of emergent salmon juveniles is density dependent, understanding how climate‐driven changes in flow affect the location of salmon redds is important for future conservation efforts. This study examined how flow conditions affect the distribution of redds within a UK chalk stream, the river Frome in Dorset. Sixteen years of redd distribution and flow data between 1980 and 2015 were analysed using linear mixed‐effects modelling. Generally, highest redd densities occurred within middle reaches of the main river. Mean flow during the river Frome critical migration period (October–December) did not affect the density of redds directly but affected the relationship between redd density and distance from tidal limit: redd densities were spread more uniformly throughout the river under high flow conditions, whereas redds were more aggregated in the middle river reaches under low flow conditions. Together, these findings suggest that access to upstream spawning grounds was limited under low flow conditions, which could have negative repercussions on juvenile survival. This study has revealed the distribution of redds along the river Frome for the first time and provided a basis for further study into the effects of redd distribution on subsequent juvenile life stages.     

Autumn microhabitat breadth differs between family groups of Atlantic salmon parr (Salmo salar) in a small chalk stream

The effect of family traits on the microhabitat use by six genetically distinct groups (three in each year of study) of juvenile Atlantic salmon tagged with passive integrated transponder (PIT) tags was studied via PIT‐tag detectors installed on the river bed in a small chalk stream of southern England, during Autumn in 2006 and 2007. Canonical correspondence analysis of the molecular and microhabitat data revealed considerable overlap in the microhabitat use of the family groups and notable differences in microhabitat breadth, which was partly influenced by sample size. The data suggest that microhabitat breadth and preferences of wild salmon are influenced by family of origin.     

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.     

Comparing the behavioural thermoregulation response to heat stress by Atlantic salmon parr (Salmo salar) in two rivers

Climate change is expected to increase the frequency and magnitude of extreme thermal events in rivers. The Little Southwest Miramichi River (LSWM) and the Ouelle River (OR) are two Atlantic salmon (Salmo salar) rivers located in eastern Canada, where in recent years, water temperatures have exceeded known thermal limits (~23°C). Once temperature surpasses this threshold, juvenile salmon exploit thermal heterogeneity to behaviourally thermoregulate, forming aggregations in coolwater refuges. This study aimed to determine whether the behavioural thermoregulation response is universal across rivers, arising from common thermal cues. We detailed the temperature and discharge patterns of two geographically distinct rivers from 2010 to 2012 and compared these with aggregation onset temperature. PIT telemetry and snorkelling were used to confirm the presence of aggregations. Mean daily maximum temperature in 2010 was significantly greater in the OR versus the LSWM (p = 0.005), but not in other years (p = 0.090–0.353). Aggregations occurred on 14 and 9 occasions in the OR and LSWM respectively. Temperature at onset of aggregation was significantly greater in the OR (T_onset = 28.3°C) than in the LSWM (T_onset = 27.3°C; p = 0.049). Logistic regression models varied by river and were able to predict the probability of aggregation based on the preceding number of hours >23°C (R² = 0.61 & 0.65; P₅₀ = 27.4°C & 28.9°C; in the OR and LSWM respectively). These results imply the preceding local thermal regime may influence behaviour and indicate a degree of phenotypic plasticity, illustrating a need for localised management strategies.     

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.     

Variation in wind and piscivorous predator fields affecting the survival of Atlantic salmon,Salmo salar,in the Gulf of Maine

Abstract Observations relevant to the North American stock complex of Atlantic salmon, Salmo salar L., suggest that marine mortality is influenced by variation in predation pressure affecting post‐smolts during the first months at sea. This hypothesis was tested for Gulf of Maine (GOM) stocks by examining wind pseudostress and the distribution of piscivorous predator fields potentially affecting post‐smolts. Marine survival has declined over recent decades with a change in the direction of spring winds, which is likely extending the migration of post‐smolts by favouring routes using the western GOM. In addition to changes in spring wind patterns, higher spring sea surface temperatures have been associated with shifting distributions of a range of fish species. The abundance of several pelagic piscivores, which based on their feeding habits may predate on salmon post‐smolts, has increased in the areas that serve as migration corridors for post‐smolts. In particular, populations of silver hake, Merluccius bilinearis (Mitchell), red hake, Urophycis chuss (Walbaum), and spiny dogfish, Squalus acanthias L., increased in size in the portion of the GOM used by post‐smolts. Climate variation and shifting predator distributions in the GOM are consistent with the predator hypothesis of recruitment control suggested for the stock complex.     

