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Martin H. Larsen, Jörgen I. Johnsson, Svante Winberg, Alexander D. M. Wilson, David Hammenstig, Per-Ove Thörnqvist, Jonathan D. Midwood, Kim Aarestrup, Erik Höglund. 2015. Effects of Emergence Time and Early Social Rearing Environment on Behaviour of Atlantic Salmon: Consequences for Juvenile Fitness and Smolt Migration. PLoS ONE. doi:10.1371/journal.pone.0119127

Consistent individual differences in behaviour have been well documented in a variety of animal taxa, but surprisingly little is known about the fitness and life-history consequences of such individual variation. In wild salmonids, the timing of fry emergence from gravel spawning nests has been suggested to be coupled with individual behavioural traits. Here, we further investigate the link between timing of spawning nest emergence and behaviour of Atlantic salmon (Salmo salar), test effects of social rearing environment on behavioural traits in fish with different emergence times, and assess whether behavioural traits measured in the laboratory predict growth, survival, and migration status in the wild. Atlantic salmon fry were sorted with respect to emergence time from artificial spawning nest into three groups: early, intermediate, and late. These emergence groups were hatchery-reared separately or in co-culture for four months to test effects of social rearing environment on behavioural traits. Twenty fish from each of the six treatment groups were then subjected to three individual-based behavioural tests: basal locomotor activity, boldness, and escape response. Following behavioural characterization, the fish were released into a near-natural experimental stream. Results showed differences in escape behaviour between emergence groups in a net restraining test, but the social rearing environment did not affect individual behavioural expression. Emergence time and social environment had no significant effects on survival, growth, and migration status in the stream, although migration propensity was 1.4 to 1.9 times higher for early emerging individuals that were reared separately. In addition, despite individuals showing considerable variation in behaviour across treatment groups, this was not translated into differences in growth, survival, and migration status. Hence, our study adds to the view that fitness (i.e., growth and survival) and life-history predictions from laboratory measures of behaviour should be made with caution and ideally tested in nature.



Per-Ove Thörnqvist, Erik Höglund, and Svante Winberg. 2015. Natural selection constrain personality and brain gene expression differences in Atlantic salmon (Salmo salar). The Journal of Experimental Biology. doi: 10.1242/​jeb.114314

In stream spawning salmonid fishes there is a considerable variation in the timing of when fry leave the spawning nests and establish a feeding territory. The timing of emergence from spawning nests appears to be related to behavioural and physiological traits, e.g. early emerging fish being bolder and more aggressive. In the present study, emerging Atlantic salmon (Salmo salar L.) alevins were sorted into three fractions early, intermediate and late emerging. At the parr stage, behaviour, stress responses, hindbrain monoaminergic activity and forebrain gene expression, were explored in fish from the early and late emerging fractions (first and last 25%). The results show that when subjected to confinement stress, fish from the late emerging fraction respond with a larger activation of the brain serotonergic system than fish from the early fraction. Similarly, in late emerging fish stress resulted in elevated expression of mRNA coding for serotonin 1A receptors, GABA-A receptor associated protein and ependymin, effects not observed in fish from the early emerging fraction. Moreover, fish from the early emerging fraction displayed bolder behaviour than late emerging littermates. Taken together these results suggest that time of emergence, boldness and aggression are linked to each other, forming a behavioural syndrome in juvenile salmon. Differences in brain gene expression between early and late emerging salmon put further support to a relationship between stress coping style and timing of emergence. However, early and late emerging salmon do not appear to differ in HPI axis reactivity, another characteristic of divergent stress coping styles




J. I. Johnsson, S. Brockmark and J. Näslund. 2014. Environmental effects on behavioural development: consequences for fitness of captive-reared fishes in the wild. Journal of Fish Biology. doi:10.1111/jfb.12547

Why do captive-reared fishes generally have lower fitness in natural environments than wild conspecifics, even when the hatchery fishes are derived from wild parents from the local population? A thorough understanding of this question is the key to design artificial rearing environments that optimize post-release performance, as well as to recognize the limitations of what can be achieved by modifying hatchery rearing methods. Fishes are generally very plastic in their development and through gene–environment interactions, epigenetic and maternal effects their phenotypes will develop differently depending on their rearing environment. This suggests that there is scope formodifying conventional rearing environments to better prepare fishes for release into the wild. The complexity of the
natural environment is impossible tomimic in full-scale rearing facilities. So, in reality, the challenge is to identify key modifications of the artificial rearing environment that are practically and economically
feasible and that efficiently promote development towards a more wild-like phenotype. Do such key modifications really exist? Here, attempts to use physical enrichment and density reduction to improve the performance of hatchery fishes are discussed and evaluated. These manipulations show potential to increase the fitness of hatchery fishes released into natural environments, but the success is strongly dependent on adequately adapting methods to species and life stage-specific conditions.



