Contemporary divergence in early life history in grayling (Thymallus thymallus)

<p>Abstract</p> <p>Background</p> <p>Following colonization of new habitats and subsequent selection, adaptation to environmental conditions might be expected to be rapid. In a mountain lake in Norway, Lesjaskogsvatnet, more than 20 distinct spawning demes of grayling have been established since the lake was colonized, some 20-25 generations ago. The demes spawn in tributaries consistently exhibiting either colder or warmer temperature conditions during spawning in spring and subsequent early development during early summer. In order to explore the degree of temperature-related divergence in early development, a multi-temperature common-garden experiment was performed on embryos from four different demes experiencing different spring temperatures.</p> <p>Results</p> <p>Early developmental characters were measured to test if individuals from the four demes respond differently to the treatment temperatures. There was clear evidence of among-deme differences (genotype - environment interactions) in larval growth and yolk-to-body-size conversion efficiency. Under the cold treatment regime, larval growth rates were highest for individuals belonging to cold streams. Individuals from warm streams had the highest yolk-consumption rate under cold conditions. As a consequence, yolk-to-body-mass conversion efficiency was highest for cold-deme individuals under cold conditions. As we observed response parallelism between individuals from demes belonging to similar thermal groups for these traits, some of the differentiation seems likely to result from local adaptation</p> <p>Conclusion</p> <p>The observed differences in length at age during early larval development most likely have a genetic component, even though both directional and random processes are likely to have influenced evolutionary change in the demes under study.</p

Contemporary temperature-driven divergence in a Nordic freshwater fish under conditions commonly thought to hinder adaptation

BACKGROUND: Evaluating the limits of adaptation to temperature is important given the IPCC-predicted rise in global temperatures. The rate and scope of evolutionary adaptation can be limited by low genetic diversity, gene flow, and costs associated with adaptive change. Freshwater organisms are physically confined to lakes and rivers, and must therefore deal directly with climate variation and change. In this study, we take advantage of a system characterised by low genetic variation, small population size, gene flow and between-trait trade-offs to study how such conditions affect the ability of a freshwater fish to adapt to climate change. We test for genetically-based differences in developmental traits indicating local adaptation, by conducting a common-garden experiment using embryos and larvae from replicate pairs of sympatric grayling demes that spawn and develop in natural cold and warm water, respectively. These demes have common ancestors from a colonization event 22 generations ago. Consequently, we explore if diversification may occur under severely constraining conditions. RESULTS: We found evidence for divergence in ontogenetic rates. The divergence pattern followed adaptation predictions as cold-deme individuals displayed higher growth rates and yolk conversion efficiency than warm-deme individuals at the same temperature. The cold-deme embryos had a higher rate of muscle mass development. Most of the growth- and development differences occurred prior to hatch. The divergence was probably not caused by genetic drift as there was a strong degree of parallelism in the divergence pattern and because phenotypic differentiation (Q(ST)) was larger than estimated genetic drift levels (microsatellite F(ST)) between demes from different temperature groups. We also document that these particular grayling populations cannot develop successfully at temperatures above 12°C, whereas other European populations can, and that increasing the muscle mass development rate comes at the cost of some skeletal trait development rates. CONCLUSIONS: This study shows that genetically based phenotypic divergence can prevail even under conditions of low genetic variation and ongoing gene flow. Furthermore, population-specific maximum development temperatures along with musculoskeletal developmental trade-offs may constrain adaptation

Empirical support for sequential imprinting during downstream migration in Atlantic salmon (Salmo salar) smolts

The precise homing of Atlantic salmon to their natal river and spawning grounds is the foundation for locally adapted genetically differentiated populations across rivers or across river sections. A sequential imprinting hypothesis states that salmon smolts may imprint on environmental clues along the outward migration route and then use this in reverse order to direct the spawning migration later in life. In this study, we provide empirical support for this hypothesis. PIT-tagged wild Atlantic salmon using a 2 km hydropower tunnel as downstream migrating smolts had a 18% (1SW) and 23% (2SW) lower probability of successfully migrating through the parallel river stretch as adult spawners compared to spawners that migrated through the same river stretch as smolts. These findings highlight how a fine-scale riverine migration route may be imprinted in wild Atlantic salmon smolts. From an applied perspective, these results stress the importance of not depriving smolts from parts of their migration route to ensure successful return of adults to their natal spawning grounds.publishedVersio

