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.     

Population structuring in Atlantic salmon (Salmo salar): evidence of genetic influence on the timing of smolt migration in sub-catchment stocks

Abstract –  We compared timing of smolt migration for two populations of naturally spawned Atlantic salmon, Salmo salar , originating from an upper catchment (River Tummel) and a lower catchment (River Almond) tributary of the River Tay, Scotland. Smolts from the upper tributary began migration earlier than those from the lower tributary. On both occasions when fry derived from River Isla (lower tributary) stock were transferred to a location in the upper catchment, smolt migration was later than for native fish. Similarly, when fry from Tummel and Isla stocks were stocked in a common, upper catchment location, Isla origin fish migrated at a later date, in both of the two following smolt years. These differences are indicative of a genetic basis for the timing of smolt migration and suggestive of local adaptation. Mounting evidence points towards local genetic adaptation for the timing of expression of behaviours associated with migration. These aspects of variation should be accommodated in management theory and practices.     

Diel migration patterns of Atlantic salmon smolts with particular reference to the absence of crepuscular migration

Abstract –  The real-time diel pattern of Atlantic salmon smolt migration was observed for 8 years using automatic resistivity counters verified by video surveillance. A clear dominant nocturnal migration was demonstrated early in the migration period, later becoming increasingly diurnal, until rates became approximately equal at day and night. Migration patterns were related to water temperature, such that when mean daily temperatures were below 12 °C, hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12 °C, there was no significant difference between diurnal and nocturnal migration rates. Migration patterns showed a distinct suppression of migration at dawn and dusk throughout the migration period. It is hypothesised that this behaviour is an active decision and/or an adaptive strategy either to take advantage of increased food in the form of invertebrate drift or to reduce predation risk from actively feeding piscivores or both.     

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.     

Atlantic salmon smolts in the Irish Sea: First evidence of a northerly migration trajectory

Results from an acoustic telemetry study revealed for the first time a northerly migration route for Atlantic salmon (Salmo salar L.) smolts leaving the east coast of Ireland. Atlantic salmon smolts were tagged in spring 2019 in the Castletown and Boyne rivers. Three tagged smolts registered on disparate deep‐water offshore marine receivers as they travelled northwards out of the Irish Sea through the North Channel. One fish had migrated an estimated 250 km in a period of 32 days. The remaining two individuals were detected on receivers located off the Northern Ireland coast, further corroborating the northward migration of salmon smolts through the Irish Sea.     

Straying of Atlantic salmon, Salmo salar, from delayed and coastal releases in the Baltic Sea, with special focus on the Swedish west coast

Abstract  Atlantic salmon, Salmo salar L., reared from two Baltic strains were released around the islands Bornholm and Møn in the Baltic Sea between 1995 and 1999. A total 600 000 reared salmon were released from net pens using the delayed release technique, keeping the salmon in net pens for approximately 3 months after smolting, and 208 000 were released directly from the hatchery. Of these, 15 958 were tagged with Carlin tags. Additionally, 65 300 coded wire tagged salmon were released as delayed release salmon close to Bornholm in 2000. Recaptures from the five years of Carlin tagged releases varied between 2.8% and 21.2% (average 13.1%). Most recaptures were from within the Baltic Sea (average 98%), but some were recaptured outside the Baltic Sea, either in the sea (1%) or in fresh water (1%). Recaptures outside the Baltic Sea and in fresh water were higher for releases at Møn in the western part of the Baltic, than releases at Bornholm. Straying rates from the releases into six rivers on the Swedish west coast were estimated using information from capture in traps and sport and broodstock fisheries. The proportion of straying salmon in rivers on the Swedish west coast was about 3.8% of the salmon run, but with large variations between rivers. Releases were discontinued because of possible deleterious effect on the local wild salmon populations.     

Entry of adult Atlantic salmon into a tributary of the Aberdeenshire Dee,Scotland

Upstream counts of adult salmon in a tributary to the River Dee during autumn and winter were examined over a 13‐year period using an optical fish counter. Statistical analysis indicated that salmon were mainly primed to enter the tributary at a particular time of year, peaking at early December. However, environmental factors also had a large influence. Entry of salmon was initiated by high flow rates in the main stem of the River Dee. Increases in water level in the tributary itself were not needed to stimulate the entry of salmon. Moreover, a change in flow from 1 day to the next did not result in a response from salmon. Rather, they reacted only after more than a day of flow increase. There was no consistent threshold level of flow that triggered tributary entry. The upstream passage of salmon was reduced at low temperatures (below 3°C). The numbers of salmon migrating upstream showed a gradual increase as the temperatures increased up to 11°C, and then sharply decreased at higher temperatures. Female salmon migrated earlier by about 2 weeks on average than males. Increasingly, counters and traps are being installed on spawning tributaries to examine the state of salmon stocks. Caution should be exercised, however, in using upstream entry data from individual tributaries to provide an overall assessment of salmon abundance, as local conditions in the tributaries may affect the salmon counts in particular years.     