Joacim Näslund and Jörgen I Johnsson. 2014. Environmental enrichment for fish in captive environments: effects of physical structures and substrates. Fish and Fisheries. DOI: 10.1111/faf.12088

Structural environmental enrichment, that is, a deliberate addition of physical complexity to the rearing environment, is sometimes utilized to reduce the expression of the undesirable traits that fish develop in captivity. Increasing demands and regulations regarding usage of enrichment to promote fish welfare also make investigations on the effects of enrichment important. Here, we sythesize the current state-of-the-art knowledge about the effects of structural environmental enrichment for fish in captive environments. We find that enrichment can affect several aspects of the biology of captive fish, for example, aggression, stress, energy expenditure, injury and disease susceptibility. Importantly, these effects are often varying in direction and magnitude, and it is clear that just addition of structure is not a solution to all problems in fish rearing. Each species and life stage needs special consideration with respect to its natural history and preferences. A multitude of different enrichment types has been investigated and many studies investigate several enrichment components at the same time, making comparisons among studies difficult. To the present date, the majority of efforts have been directed to investigate salmonid fish in stock-fish hatcheries and cichlids from a basic research perspective. Some contexts are under-studied with respect to environmental enrichment, for instance effects on results in basic research and welfare effects in display aquaria. There are many research opportunities left within this field. However, future studies could utilize experimental designs which make it possible to discriminate between effects of different environmental manipulations to a higher degree than what has been performed to this date.



L. Persson and A. Alanärä. 2014. The effect of shelter on welfare of juvenile Atlantic salmon Salmo salar reared under a feed restriction regimen. Journal of Fish Biology. doi:10.1111/jfb.12443

This study investigated whether shelter (presence or absence) affected the frequency of fin damage in Atlantic salmon Salmo salar exposed to feed restrictions (0·73 or 0·33% of body mass day−1). The presence of shelter had a positive effect on the pectoral fins at both feed ration levels and on the dorsal fin at the higher ration level but it had a negative effect on survival. The reduced feed rations resulted in fish of the same size and nutritional status as wild fish. The provision of shelter has potential to mitigate the negative effects of feed restrictions on fin quality, but the optimal shelter design requires some additional investigation.



Jonathan D. Midwood, Martin Hage Larsen, Mikkel Boel, Niels Jepsen, Kim Aarestrup, and Steven J. Cooke. 2014. Does cortisol manipulation influence outmigration behaviour, survival and growth of sea trout? A field test of carryover effects in wild fish. Mar. Ecol. Prog. Ser. doi: 10.3354/meps10524

For anadromous brown trout Salmo trutta, the transition from life in freshwater to the marine environment is an inherently challenging and dangerous period characterized by high levels of mortality. As such, smoltification is a relevant life-history phase to examine how physiological state, in particular glucocorticoids, influences fitness-oriented endpoints such as migration timing and survival. We experimentally assessed the effect of cortisol by combining passive integrated transponder (PIT) telemetry with a physiologically relevant exogenous cortisol manipulation (i.e. intracoelomic injection) in juvenile sea trout in the Gudsø Stream, Denmark. Individual survival, migration behaviour (timing and speed), and growth were assessed for 4 treatment categories: control (CO), sham (SH), and low- (LW; 25 mg kg-1) and high-dose (HI; 100 mg kg-1) cortisol. There was no difference in the timing of migration among treatments, but trout in the HI treatment had lower survival rates to the lower station (41.6%) when compared to the CO (53.9%) and SH (52.3%) groups. After migration, the system was electroshocked again to contrast growth of trout that remained in the system. HI, LW and SH individuals recaptured in the stream had lower growth rates for length than the CO treatments; HI and LW also had significantly lower growth rates for mass than CO trout. Future monitoring of this population may demonstrate the long-term repercussions of chronic stress as trout return from the ocean. This study provides contributions to our understanding of the relationship between organismal condition and fitness while elucidating the potential for carryover effects, i.e. lasting effects that influence future success.



