The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Increasing temperature associated with increasing grilse proportion and smaller grilse size of Atlantic salmon

Effects of temperature on Atlantic salmon (Salmo salar) were analysed using Carlin tag recovery data (1985–2014), and mixed-stock catch data (smolt years from 2001 to 2012) in northern parts of the Baltic Sea. During warmer summers, the mean smolt length of the recaptured salmon tended to be smaller, and salmon were recaptured more frequently in feeding grounds closer to the home rivers in the Gulf of Bothnia, while colder summers were associated with more recaptures further south, in the Baltic Main Basin. Moreover, a warmer spring in the smolt year was associated with decreased weight of male grilses in mixed stock data. Further, warmer spring temperatures during the smolt year were associated with a higher proportion of one-sea-winter (1SW) males during the return migration in mixed stock data. These results suggest that the increasing global temperature may affect Atlantic salmon life history demographics.     

Post-smolt migration of Atlantic salmon, Salmo salar L., from the Simojoki river to the Baltic Sea

The migration pattern of Atlantic salmon ( Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972–2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river ( R ² = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatchery-reared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.     

The timing, sex and age composition of the wild and reared Atlantic salmon ascending the Simojoki River, northern Finland

In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced considerable numbers of ascending one‐sea‐winter (1 SW) males, whereas the proportion of male 1 SW salmon was low among spawning migrants of wild or reared parr origin. The sex ratio among ascending wild fish and reared salmon stocked as parr was similar, with females predominating, while reared salmon stocked as smolts were mainly males. The multi‐sea‐winter (MSW) salmon entered the river annually within a fairly short time period from the beginning of the migration season, independent of their sex or origin. 1 SW males migrated into the river significantly later in the season than MSW males. The results indicate that the delayed opening of the fishing season in the Gulf of Bothnia is effective in reducing the harvest of MSW salmon at sea. However, as the timing of the ascent may vary by several weeks from year to year, the effect of this regulation bound to certain calendar days may also vary considerably from year to year.     

Survival and migration patterns of naturally and hatchery-reared Atlantic salmon (Salmo salar) smolts in a Lake Ontario tributary using acoustic telemetry

1. Atlantic salmon (Salmo salar) smolts are often stocked into rivers to supplement natural reproduction, yet hatchery-reared fish have lower survival compared to wild conspecifics. However, few studies have assessed riverine migratory performance and survival differences in hatchery and wild smolts, or more specifically naturally reared smolts (hatchery fish released earlier as parr), particularly in rivers with weirs which may further reduce survival.
2. Using acoustic telemetry, including a subset of fish with novel transmitters that identify predation events, we assessed survival and migration patterns of hatchery- (2017: n = 32; 2018: n = 30) and naturally reared Atlantic salmon smolts (2017: n = 8; 2018: n = 30) in a Lake Ontario tributary with two weirs to better understand their ecology and assess the influence of environmental parameters on migration.
3. Naturally reared smolts were 13.9 times more likely to survive than hatchery-reared smolts and mark–recapture models indicated that weirs did not reduce survival for either group. Survival per km was lowest at the release site, indicating pre-migration mortality, and specifically high stocking-related mortality of hatchery-reared smolts. Speed and times of day fish migrated (i.e. migratory performance) did not vary by rearing group, suggesting that the high mortality of hatchery-reared smolts may be due to other factors related to hatchery and stocking operations. Overall mean (± SD) migration speed for smolts was 0.70 ± 0.39 km/hr and movements occurred significantly more frequently at night (18:00–06:00).
4. Smolts were detected in Lake Ontario after they left the river; however, the array in Lake Ontario was not conducive to providing much detail regarding movement patterns. There was no predation of the two predation tags detected in Lake Ontario, indicating that movements were made by smolts and not predators.
5. With ongoing restoration efforts of Atlantic salmon in Lake Ontario, it was important to understand the smolt migration patterns and success of the stocked fish. Our findings of similar migratory performance yet different relative survival of hatchery- and naturally reared smolts help inform management with regards to stocking strategies that could improve Atlantic salmon reintroduction success.