Spatially structured interactions between lobsters and lobster fishers in a coastal habitat: fine-scale behaviour and survival estimated from acoustic telemetry

Fishing can have profound impacts on the ecology and evolution of marine populations. Understanding population-level changes ultimately depends on knowledge about individual survival and how it varies in time and space. We used acoustic tags and a network of receivers to monitor individual behaviour and fate of European lobster (Homarus gammarus) exposed to commercial and recreational trap fisheries on the Norwegian Skagerrak coast. In August 2011, 50 male lobsters above minimum legal size were tagged and monitored before and during the lobster fishing season. We also quantified the spatial and temporal variation in fishing activity. There was no significant effect of home-range size on the probability of surviving the fishery. However, there was substantial fine-scale spatial variation in fishing activity, and lobsters with short-term home ranges positioned away from trap-dense areas had a significantly higher survival probability. Also, the overall survival probability of 16.7% suggests that fishing depletes the catchable lobster population at a high rate. The current harvesting regime may drive selection in favour of movement behaviours avoiding habitats typically targeted by fishers

Body downsizing caused by non-consumptive social stress severely depresses population growth rate

Chronic social stress diverts energy away from growth, reproduction and immunity, and is thus a potential driver of population dynamics. However, the effects of social stress on demographic density dependence remain largely overlooked in ecological theory. Here we combine behavioural experiments, physiology and population modelling to show in a top predator (pike Esox lucius) that social stress alone may be a primary driver of demographic density dependence. Doubling pike density in experimental ponds under controlled prey availability did not significantly change prey intake by pike (i.e. did not significantly change interference or exploitative competition), but induced a neuroendocrine stress response reflecting a size-dependent dominance hierarchy, depressed pike energetic status and lowered pike body growth rate by 23 per cent. Assuming fixed size-dependent survival and fecundity functions parameterized for the Windermere (UK) pike population, stress-induced smaller body size shifts age-specific survival rates and lowers age-specific fecundity, which in Leslie matrices projects into reduced population rate of increase (λ) by 37–56%. Our models also predict that social stress flattens elasticity profiles of λ to age-specific survival and fecundity, thus making population persistence more dependent on old individuals. Our results suggest that accounting for non-consumptive social stress from competitors and predators is necessary to accurately understand, predict and manage food-web dynamics

Sjøauren i Lærdalselva; vekstmønster og fjordvandringar før og no

Forsknings- og utgreiingsarbeid knytt til behandlingar mot Gyrodactylus salaris i Lærdalselva har framskaffa interessante data på sjøaurestamma sin biologi. Telemetristudiar av vandringsåtferd i elv og fjord er gjennomførde. Det er i tillegg innsamla skjelprøvar av all merka fisk. Samstundes eksisterar unike historiske data frå fjordsystemet. Frå byrjinga av 50- til midten av 60-talet er merke-gjenfangsstudie og skjellanalysar utførd. Vekst i ferskvatn og alder/storleik ved smoltifisering synast å ha endra seg mellom dei to periodane, med ein betre vekst og høgre alder/storleik ved første gongs sjøvandring i dag. Vandringane i sjøen er lange, og til dels lengre enn rapportert i andre studie på aure. Store delar av bestanden nyttar seg av ytre fjordstrøk og fjordmunningen som sitt marine fødeområde. Vandringsfarta kan være høg. Opphaldstida i det marine miljø er varierande, og ein ser døme på fisk som oppheld seg i sjøen i vinterhalvåret. Resultata viser at Lærdalsauren nyttar seg av store delar av fjordsystemet og kystnære strøk, og at tiltak knytt til å ivareta stamma bør omfatta vurderingar av heile Sognefjorden med tilgrensande områd

Vandringsmønsteret til laksesmolt i Vossovassdraget med vekt på detaljert kartlegging av åtferd i innsjøsystema og effektar av miljøtilhøve