D. Hammenstig, E. Sandblom, M. Axelsson, and J. I. Johnsson. 2014. Effects of rearing density and dietary fat content on burst-swim performance and oxygen transport capacity in juvenile Atlantic salmon Salmo salar. Journal of Fish Biology. doi:10.1111/jfb.12511

The effects of hatchery rearing density (conventional or one third of conventional density) and feeding regime (high or reduced dietary fat levels) on burst-swim performance and oxygen transport capacity were studied in hatchery-reared Atlantic salmon Salmo salar, using wild fish as a reference group. There was no effect of rearing density or food regime on swimming performance in parr and smolts. The maximum swimming speed of wild parr was significantly higher than that of hatchery-reared conspecifics, while no such difference remained at the smolt stage. In smolts, relative ventricle mass was higher in wild S. salar compared with hatchery-reared fish. Moreover, wild S. salar had lower maximum oxygen consumption following a burst-swim challenge than hatchery fish. There were no effects of hatchery treatment on maximum oxygen consumption or relative ventricle mass. Haemoglobin and haematocrit levels, however, were lower in low-density fish than in fish reared at conventional density. Furthermore, dorsal-fin damage, an indicator of aggression, was similar in low-density reared and wild fish and lower than in S. salar reared at conventional density. Together, these results suggest that reduced rearing density is more important than reduced dietary fat levels in producing an S. salar smolt suitable for supplementary release.



Kim Aarestrup, Henrik Baktoft, Anders Koed. Diego del Villar-Guerra, and Eva B. Thorstad. 2014. Comparison of the riverine and early marine migration behaviour and survival of wild and hatchery-reared sea trout Salmo trutta smolts. Mar. Ecol. Prog. Ser. doi:10.3354/meps10614

The seaward migration of wild (n = 61) and hatchery-reared (n = 46) sea trout smolts was investigated in the Danish River Gudenaa and Randers Fjord (17.3 and 28.6 km stretch, respectively) using acoustic telemetry. Their riverine and early marine migration was monitored by deploying automatic listening stations (ALS) at 4 locations in the river and fjord. Migration speeds were approximately 3 to 11 times faster in the river than in the early marine environment. Hatchery-reared smolts migrated faster than wild smolts, but the difference was small, especially compared with the large differences in migration speeds among habitats. There was no difference in the diurnal activity pattern between wild and hatchery-reared smolts. Both the riverine and early marine migration activity were primarily nocturnal, although some individuals were also recorded by the ALSs during the daytime. The survival of the wild smolts from release in the river to the outermost marine ALS site, 46 km from the release site, was 1.8 and 2.9 times higher than that of the hatchery-reared smolts in the 2 study years, respectively. Overall, survival from release to the outermost ALS site was 79% for wild and 39% for hatchery-reared smolts. Since the lower survival of the hatchery-reared compared with the wild smolts could not be explained by differences in migration speeds or diurnal migration patterns, behavioural differences on a smaller scale than those recorded in the present study may explain the difference in survival.





Martin H Larsen, Aske N Thorn, Christian Skov, and Kim Aarestrup. 2013. Effects of passive integrated transponder tags on survival and growth of juvenile Atlantic salmon Salmo salar. Animal Biotelemetry. doi:10.1186/2050-3385-1-19

A laboratory experiment was conducted to assess the potential impacts of surgically implanted 23 and 32 mm passive integrated transponder (PIT) tags on survival, growth, and body condition of juvenile Atlantic salmon Salmo salar. Rate of tag retention and healing of the tagging incision were also evaluated. Atlantic salmon of three different size classes (I: 80 to 99 mm fork length (FL), II: 100 to 119 mm FL, III: 120 to 135 mm FL) were allocated to each of five experimental treatment groups: control, sham-operated (surgery without PIT-tag implantation), 23 mm PIT-tag implantation with and without suture closure of the incision, and 32 mm PIT-tag implantation without suture closure.