     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

The effect of stocking with 0+ year age‐class Atlantic salmon Salmo salar fry: a case study from the River Bush,Northern Ireland

An enhancement programme based on stocking 0+ year age‐class Atlantic salmon Salmo salar, conducted in the River Bush, Northern Ireland, U.K. over the period 1996–2005, was reviewed with reference to the performance and biological characteristics of wild fish. Wild ova to 0+ year fry (summer) survival was c. 8% with subsequent wild 0+ year fry‐to‐smolt survival c. 9%. Stocked unfed 0+ year juveniles gave c. 1% survival to smolt whilst fed 0+ year S. salar stocked in late summer exhibited survival at c. 5%. Stocking with unfed and fed fry contributed to increased smolt production and helped attain local management objectives between 2001 and 2005. Significant differences in biological characteristics were observed between wild and stocked‐origin fish. Wild‐smolt cohorts were dominated by 2+ year age‐class fish on the River Bush whilst smolts originating from fed fry mostly comprised younger 1+ year individuals. The mean mass of 1+ year smolts derived from stocked fed fry was significantly lower than that of wild 1+ year smolts, although these differences were not evident between older age classes. Differences in run timing between wild smolts and smolts derived from stocked fry were also apparent with the stocked‐origin fish tending to run earlier than wild fish. Although the stocking exercise was useful in terms of maximizing freshwater production, concerns over the quality of stocked‐origin recruits and the long term consequences for productivity are highlighted.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

Environmental change influences the life history of salmon Salmo salar in the North Atlantic Ocean

Annual mean total length (L_T) of wild one‐sea‐winter (1SW) Atlantic salmon Salmo salar of the Norwegian River Imsa decreased from 63 to 54 cm with a corresponding decrease in condition factor (K) for cohorts migrating to sea from 1976 to 2010. The reduction in L_T is associated with a 40% decline in mean individual mass, from 2 to 1·2 kg. Hatchery fish reared from parental fish of the same population exhibited similar changes from 1981 onwards. The decrease in L_T correlated negatively with near‐surface temperatures in the eastern Norwegian Sea, thought to be the main feeding area of the present stock. Furthermore, S. salar exhibited significant variations in the proportion of cohorts attaining maturity after only one winter in the ocean. The proportion of S. salar spawning as 1SW fish was lower both in the 1970s and after 2000 than in the 1980s and 1990s associated with a gradual decline in post‐smolt growth and smaller amounts of reserve energy in the fish. In wild S. salar, there was a positive association between post‐smolt growth and the sea survival back to the River Imsa for spawning. In addition, among smolt year‐classes, there were significant positive correlations between wild and hatchery S. salar in L_T, K and age at maturity. The present changes may be caused by ecosystem changes following the collapse and rebuilding of the pelagic fish abundance in the North Atlantic Ocean, a gradual decrease in zooplankton abundance and climate change with increasing surface temperature in the Norwegian Sea. Thus, the observed variation in the life‐history traits of S. salar appears primarily associated with major changes in the pelagic food web in the ocean.     

Evidence of changing migratory patterns of wild Atlantic salmon Salmo salar smolts in the River Bush,Northern Ireland,and possible associations with climate change

The migration patterns, timing and biological characteristics of wild Atlantic salmon Salmo salar smolts in the River Bush, Northern Ireland, were examined over the period 1978–2008. A distinct change in the timing of the smolt run was detected with progressively earlier emigration periods evident across the time series. The shift in run timing ranged from 3·6 to 4·8 days 10 years^?1 for a range of standard migratory audit points. The timing of smolt emigration has been linked to ambient river temperature patterns. Distinct seasonal patterns were evident for biological characteristics of River Bush smolts with mean age and fork length decreasing throughout the emigration period. Marine survival patterns in 1 sea winter River Bush S. salar were strongly influenced by the run timing of the preceding smolt year such that later emigrating cohorts demonstrated increased survival. Possible mechanisms for this relationship based on local climatic variation have been explored, including the effect of potential thermal mismatch between freshwater and marine environments.     