Fat dynamics of Atlantic salmon Salmo salar L. smolt during early seawater growth

Growth of Atlantic salmon Salmo salar L. smolt is poor in the period immediately following transfer to seawater, and the fish may use endogenous reserves to meet metabolic requirements at this time. Fat dynamics of smolt that differed in ‘fat status’ (10–12 versus 5–7% body fat) at the time of transfer to seawater were examined in fish fed either high‐(31% fat, 41% protein) or low‐fat (18% fat, 49% protein) feeds during seawater rearing. Samples were taken at intervals over 14 weeks to monitor changes in fat of the fillet, viscera and remaining carcass (head, skeleton, skin and ‘belly flap’). Growth rates (SGRs) were low during the first 3–6 weeks, but improved with time and SGRs for the 14 weeks were approximately 1% d^?1. Fat status of the smolt appeared to influence growth in seawater, because the fish that had been held on the high‐fat feed in fresh water grew less well than those given low‐fat feed at that time. At transfer to seawater, the fillet housed 20–25% of the body fat, and the carcass over 50%. After 14 weeks, the fillet held 32–35% of the body fat, and viscera 19–26%, but the carcass, with 40–49%, was still the major fat depot irrespective of dietary treatment. Thus, the carcass is a major fat storage depot in Atlantic salmon smolt, but the fillet appears to become more important as the fish increase in size.     

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.     

Effect of long-term pinealectomy on growth and precocious maturationin Atlantic salmon,Salmo salar parr

While the pineal organ in poikilothermic vertebrates consistently influences a number of physiological processes, its precise role, and what controlling signal (hormonal and/or neural) is abolished by its removal, is still uncertain. For this reason, the effect of long-term pinealectomy (PINX) on seasonal growth and reproductive development was investigated in Atlantic salmon, Salmo salar, parr. Mean daytime plasma melatonin level in the control, sham-operated and PINX fish was 35 pg·mL^–1 in all groups, while mean night-time levels were 296, 255 and 25 pg·mL^–1, respectively, indicating that PINX abolished the natural nocturnal rise in melatonin. Pinealectomy did not influence the incidence or timing of early sexual maturation in the male parr. However, pinealectomy significantly affected growth, its effect being strongly seasonally dependent. Compared to sham-operated and control fish, pinealectomized fish showed significantly lower specific growth rates (SGRs) during the period of lengthening photoperiod up until the summer solstice. Thereafter, corresponding to the season of decreasing photoperiods, the pinealectomized fish exhibited higher SGRs. These results suggest a functional relationship between the pineal organ and somatic growth in the Atlantic salmon, although what controlling signal from the pineal (melatonin or neural) is involved has yet to be determined.     

Seasonal differences in smolt traits and post-smolt survival of wild Atlantic salmon, Salmo salar, migrating from a northern boreal river

Abstract  Seasonal differences in smolt traits and the post-smolt survival of wild Atlantic salmon, Salmo salar L., were investigated by smolt trapping and Carlin-tagging in the River Simojoki, northern Finland between 1991 and 2004. The annual trapping season was divided into two halves based on the median catch date. Smolt length was significantly higher during the first half of the season, while differences in smolt weight were typically small. Smolt age was always significantly higher during the first half of the season because older smolts tended to migrate earlier in the season. Many smolts migrating during the early season and almost all smolts migrating later had started their new growth, indicating that smolts grow in the spring before sea entry. The differences in recaptures between smolts tagged during the first and second halves were insignificant. Although variations in smolt traits and environmental conditions can produce inter-annual variation in post-smolt survival, their seasonal differences seem to be too small to have an effect.     

Influence of long term ammonia exposure on Atlantic salmon (Salmo salar L.) parr growth and welfare

The objective of this study was to determine the long‐term effects of ambient unionized ammonia nitrogen (NH₃‐N) combined with different feeding regimes on Atlantic salmon Salmo salar L parr growth, welfare and smoltification. Previous studies on the parr stage of Atlantic salmon have mostly focused on acute exposure, or at low temperatures. Atlantic salmon parr were exposed for 105 days (at 12°C, pH 6.8) to four sublethal ammonia concentrations ranging from 0.1 to 35 μg L^?1 NH₃‐N (0.1–25 mg L^?1 TAN) at two feeding levels: full feed strength (+20% overfeeding) and 1/3 of full feed strength. After 21 days, it was observed that 32 μg L^?1 NH₃‐N reduced growth rate of parr fed full ration, but this effect was not evident at the end of the exposure. Feed utilization was not affected by ammonia exposure at any sampling point. Increasing ammonia levels were associated with a higher prevalence and severity of gill damage at 22 days but not at the end of the exposure. The examination of welfare indicators revealed only a few pathologies, not related to ammonia exposure. In addition, higher ammonia concentrations did not appear to influence the development of hypo‐osmoregulatory ability during parr‐smolt transformation.     