Over the 35-day experiment, mortality occurred only among fish tagged with 32 mm PIT tags (14%) and all fish larger than 103 mm FL survived. Non-sutured Atlantic salmon between 80 and 99 mm FL implanted with 23 mm PIT tags had a significantly lower mean specific growth rate of mass compared with untagged (control and sham-operated) and sutured conspecifics. However, no significant difference in growth was found between untagged fish and 23 mm PIT-tagged fish 100 to 135 mm FL. Implantation of 32 mm PIT tags decreased growth in all size classes. Regardless of size class, body condition of the fish was not affected by PIT tagging. Retention rates of 23 mm PIT tags with and without suture closure were 100% and 97%, respectively; retention of 32 mm PIT tags without suture closure was 69%. At the end of the experiment, tagging incisions without suture closure were generally well-healed. Fungal infection and inflammation around the incision site occurred only when suture was used, in 46% of size class I, 21% of size class II and 38% of size class III.

Although suture closure of the incision following 23 mm PIT-tag implantation had a positive impact on growth of fish smaller than 100 mm FL, we advise against the use of sutures due to high rates of fungal infection around the incision site. Hence, results suggest that surgical implantation of 23 mm PIT tags without suture closure of the incision is a feasible method for marking juvenile Atlantic salmon 100 to 135 mm FL. Further, we caution researchers about the use of 32 mm PIT tags in juvenile Atlantic salmon 80 to 135 mm FL due to high rate of tag rejection and reduced survival and growth.



Diego del Villar-Guerra, Kim Aarestrup, Christian Skov, and Anders Koed. 2013. Marine migrations in anadromous brown trout (Salmo trutta). Fjord residency as a possible alternative in the continuum of migration to the open sea. Ecology of Freshwater Fish. DOI: 10.1111/eff.12110

Partial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater. In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT-tags in the river Villestrup (Denmark) to study the initial postsmolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord, while others migrate to the sea, suggesting that partial migration occurs even in the marine environments. Overall, a total of 53% of the tagged smolts migrated from the fjord to the sea, and 47% stayed (or potentially died) in the fjord. The ratios of fjord-resident versus sea-migrating postsmolts were consistent at the study times, and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout postsmolts encounter a fjord system. At this point, postsmolts will assess the possibility of migration versus the alternative of fjord residency.



Joacim Näslund, Malin Rosengren, Diego Del Villar, Lars Gansel, Johnny R. Norrgård, Lo Persson, John James Winkowski, and Eli Kvingedal. 2013. Hatchery tank enrichment affects cortisol levels and shelter-seeking in Atlantic salmon (Salmo salar). Can. J. Fish. Aquat. Sci. dx.doi.org/10.1139/cjfas-2012-0302

Stocking programs using hatchery-reared salmon are often implemented for augmenting natural populations. However, survival of these fish is often low compared with wild conspecifics, possibly because of genetic, physiological, and behavioural deficiencies. Here, we compared presmolt Atlantic salmon (Salmo salar) from three different environmental treatments (barren environment, plastic tube enrichment, and plastic shredding enrichment) with regard to plasma cortisol levels, shelter-seeking behaviour, and fin deterioration. Basal plasma cortisol levels were higher in barren-reared fish, indicating higher stress levels, while no differences were found in acute cortisol response after a 30 min confinement test. Shelter-seeking was higher in salmon reared in enriched tanks when tested alone, but not when tested in small groups. Barren-reared fish had higher levels of fin deterioration over winter, potentially owing to higher aggression levels. These results suggest that enrichment can
reduce the impact of stressors experienced in the hatchery and thus increase fish welfare. Tank enrichment may also be used to produce salmon better adapted for the more complex environment encountered after release.




J. Näslund, K. Aarestrup, S. T. Thomassen, and J. I. Johnsson. 2012. Early enrichment effects on brain development in hatchery-reared Atlantic salmon (Salmo salar): no evidence for a critical period. Can. J. Fish. Aquat. Sci. doi:10.1139/F2012-074.