Fjord migration and survival of wild and hatchery-reared Atlantic salmon and wild brown trout post-smolts

The behaviour of wild (n = 43, mean L_T = 152 mm) and hatchery-reared (n = 71, mean L_T = 198 mm) Atlantic salmon and wild anadromous brown trout (n = 34, mean L_T = 171 mm) post-smolts with acoustic transmitters was compared in a Norwegian fjord system. There was no difference in survival between wild and hatchery reared salmon from release in the river mouth to passing receiver sites 9.5 km and 37.0 km from the release site. Mortality approached 65% during the first 37 km of the marine migration for both groups. There was no difference between wild and hatchery-reared salmon either in time from release to first recording at 9.5 km (mean 135 and 80 h), or in the rate of movement through the fjord (mean 0.53 and 0.56 bl s⁻¹). Hatchery-reared salmon reached the 37 km site sooner after release than the wild salmon (mean 168 and 450 h), but rate of movement in terms of body lengths per second did not differ (mean 0.56 and 0.77 bl s⁻¹). The brown trout remained a longer period in the inner part of the fjord system, with much slower rates of movement during the first 9.5 km (mean 0.06 bl s⁻¹).     

Genetic versus Rearing-Environment Effects on Phenotype: Hatchery and Natural Rearing Effects on Hatchery- and Wild-Born Coho Salmon

With the current trends in climate and fisheries, well-designed mitigative strategies for conserving fish stocks may become increasingly necessary. The poor post-release survival of hatchery-reared Pacific salmon indicates that salmon enhancement programs require assessment. The objective of this study was to determine the relative roles that genotype and rearing environment play in the phenotypic expression of young salmon, including their survival, growth, physiology, swimming endurance, predator avoidance and migratory behaviour. Wild- and hatchery-born coho salmon adults (Oncorhynchus kisutch) returning to the Chehalis River in British Columbia, Canada, were crossed to create pure hatchery, pure wild, and hybrid offspring. A proportion of the progeny from each cross was reared in a traditional hatchery environment, whereas the remaining fry were reared naturally in a contained side channel. The resulting phenotypic differences between replicates, between rearing environments, and between cross types were compared. While there were few phenotypic differences noted between genetic groups reared in the same habitat, rearing environment played a significant role in smolt size, survival, swimming endurance, predator avoidance and migratory behaviour. The lack of any observed genetic differences between wild- and hatchery-born salmon may be due to the long-term mixing of these genotypes from hatchery introgression into wild populations, or conversely, due to strong selection in nature—capable of maintaining highly fit genotypes whether or not fish have experienced part of their life history under cultured conditions.     

Thiaminase activity of Baltic salmon prey species: a comparision of net- and predator-caught samples

Thiaminase activity was determined for Gulf of Bothnia (GB) and Gulf of Finland (GF) Baltic herring Clupea harengus membras , sprat Sprattus sprattus and three-spined stickleback Gasterosteus aculeatus sampled from either trawl or gillnet catches or from Baltic salmon Salmo salar stomachs. The thiaminase activity in Baltic herring was about 10-fold higher than that in sprat, and there was almost no thiaminase activity in three-spined stickleback. Thiaminase activity of undigested Baltic herring found in Baltic salmon stomachs was significantly higher than that of trawl-caught Baltic herring from the same sea area, suggesting that there may be a higher risk of predation for Baltic herring with high thiaminase activity, possibly linked to their health. Thiaminase activity of the gastrointestinal contents of Baltic salmon, feeding almost entirely on Baltic herring in the GB, was significantly higher than for Baltic salmon feeding on both Baltic herring and sprat in the GF. Therefore, Baltic herring may be the major source of thiaminase for Baltic salmon. A tank experiment demonstrated that thiaminase activity in Baltic herring may vary, even within very short time periods. The results were consistent with the hypothesis that the thiaminase content in Baltic salmon forage fish may be an important link in the aetiology of the thiamine deficiency syndrome, M74, in Baltic salmon.     