Effect of cadmium on the cytoskeleton and morphology of gill chloride cells in parr and smolt Atlantic salmon (Salmo salar)

This study documented the effect of cadmium on salmon parr and smolt gill morphology. Cadmium-induced changes in chloride cell (CC) cytoskeletal elements were investigated, as well as the modifications of CC surface area and density. In cadmium-treated parr (10 µg Cd l^-1 for 2 days), immunofluorescent light microscopy revealed the appearance of an intense actin staining located in the CC apical part. Transmission electron microscopic observations revealed a change in the organization of the microfilaments at the CC apex, with the appearance of numerous aggregates of filamentous actin. Higher cadmium concentrations (30 and 50 µg l^-1) and prolonged treatment times (7 to 14 days) did not modify such reorganisation. Microtubules were not significantly affected by similar treatments. Further, scanning electron microscopic analysis revealed that cadmium induces a significant increase of parr CC surface area as early as the second day of exposure. After 2 days, mature CC density had also increased. In smolt, a rise in CC surface area was observed, although CC density did not significantly increase.     

The onset of downstream movement of juvenile Atlantic salmon, Salmo salar L., in a chalk stream

The downstream movements of wild Atlantic salmon, Salmo salar L., from their established feeding territories in the River Itchen, Hampshire, UK, were logged continuously over an 11-month period using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. Initial downstream movement of smolts in April was correlated with the onset of darkness, at which time salmon moved from their established feeding territories alone. No relationship was found between the number of smolts migrating and river flow or maximum daily water temperature. The timing of downstream movement of parr between October and March was random with regard to sunset and time of maximum daily water temperature, suggesting the environmental cue that initiates movement may be different outside the spring smolt period.     

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.     

Temporal spawning migration patterns of landlocked Atlantic salmon (Salmo salar) in a constructed stream

Landlocked Atlantic salmon (ouananiche, Salmo salar) spawning migration timing varies throughout its range and is influenced by a variety of environmental variables. This study examined the spawning migration time of landlocked Atlantic salmon in a small (4 ha) constructed stream linked to a large reservoir (32,600 ha) in Newfoundland, Canada. Salmon use of the stream was passively monitored using Passive Integrated Transponder (PIT) Tags from 2006 to 2012. Of the salmon tagged throughout the reservoir, 48% entered the stream during the spawning season and remained there for 18–36 days from late September to November before they returned to the reservoir. Most fish entered the stream at dusk (32%). The lunar cycle influenced the timing of migration each year. Mean stream discharge (cubic metres per second) influenced migration timing in some years, and water temperature appeared to have minimal impact on migration timing. Twenty per cent of fish migrated to the stream in multiple years and 51% of these individuals appeared to use the stream on a biennial or alternate year cycle. These results demonstrate that small constructed streams can attract salmon from throughout a large reservoir and can provide valuable habitat in areas where spawning habitat is limited.     

Adiposity and growth of post-smolt Atlantic salmon Salmo salar L.

The long‐term adipose homeostasis seen in mammals gives rise to a ‘lipostatic’ model in which signals produced in proportion to fat stores serve to regulate energy intake. An extension of this predicts an impact of these signals on growth; downregulation of feeding in animals with increased adiposity should result in reduced growth. This was tested by monitoring fat deposition and growth in Atlantic salmon Salmo salar L. provided with feeds that differed in fat content. Salmon parr (mass c. 20 g) were fed either high‐ (H: 34%) or low‐fat (L: 22%) feeds, based on either fish (F) or vegetable (V) oils for 6 months to create groups of fish that differed in adiposity (10–12% and 5–7% body fat) at parr–smolt transformation (mass c. 130 g). Fish fed the high‐fat feeds deposited more body fat, and this was confirmed by measurement of fat concentrations in the fillet, viscera and remaining carcass. The fish were then grown‐on in sea water (c. 35 g L^?1, 8 °C, 24L:0D) for 14 weeks while being fed either high‐ or low‐fat feed formulated with fish oil to give the following treatments: HF→ H, HF→ L, LF→ L, LF→ H, HV→ H, HV→ L, LV→ L, LV→ H. Although fish exposed to the various feed treatments did not differ markedly in growth rate (SGR range 1–1.14% day^?1) over the 14 weeks of rearing in sea water, the results were in general agreement with predictions from the ‘lipostatic’ model, i.e. fish with the greatest fat reserves after the parr–smolt transformation grew more slowly than fish that were ‘leaner’ at this time. This suggests that adiposity, or ‘fatness’, may exert a negative feedback on feeding in salmon, thereby having an influence upon growth.     

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.