In hatcheries, fish are normally reared in barren environments, which have been reported to affect their phenotypic development compared with wild conspecifics. In this study, Atlantic salmon (Salmo salar) alevins were reared in conventional barren hatchery trays or in either of two types of structurally enriched trays. We show that increased structural complexity during early rearing increased brain size in all investigated brain substructures. However, these effects disappeared over time after transfer to barren tanks for external feeding. Parallel to the hatchery study, a group of salmon parr was released into nature and recaptured at smoltification. These stream-reared smolts developed smaller brains than the hatchery reared smolts, irrespective of initial enrichment treatment. These novel findings do not support the hypothesis that there is a critical early period determining the brain growth trajectory. In contrast, our results indicate that brain growth is plastic in relation to environment. In addition, we show allometric growth in brain substructures over juvenile development, which suggests that comparisons between groups of different body size should be made with caution. These results can aid the development of ecologically sound rearing methods for conservational fish-stocking programs.



L. Sundt-Hansen, S. Einum, L. Neregård, B.T. Björnsson,
J.I. Johnsson, I.A. Fleming, R.H. Devlin and K. Hindar
. 2012. Growth hormone reduces growth in free-living Atlantic salmon fry. Funct. Ecol. doi: 10.1111/j.1365-2435.2012.01999.x.
1. Although life-history theory predicts that juvenile growth rates should be high, there is substantial evidence that most juveniles grow below their physiological maximum. The endocrine system plays an important role in the determination of fundamental life-history traits, and hormones often serve as a link between an organism’s environment and the expression of a trait. Particularly, growth is a life-history trait, which is strongly associated with growth hormone
(GH) in fish, as well as most vertebrates.
2. To elucidate trade-offs related to elevated GH in fish in a natural environment, we experimentally administrated GH exogenously to juvenile Atlantic salmon using sustained-release GH implants, at an earlier ontogenetic stage than previously achieved (1Æ5 months). We assessed the effects on growth, dispersal and survival in contrasting environments.
3. Exogenous GH treatment increased the growth rate when fish were fed ad libitum in captivity. However, in a natural stream, GH treatment had a significant negative effect on growth and no apparent effect on survival or dispersal. This contrasts with previous studies conducted at later developmental stages, which show either a positive growth effect or no effect of elevated GH levels.
4. This study shows that environmental conditions strongly affect the response to GH and that under some natural conditions, it may also reduce growth. We suggest that the endogenous plasma GH levels may be maximizing growth during early, but not later, juvenile stages
in nature.




J. Vaz-Serrano, M. L. Ruiz-Gomez, H. M. Gjøen, P. V. Skov, F. A. Huntingford, Ø. Øverli and E. Höglund. 2011. Consistent boldness behaviour in early emerging fry of domesticated Atlantic salmon (Salmo salar): Decoupling of behavioural and physiological traits of the proactive stress coping style. Physiol. Behav. doi: 10.1016/j.physbeh.2011.02.025.

Individual variation in the way animals cope with stressors has been documented in a number of animal groups. In general, two distinct sets of behavioural and physiological responses to stress have been described: the proactive and the reactive coping styles. Some characteristics of stress coping style seem to be coupled to the time to emerge of fry from spawning redds in natural populations of salmonid fishes. In the present study, behavioural and physiological traits of stress coping styles were compared two and five months after emergence in farmed Atlantic salmon (Salmo salar), using individuals with an early or late time to emerge. Initially, compared to late emerging individuals, early emerging individuals showed a shorter time to resume feeding after transfer to rearing in isolation. Resumption of feeding after isolation was suggested to be related to boldness behaviour, rather than hunger, in the present study. This observation was repeated five months after emergence, demonstrating behavioural consistency over time in this trait. However, in other traits of proactive and reactive stress coping styles, such as social status, resting metabolism or post stress cortisol concentrations, early and late emerging individuals did not differ. Therefore, this study demonstrates
that boldness in a novel environment is uncoupled from other traits of the proactive and reactive stress coping styles in farmed salmonids. It is possible that this decoupling is caused by the low competitive environment in which fish were reared. In natural populations of salmonids, however, the higher selection pressure at emergence could select for early emerging individuals with a proactive coping style.




Adriaenssens, B. and J. I. Johnsson. 2010. Shy trout grow faster: exploring links between personality and fitness-related traits in the wild. Behav. Ecol. doi: 10.1093/beheco/arq185.