Efficacy of releasing captive reared broodstock into an imperilled wild Atlantic salmon population as a recovery strategy

The strategy of releasing captive reared adult Atlantic salmon Salmo salar into the Magaguadavic River, New Brunswick, Canada, to spawn, was not an effective tool for rebuilding a seriously depressed wild population. The fish were first generation progeny from wild parents, and had spent their entire lives in captivity in either sea or fresh water. No differences in movement or behaviour patterns were observed between freshwater and seawater reared groups. Fish released in the lower river early (35 to 80 days prior to the natural spawning period) moved into a lake low in the system, and most stayed there near the commercial hatchery where they had been reared from egg to smolt. During the spawning season, none moved to the upper river reaches where most spawning habitat exists. Most broodstock released in the upper river reaches near the time of spawning stayed there during the spawning period. The following year few to no Atlantic salmon fry were found, and most appeared not to be offspring of released adults.     

Timing of Atlantic salmon Salmo salar smolt migration predicts successful passage through a reservoir

Around 30% of Atlantic salmon Salmo salar smolts successfully survived passage through Loch Meig, a reservoir in the north of Scotland, en route to the sea. However, this survival rate was in turn dependent on the timing of migration, with the earliest migrants in the spring having the best chance of survival. This could have implication for fisheries management, since the estimation of smolt downstream survival may be influenced by which time period of the smolt run is analysed.     

Diet of post-smolt and one-sea-winter Atlantic salmon in the Bothnian Sea, Northern Baltic

Until July, post-smolt salmon Salmo salar ( n =337; 129–375 mm L _T, mean 225 mm) in the Bothnian Sea relied largely on surface fauna (mainly terrestrial insects). From August onwards, fish was the principal food type. The smallest piscivorous post-smolts were <200 mm, but the main shift to piscivory occurred at sizes of 240–320 mm. Piscivory was promoted by a large smolt size. Almost all one-sea-winter (1-SW) salmon ( n =316; 278–524 mm, mean 397 mm) were piscivorous. Over 70% of the post-smolt and 96% of the 1-SW salmon with identifiable fish species in their stomachs had preyed on herring Clupea harengus. Other fish prey included the ten-spined Pungitius pungitius and three-spined sticklebacks Gasterosteusaculeatus but no sprats Sprattus sprattus. The results support earlier observations of a close relationship between recruitment of herring and production of salmon in the Bothnian Sea, and of the crucial role of smolt size in determining the ability of feeding salmon for utilizing the food resources of the area.     

Stocking experiment with Atlantic salmon and sea trout parr reared on either live prey or a pellet diet

Survival rate and growth parameters of Atlantic salmon fry and sea trout fry were determined after stocking in the wild. Before release (22 May 2009) into the wild the larvae were reared for 10 weeks in the hatchery in three groups: (i) fry fed on live zooplankton , (ii) fry fed on larvae of live nekton, and (iii) fry fed on prepared pellet food. In autumn (15 September 2010) the fish were caught in the wild; the survival rate and growth parameters of both Atlantic salmon and sea trout were the highest in the zooplankton‐fed group, whilst the pellet‐fed group had the lowest survival rate and growth value parameters. Most effective food for hatchery‐reared fishes to be used as stock was the natural living zooplankton. The general conclusion is that the live diet supplied in the rearing period has a positively impact on fish survival in the wild.     

Retrospective growth analysis of Atlantic salmon Salmo salar and implications for abundance trends

Scale archives of Atlantic salmon Salmo salar from Maine, U.S.A., were examined to determine whether ocean conditions affected the long‐term trends in S. salar populations in the southern tier of the species' range in North America. To date, scale analyses of southern tier populations have been limited to hatchery fish; previous studies suggest that post‐smolt growth does not influence recruitment, with the exception that winter growth may play a role in stock maturation rate. A time series of scales from the Machias and Narraguagus Rivers spanning the years 1946 to 1999 was analysed. Image analysis was used to measure intercirculi spacing, which provided proxy variables of growth rate. Post‐smolt growth increment has increased since the early 1990s, as returns have decreased, suggesting that survival factors act on post‐smolts independent of growth. The data support the hypothesis of a decoupling between freshwater size and early marine growth. Growth during the second sea winter was independent of post‐smolt growth, suggesting that individuals are capable of significant compensatory growth. Southern tier North American stocks exhibit a similar pattern of independence between growth and survival as observed for northern tier North American stocks. These data support the inference that the recruitment of the North American and European subspecies is governed by fundamentally different mechanisms.