Effect of fishing regulation on the occurrence of repeat spawners and age distribution of Atlantic salmon in a northern Baltic river

Abstract  Upstream migrating Atlantic salmon, Salmo salar L., were trapped annually between 1990 and 2003 at the mouth of the River Simojoki in the northern part of the Gulf of Bothnia, the Baltic Sea, to collect hatchery brood fish. They were also collected between 1996 and 1998 in the river to estimate the number of ascending salmon. These data on spawners were used to study the effect of changing fishing regulations in the Finnish coastal fishery in the mid-1990s on the structure of the spawning stock. In total, 1.9% of 2290 multi-sea-winter adults had previously spawned. The average annual proportion of repeat spawners was 0.5% between 1990 and 1996, but significantly higher (2.8%) between 1997 and 2003. The age distribution also changed significantly, as 4SW and 5SW salmon appeared in the population in the latter years. These changes were most probably due to the application of stricter fishing regulations. The potential of salmon to spawn repeatedly was thus preserved, despite a severe decline in natural production in the 1980s and early 1990s.     

Interspecific hybridization between Atlantic salmon and brown trout introduced in the subantarctic Kerguelen Islands

In the subantarctic French territory of the Kerguelen Islands, Atlantic salmon Salmo salar was stocked to create a new population in the Korrigans drainage system. The analysis of historical scale collections showed that hybrids were inadvertently introduced with the original Atlantic salmon stocks. Later colonization of the Korrigans system by brown trout Salmo trutta was followed by interspecific hybridization between the two species. Our results highlight the importance of both routine genetic monitoring of hatchery stocks and considering the possible influence of other species when stocking is envisaged.     

Short feeding migration associated with a lower mean size of whitefish in the River Tornionjoki,northern Baltic Sea

The size of anadromous whitefish caught with dip nets in the River Tornionjoki in the northern part of the Gulf of Bothnia has decreased during recent decades. To find an explanation for this trend, ascending whitefish were sampled in the lower part of the river in August 2016, and they were divided into two groups according to fish length. The length of the SMALL group was ≤ 32 cm (n = 32) and that of the LARGE group ≥ 33 cm (n = 65). The elemental analysis of otoliths revealed that the barium concentration was clearly higher in otoliths from the SMALL group compared to the LARGE group. This indicates that the fish in the LARGE group had migrated to feed further south in the Gulf of Bothnia, with a higher salinity, than the fish in the SMALL group. Because of this migration difference, and a decline in the gillnet effort, especially in the feeding areas near the river mouth, it is assumed that proportionally more fish in the SMALL group are able to ascend the river to spawn than previously, thus decreasing the mean size. At the same time, the heavy gillnet fishery in the south removes fish from the LARGE group.     

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.     

Vertical movements of Atlantic salmon post-smolts relative to measures of salinity and water temperature during the first phase of the marine migration

Abstract  The migratory behaviour of hatchery-reared Atlantic salmon, Salmo salar L., post-smolts during the first phase of the marine migration was examined to assess their susceptibility to salmon lice, Lepeophtheirus salmonis (Krøyer), infestations. Swimming depths of eight post-smolts relative to the measured salinity and temperature were monitored for an average of 11.4 h following release outside the mouth of the River Eio using depth sensitive acoustic transmitters. Vertical salinity and temperature distributions were simultaneously recorded along the migratory route. Mean swimming depth was 1.7 m (individual mean 0.5–2.1 m). There was no overall preference among all the post-smolts for specific salinity concentrations. Typically post-smolts migrated the majority (68%) of their time at salinities <20 psu (brackish water), and as a result outside the reported salinity tolerances of sea lice. Furthermore, post-smolts chose the warmest water layer during their coastal migration.     

Upstream migration of Atlantic salmon at a power station on the River Nidelva, Southern Norway

The upstream migration of 17 radio-tagged adult Atlantic salmon, Salmo salar L., past hydroelectric developments on the River Nidelva, Southern Norway, was examined. Salmon migrated quickly from the site of release in the lower part of the river up to the tunnel outlet of Rygene power station, but were substantially delayed at the outlet. The salmon stayed in the outlet area for 0–71 days (median = 20), and mainly took up a position inside the dark power station tunnel. Water discharge in the tunnel was 57–176 m³ s⁻¹, while residual flow in the river between the outlet and the dam 2.5 km further upstream was 3 m³ s⁻¹. Ten salmon passed the outlet and entered the residual flow stretch, but none passed the dam. Six of the 10 salmon returned to the tunnel outlet. No major migration barriers were identified in the residual flow stretch, suggesting lack of motivation among the salmon to migrate due to either low water discharge compared with the main river, or several minor migration barriers along the river stretch.     