In many animals, individual differences in behavior show remarkable consistency across situations and contexts (i.e., animal personality and behavioral syndromes). Studies on the association between personality traits and fitness-related measures in nature are, however, important to clarify the causes and consequences of this phenomenon. Here, we tested for correlations between 3 behavioral axes in brown trout (Salmo trutta) parr: exploration tendency, behavioral flexibility, and aggressiveness. Next, we tested how these individual behaviors relate to social dominance and performance under natural conditions (growth, survival, and movement). We found support for behavioral syndromes in brown trout with less explorative individuals being less aggressive and showing more flexible behavior. In addition, these low-explorative personality types grew faster than bolder conspecifics in the wild. Standardized aggression in the laboratory was a poor indicator of social dominance, and neither of these 2 traits affected performance in the wild. These results challenge the view that personality traits can be predicted by constant associations with life-history trade-offs (e.g., boldness is linked with rapid growth). Moreover, our findings suggest that fitness predictions from laboratory measures of behavior should be made with caution and ideally tested in nature



Brockmark, S., B. Adriaenssens, and J. I. Johnsson. 2010. Less is more: density influences the development of behavioural life skills in trout. Proc. R. Soc. B. doi: 10.1098/rspb.2010.0561.

Theory suggests that habitat structure and population density profoundly influence the phenotypic development of animals. Here, we predicted that reduced rearing density and increased structural complexity promote food search ability, anti-predator response and the ability to forage on novel prey, all behavioural skills important for surviving in the wild. Brown trout were reared at three densities (conventional hatchery density, a fourth of conventional hatchery density and natural densit" in tanks with or without structure. Treatment effects on behaviour were studied on trout fry and parr, whereupon 20 trout from each of the six treatment groups were released in an enclosed natural stream and recaptured after 36 days. Fry reared at natural density were faster to find prey in a maze. Moreover, parr reared at natural density were faster to eat novel prey, and showed more efficient anti-predator behaviour than fish reared at higher densities. Furthermore, parr reared at reduced densities were twice as likely to survive in the stream as trout reared at high density. In contrast, we found no clear treatment effects of structure. These novel results suggest that reduced rearing densities can facilitate the development of behavioural life skills in captive animals, thereby increasing their contribution to natural production.



Brockmark, S., and J. I. Johnsson. 2010. Reduced hatchery rearing density increases social dominance, postrelease growth, and survival in brown trout (Salmo trutta). Can. J. Fish. Aquat. Sci. 67:288-295.


Hatchery fish reared for conservation or supplementation often have difficulties adapting to natural conditions, resulting in poor performance in the wild. In a standard hatchery, fish are confined at high densities, which creates a social environment different from that experienced after release. Here we investigated how rearing density influences social dominance, postrelease growth, and survival in brown trout (Salmo trutta). Fish were reared at three density treatments: conventional hatchery density, half of conventional hatchery density, and natural density. Four months after hatching, dominance status was determined, and 36 fish from each treatment were released into an enclosed stream and recaptured after 36 days. Trout reared at natural density had higher dominance status and grew faster, both in the hatchery and in the natural stream, than trout from higher densities. Moreover, trout reared at natural density were twice as likely to survive in the stream as trout from higher densities. These novel results suggest that more natural rearing densities would facilitate the development of adaptive behaviour in hatchery salmonids and, thereby, their contribution to natural production.



Lundqvist, H., K. Leonardsson, U. Carlsson, S. Larsson, J. Nilsson, J. Östergren, L. Karlsson, P. Rivinoja, I. Serrano, D. Palm, and J. Ferguson. 2010. Monitoring Juvenile Atlantic Salmon and Sea Trout in the River Sävarån, Northern Sweden. Pp. 207-218 in C. Hurford, M. Schneider, and I. Cowx, eds. Conservation Monitoring in Freshwater Habitats. Springer Netherlands.