The River Vosso population of Atlantic salmon (Salmo salar) was strongly reduced over the course of a few years during late 1980s, and has not recovered since, despite substantial mitigation efforts. The ultimate reason(s) for the population decline remains enigmatic, as are the causes for lack of recovery. After decades of poor returning numbers of naturally produced salmon, it seems likely that the causative factors are still in operation. Former studies indicate heavy mortalities of hatchery-produced smolts during migration through the inner fjord, and especially in the Bolstadfjord estuary. A large-scale acoustic telemetry study conducted during 2015 revealed high smolt mortality in both Evangervatnet and the Bolstadfjord estuary. Most of the Evangervatnet mortality was located close to the Evanger power plant. Analyses of power plant outlet water showed supersaturation of nitrogen and oxygen and was suggested as a possible mortality factor for the smolt. Motivated by these findings, a large-scale telemetry study was set up for the 2016 smolt descent. Passive receivers (n=89) were organized into 19 cross-sectional transects dividing the migratory route into 20 zones from the Upper Vosso area to the Nordhordlandsbrua area in outer reaches of Osterfjorden. River mouths and the Evanger power plant area were covered with triangulation networks of receivers, allowing for detailed positioning. In total, 50 hatchery-reared and 150 wild salmon smolt were tagged with acoustic transmitters. All hatchery-reared smolt were tagged with ordinary 7.3×18 mm Id-transmitters and released into Upper Vosso area. Wild smolt were caught in and released into three river sections: Upper Vosso, Vosso and Bolstadelva. One-half of the 50 wild smolt from each river section was tagged with id-transmitters (IDT), and the other half with 7.3×22 mm depth-sensors transmitters (DST). Altogether, more than 8 million tag detections were recorded during the mid-April to late August period, where 91.4% of the wild smolt were detected, and 46% of the hatchery-reared ones. All hatchery-reared smolt died prior to reaching Evangervatnet, and displayed highly differentiated behavior when compared to wild smolt in terms of time-of-day and day-of-year timing of migration. Furthermore, migration in this group was not correlated with environmental factors. The wild smolt from all three release sections displayed a strong positive correlation between onset of river descent and river discharge and/or water temperature. In all wild smolt groups, the initiation of river descent was largely synchronous and took place during nighttime. River descent started in mid-April for the earliest individuals in Upper Vosso, but ca 80% descended during the 3-12 May period that coincided with a flooding event. Because the migration of smolt from both Upper Vosso and Vosso was severely delayed by the unavoidable lake passage(s) of Vangsvatnet and Evangervatnet, these smolts experienced an up to two week later fjord arrival than smolt from Bolstadelva. The progression rate through the lakes were on average 0.23 body lengths/sec in Vangsvatnet and 0.06 body length per sec in Evangervatnet. Fjord migration rates were more similar among the three groups, ranging between one to three body lengths per sec. The first 25% percentile of tagged smolt arriving at Sørfjorden/Nordhordlandsbrua arrived at May 8 (Bolstadelva smolt), May 15 (Vosso) and May 21 (Upper Vosso). The smolt survival was low in both lakes and in the Straume-Stamnes estuary area. Smolt from Upper Vosso and Vosso tagged with IDT experienced higher mortality than individuals tagged with DST. A transmitter type effect on survival was not found in smolt from Bolstadelva. Survival through Vangsvatnet was 47% and 55% for wild smolt tagged with IDT and DST, respectively. The same numbers were 18% and 51% for Evangervatnet, and 54% and 57% for the Straume-Stamnes estuary area. Apart from the estuary survival, these numbers are high compared to other Atlantic salmon smolt migration studies, however most other studies constitute systems without lakes included in their migration route. Altogether, using DST data only, about 17% of the tagged smolt from Upper Vosso reach the sea (i.e., Stamnes), 25% from Vosso and 50% from Bolstadelva. These numbers are higher than in the 2015 study, and comparable to other lake-holding study systems. Capture-mark-recapture analyses revealed that mortality per km was highest in inlet areas to the lakes and right “downstream” Evanger power station. A backtracking survey conducted in late August documented aggregation of non-moving pinging transmitters in these areas. Interpretations of the migration pattern and depth use indicated that predators ate a large fraction of the dead tagged smolt. Due to lack of predator behavior data from the system, these interpretations have not been verified. However, stomach-content data retrieved from large brown trout (Salmo trutta) and Artic charr (Salvelinus alpinus) caught during the descent period (trap nets and rod-and-reel) showed smolt to be frequently eaten by the larger brown trout, but not by Arctic charr. Smolt with DST displayed diel vertical migration throughout the migration route, with the largest amplitude in the lakes and in Bolstadfjorden (mean 7 m in Vangsvatnet; 12 m Evangervatnet; 8 m Bolstadfjorden). The smolt used deep water during daytime and shallow