Preferences of mature male brown trout and Atlantic salmon parr for orifice and weir fish pass entrances matched for peak velocities and turbulence

Abstract –  Mature male brown trout and Atlantic salmon parr were offered a choice of two fish pass entrances: a weir and an orifice. Peak velocities and turbulence were closely matched between the pass entrances. There was very strong selection for the orifice among both salmon (20 of 20) and trout (19 of 20). Combined with data in the literature, these results add to the impression that the main behavioural characteristics of salmonid fish migrating upstream through fish passes may largely be independent of size and life stage.     

Survival of Atlantic salmon captured in and released from a commercial trap-net: Potential for selective harvesting of stocked salmon

The major wild Atlantic salmon stocks in the Baltic Sea began to recover in the late 1990s. This recovery has been partly due to strict regulations in the Gulf of Bothnia that effectively prevent salmon fisheries during the peak migration. About half of the migrating salmon, however, are reared fish that could be harvested. We simulated a limited trap-net fishery that selectively harvested reared salmon and released wild fish, and studied the survival and migration of the released salmon. We tagged and released 1970 salmon caught in the trap-nets along the coast in 2001 and 2002. The mean maximum capture and release induced mortality of salmon was 11%, ranging between 4% and 21% in different release groups by year, sea age and number of releases. The cumulative mortality for the total salmon population on their spawning migration in the Gulf of Bothnia was below 5%, and it would not increase considerably after the first capture and release events, provided fishing effort is not excessive and fish are handled properly. Survival of trap-net captured and released Baltic salmon appears high and their migration behavior is not altered due to this handling. Several preconditions, however, should be considered before selective fishing is introduced in the Gulf of Bothnia salmon fishery.     

Environmental threats and conservation implications for Atlantic salmon and brown trout during their critical freshwater phases of spawning,egg development and juvenile emergence

Atlantic salmon Salmo salar L. and brown trout Salmo trutta fario L. are species of high socio-economic and ecological value. Declining populations make them target species of fisheries management. This paper reviews the direct effects of deficient longitudinal connectivity, changes in discharge, high water temperatures, oxygen depletion, changes in water chemistry and increasing loads of fine sediment on the critical life stages of spawning, egg incubation and emergence. It further provides an overview about the basic autecological requirements of Atlantic salmon and brown trout and summarises important thresholds of physico-chemical tolerances. This collection of information provides important baselines for assessing historical, ongoing and new threats relevant for the management of both species in fresh waters. Critical early-life stages of both species are almost identical, creating synergies in conservation and restoration. Seaward-migrating forms are exposed to further stressors, but improving starting conditions can also greatly improve their resilience.     

Pattern and prevalence of predator damage on adult Atlantic salmon,Salmo salar L., returning to a river system in north-east Scotland

Abstract There remains considerable debate over the impact of predation upon Atlantic salmon, Salmo salar L., populations. The pattern and prevalence of predator damage on salmon returning to a river system in North-east Scotland were recorded. Overall, 214 (19.5%) out of 1099 fish sampled were damaged, and 10 distinct categories of damage were identified. Measurements of scratches indicated that the damage was caused by a number of different factors. In particular, damage typically attributed to seals was suggested to be the result of attacks by odontocete cetaceans. These results highlight the multispecies nature of interactions between salmon and their predators, but suggest that damage levels may be of limited use in assessing the impact of different predators on salmonid population dynamics.     

Effects of release timing on migration behaviour and survival of hatchery‐reared Atlantic salmon smolts in a regulated river

In this study, 221 two‐year‐old hatchery‐reared salmon, Salmo salar, smolts were tagged with radio transmitters over a period of three consecutive years and released in the river in groups of 20–21 fish in various dates between late April and early June. Tagged smolts were tracked during their downstream migration in the lower 36‐km stretch of the regulated River Oulujoki, with the focus on the effects of release date, water temperature and river flow on migration behaviour and survival. The results indicate that release timing and river temperature have profound effects on the initiation of migration, swimming speed and survival of released S. salar smolts. Smolts released early in the spring in cold waters ceased migration after brief downstream movement and were vulnerable to predation, whereas the migration speed and survival rates increased markedly for smolts released later in the spring.     

Iron status and dietary levels of iron affect the bioavailability of haem and nonhaem iron in Atlantic salmon Salmo salar

The absorption of haem iron and nonhaem iron in 155-g Atlantic salmon Salmo salar was studied by simultaneous labelling of the two forms of iron with ⁵⁹Fe or ⁵⁵Fe, respectively. Iron absorption was measured after incorporation of this element into blood and tissue 2 days after oral administration. Fish with a low iron status showed a significantly greater absorption of both haem and nonhaem iron than iron-loaded fish. The low-iron group distributed a larger proportion of the iron to kidney and spleen than the iron-loaded or control fish. Absorption of both forms of iron increased with higher doses administered, but absorption relative to dose administered decreased. There was a trend suggesting that increased levels of dietary haem iron resulted in the reduced uptake of nonhaem iron, but not vice versa . The results of the present study suggest that factors affecting bioavailability of haem and nonhaem iron in Atlantic salmon are similar to those of other vertebrates.