Downstream migrations of Atlantic salmon (Salmo salar) and sea trout (S. trutta) smolts were monitored in the in northern Sweden to evaluate the river’s potential as a national index river for salmonid populations in the Baltic region, and to acquire information for stock conservation plans. A rotary screw trap was installed and operated from 2005 to 2008. Recapture rates of tagged smolts were high (from 8% to 31%), which allowed smolt abundances to be estimated. Between 2,600 and 3,900 salmon and 500–1,500 trout emigrated from the river each year. Genetic analyses made it possible to identify the origin of salmon and trout smolts and to determine whether they originated from the Sävarån stock or from stocks in neighbouring rivers. In 2005 and 2006, most of the salmon were from the Sävarån stock, which previously had been thought to be extinct. By contrast, 58% and 52% of the salmon analysed in 2007 and 2008, respectively, originated from stocks other than Sävarån. The density of salmon and trout 0 parr (i.e. recruitment success) based on electro-fishing was a poor predictor of smolt abundance 2 to 3 years later. The smolt trap data combined with genetic analyses provided valuable information to develop stock conservation plans.



Serrano, I., S. Larsson, and L. O. Eriksson. 2009. Migration performance of wild and hatchery sea trout (Salmo trutta L.) smolts-Implications for compensatory hatchery programs. Fisheries Research 99:210-215.

Migration success of hatchery-reared and wild sea trout smolts through the lower stretches and the estuary of a Baltic sea river were studied. During 3 years, wild and hatchery trout smolts were implanted with acoustic transmitters and released 14 km upstream from the river mouth. In order to monitor their out-migration pattern, acoustic receivers were deployed along the migratory route. Data on number of fish detected and date and time of detections were analysed and the migratory performance of wild and hatchery-reared fish was compared. A significantly higher proportion of wild fish (80%) successfully migrated to the coast compared to fish of hatchery origin (27.5%) and migration was faster in wild smolts. Hatchery fish were larger and had a higher condition factor and lipid concentrations, which are proposed as possible reasons for the poorer migratory performance of the hatchery-reared fish.



Serrano, I., P. Rivinoja, L. Karlsson, and S. Larsson. 2009. Riverine and early marine survival of stocked salmon smolts, Salmo salar L., descending the Testebo River, Sweden. Fish. Manag. Ecol. 16:386-394.

A combination of radio and acoustic telemetry was used to monitor the out-migration of hatchery-reared Atlantic salmon, Salmo salar L., in the River Testebo, its estuary and coastal system. As with many other Baltic rivers, the hydropower regulated River Testebo once had a self-sustaining salmon population that is now extinct. Substantial losses of smolts in the river (48-69%) and inner part of the estuary (43-47%) were found, but after leaving the estuary, the success of post-smolts moving out of the Bay was sufficiently high (83-89%) to conclude that habitat within the bay is not a factor limiting initial marine survival. The results suggest that hatchery-based recovery of a wild salmon population in the river will not be successful unless other actions, such as habitat improvement, are included. 



Brockmark, S., L. Neregard, T. Bohlin, B. T. Bjornsson, and J. I. Johnsson. 2007. Effects of rearing density and structural complexity on the pre- and postrelease performance of Atlantic salmon. Trans. Am. Fish. Soc. 136:1453-1462.

Hatchery fish released for supplementation purposes often have difficulties adapting to wild conditions and, therefore, perform poorly in the wild. This can, at least partly, be explained by differences between hatchery and wild conditions. Our objective was to evaluate the effects of rearing density and structural complexity on the culture and postrelease performance of age-0 Atlantic salmon Salmo salar. By using a 2 x 2 factorial design, we manipulated density (standard density and one-third of standard density) and structure (standard rearing tanks with and without added rocks and plastic plants) in a conventional hatchery. After 3 months of rearing, 300 fish/treatment were individually tagged with passive integrated transponders, released in a nearby stream, and recaptured in late autumn (November) and summer (June). Fish not released were retained in the hatchery until smolting occurred during the next spring. In the stream, Atlantic salmon reared at reduced density grew faster during the first period after release, but there was no difference in June. The treatments had no effect on postrelease survival estimated by the recapture rates. In the hatchery, fish kept at low density with structure grew faster than conventionally reared fish. At smoltification, fish kept at low density had higher levels of insulin-like growth factor I than those reared at standard density. Independent of size, fish kept at low density were more silvery (smolt-like) and had a lower mortality rate than fish reared at high density. There was also a density effect on dorsal fin damage; Atlantic salmon at reduced density had less-damaged fins than those at standard density. These results collectively indicate that reduced rearing density may be more important than structural complexity for improving postrelease performance of juvenile Atlantic salmon.



Top photo by Bo-Sören Wiklund

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