during night. This diel vertical migration pattern can be attributed to anti-predation behavior, as can other aspects of the descent pattern: synchronous migration (shoaling), nighttime migration and migration during high water discharge conditions. The fjord diel vertical migration amplitude was less pronounced resulting in smolt having little contact with proper seawater before arriving the Nordhordlandsbrua area (~70 km beyond the Bolstadelva river mouth). The differential effect from tag type on Vosso smolt survival (but not Bolstadelva smolt) indicates that the tag may affect the smolt negatively. IDT-tagged individuals had a higher relative tag weight compared to DST-tagged individuals (8.2% and 6.5% of body weight, respectively), resulting from larger individuals being chosen for the DST tags during the implantation process. Because DST-tagged individuals seemed less affected by the tag, we argue that individuals from this group resemble non-tagged individuals’ survival and descent behavior to the largest extent, but even this group provides conservative estimates on the real smolt survival. Due to large inter-individual and spatial variation in survival, the markrecapture analyses reveal little support for overall individual size or relative tag weight effects on survival. Owing to the observed anti-predation behavior (diel vertical migration, nighttime migration, synchronous migration) and responsiveness to environmental cues many days after tagging and release, we argue that the tagging procedure only slightly affects the smolt behavior and survival. The acoustic telemetry method therefore proves relevant for studying patterns and processes affecting Atlantic salmon smolt migration in the Vosso river system, but for future monitoring of population size and absolute survival levels a less invasive tagging method such as passive integrated transponder (PIT) telemetry (combined with trap net and rotary screw trap) should be used. This latter method will also provide data on sea survival and number of spawning episodes in returning adults. The hatchery-reared smolts, as in 2015, performed poorly during their river descent. This result indicates that the hatchery-reared smolt is not well adapted to the natural conditions and seemed to have low migration motivation. More than 50% of the individuals in this group were not detected at all after release, indicating either high predation rate or no down-stream migration (or both). The ones detected as migrators displayed suboptimal behavior. A large fraction migrated during daytime, they showed no response towards environmental cues and migrated largely asynchronous with other smolts. Most of them died in the Vangsvatnet inlet area. Analyses of hatchery-reared smolt gill enzyme activity revealed low levels Na+-K+- ATPase (NKA) values (<10 μmol ADP/mg protein/hr) throughout the migration period and higher values in early April than during early May. This maladaptive NKA development trajectory occurred despite implementation of a novel light-regime during 2016 aiming at stimulating an earlier descent time than observed in 2015. Analyses of individuals from the same population of hatchery smolt performed by another laboratory showed higher NKAvalues (mean= 12.3 μmol ADP/mg protein/hr) in early May. These latter findings are also consistent with previous years’ results, leaving our NKA-results dubious. The hatchery-reared smolt therefore seem to have adequate seawater tolerance at relevant time of descent but seem to have poorly developed migration motivation. These results indicate that the hatchery smolt may not be relevant as model organisms for understanding processes influencing natural smolt migratory behavior and migration survival in the Vosso river system. Environmental conditions and production protocols at the hatchery ought to be assessed and revised to enable migration synchronicity with wild smolt in the river. The main conclusion from the project is that the Vosso river system smolt is subjected to large predation pressure, most likely from large brown trout, during their descent and that Vangsvatnet, Evangervatnet and Straume-Stamnes area constitute key survival bottlenecks. The environmental conditions in areas directly influenced by the Evanger power plant are not acutely lethal to the smolt. However, the combined effects from altered water current conditions, dissolved gas super saturation and cold-water temperatures imposed by the power plant outlet water may increase the smolt’s lake residence time and thus exposure time to predators. The most relevant way to address this potential complex effect on smolt survival from the power plant would be to stop the power plant during parts of the smolt descent period for detailed mapping of the smolt behavior response during such periods. The project strongly recommends a multi-year study on the predator fish behavior and diet in the Vosso river system to better understand smolt-predator interactions in this system. Such a study will potentially reveal the relative importance of resident vs anadromous predator contribution to the smolt mortality. Furthermore, such data will prove pertinent for addressing if degree of brown trout anadromy is decreasing in the Vosso river system. Increased brown trout freshwater residency may increase salmon smolt predation in freshwater and challenge the ongoing salmon re-establishment program

Diel migration pattern of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) smolts: an assessment of environmental cues

The timing of smolt migration is a key phenological trait with profound implications for individual survival during both river descent and the subsequent sea sojourn of anadromous fish. We studied relationships between the time of smolt migration, water temperature and light intensity for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta). During 2006–2012, migrating smolts descending the southern Norway River Storelva were caught in a rotary screw trap located at the river mouth. The date of 50% cumulative smolt descent correlated significantly with the date when the river temperature exceeded 8°C for both Atlantic salmon and sea trout smolts. In 2010, smolts of both species were passive integrated transponder (PIT)-tagged, and the diel timing of their migration was precisely documented. The degree of night migration decreased in both species as the river temperature rose, and at temperatures above 12–13°C, more smolts migrated during day than during night. A multinomial model was fitted for estimating temperature and species effects on probabilities of migration during night, daytime, dusk and dawn. Atlantic salmon smolts preferred migrating under lower light intensities than sea trout smolts during early, but not late spring when both species migrated during bright daylight. In accordance with the early-season tendency to migrate at night, Atlantic salmon smolts migrated more during darker hours of the day than sea trout. In both species, smaller smolts migrated under dark conditions than during light conditions. Most of the findings on thermal, light and temporal effects on the observed smolt migration pattern can be explained as adaptations to predation avoidance. migration timing, phenology, temperature influence, light intensities, anadromous fish, predation avoidanc

Forsøk med trål og ekkolodd i Mjøsa, 2012

Trål har i liten grad vore brukt ved undersøkingar av fiskesamfunn i norske innsjøar. Gjennom innføringa av Vanndirektivet har Norge forplikta seg til meir regulær oppfølging av den økologiske tilstanden i større vassførekomstar. Tettleik, rekruttering, artsfordeling og vekst hjå fisk er alle parametrar der ein kan forvente relativt rask respons på miljøendringar. Det er derfor viktig å ha datainnsamling med verktøy som gjev eit mest mogeleg representativt bilete av fiskebestandane. Ekkolodd er det viktigaste instrumentet for fiskemengdemåling i dag. Samanlikna med prøvefiskegarn som berre gir relative tettleiksmål som fangst per innsats, gir ekkoloddet kvantitative mål på fiskemengde per overflateareal og vassvolum. Men ekkoloddet gir ikkje direkte informasjon om art, og data frå ekkoloddkøyringar må derfor alltid supplerast med data frå ein eller fleire fangstmetodar. I denne samanheng er trål ein viktig reiskap. Tråling gjev både biologisk materiale og kvantitativ informasjon om fisketettleik, samt at den kan fange godt på liten fisk som er vanskeleg å fange med garn. Målet med denne undersøkinga var å skaffe erfaring med tråling med ein ny og lett trål tilpassa ferskvassbruk. Samtidig ville me gjere undersøkingar med garnfangsting med multigarn, og bruk av fleire ekkolodd og frekvensar for å vurdere samsvaret dei imellom. Undersøkinga blei gjennomført i Mjøsa, sør for Mjøsbrua 11.-14. september 2012. Det vart køyrt trål og ekkolodd (70 og 200 kHz) i tre netter og fiska med garn i to netter. Trål og flytegarnfisket blei gjennomført i djupneintervalla 0-6 m og 15-22 m. Samla var fangstane 805 fisk i trålen, 305 fisk i flytegarna, og 413 fisk i botngarn. Drøftinga av resultata fokuserer på den pelagiske fisken. Trålfangsten og garnfangsten utfyller kvarandre. Trålen fanga godt på 0+ krøkle, som ikkje vart fanga på garn. Samstundes fanga trålen lite fisk >15 cm, storleikar som utgjorde ein stor del av garnfangsten. Med berre ein av metodane (trål eller garn) ville me fått eit mykje dårlegare bilete av fiskesamfunnet. Fangstane i pelagialen var dominerte av krøkle. Grovt estimerte tettleikar ut frå trålfangstane var i same storleiksorden som akustisk estimerte tettleikar. Både i fangstar og med ekkolodd observerte me ein storleiksstrukturert fiskebestand. I storleiksfordelinga frå fangstane observerte me ein topp kring 30-35 cm beståande av sik, ein kring 19-23 cm beståande av lagesild og stor krøkle, ein topp kring 12 cm (krøkle), samt ein topp kring 5 cm (0+ krøkle). Tilsvarande var det fire toppar i ekkostyrkefordelinga, men ingen av dei vanleg brukte regresjonane mellom lengde og ekkostyrke gav god tilpassing mellom fiskelengde og ekkostyrke for alle toppane. Me observerte tydelege habitatskilnader mellom ulike artar og storleiksgrupper både i fangstar og med ekkolodd, ein klar indikasjon på at observasjonar med ekkoloddet kan brukast til å indikere ulik habitatfordeling mellom ulike storleiksgrupper og artar i fiskesamfunnet. Me fann vidare godt samsvar mellom resultata frå dei ulike ekkolodda (to 70 kHz og eit 200 kHz). Som forventa kunne 200 kHz også brukast til å skilje Mysis frå fisk

Vandringsmønsteret til laksesmolt i Vossovassdraget med vekt på detaljert kartlegging av åtferd i innsjøsystema og effektar av miljøtilhøve

The River Vosso population of Atlantic salmon (Salmo salar) was strongly reduced over the course of a few years during late 1980s, and has not recovered since, despite substantial mitigation efforts. The ultimate reason(s) for the population decline remains enigmatic, as are the causes for lack of recovery. After decades of poor returning numbers of naturally produced salmon, it seems likely that the causative factors are still in operation. Former studies indicate heavy mortalities of hatchery-produced smolts during migration through the inner fjord, and especially in the Bolstadfjord estuary. A large-scale acoustic telemetry study conducted during 2015 revealed high smolt mortality in both Evangervatnet and the Bolstadfjord estuary. Most of the Evangervatnet mortality was located close to the Evanger power plant. Analyses of power plant outlet water showed supersaturation of nitrogen and oxygen and was suggested as a possible mortality factor for the smolt. Motivated by these findings, a large-scale telemetry study was set up for the 2016 smolt descent. Passive receivers (n=89) were organized into 19 cross-sectional transects dividing the migratory route into 20 zones from the Upper Vosso area to the Nordhordlandsbrua area in outer reaches of Osterfjorden. River mouths and the Evanger power plant area were covered with triangulation networks of receivers, allowing for detailed positioning. In total, 50 hatchery-reared and 150 wild salmon smolt were tagged with acoustic transmitters. All hatchery-reared smolt were tagged with ordinary 7.3×18 mm Id-transmitters and released into Upper Vosso area. Wild smolt were caught in and released into three river sections: Upper Vosso, Vosso and Bolstadelva. One-half of the 50 wild smolt from each river section was tagged with id-transmitters (IDT), and the other half with 7.3×22 mm depth-sensors transmitters (DST). Altogether, more than 8 million tag detections were recorded during the mid-April to late August period, where 91.4% of the wild smolt were detected, and 46% of the hatchery-reared ones. All hatchery-reared smolt died prior to reaching Evangervatnet, and displayed highly differentiated behavior when compared to wild smolt in terms of time-of-day and day-of-year timing of migration. Furthermore, migration in this group was not correlated with environmental factors. The wild smolt from all three release sections displayed a strong positive correlation between onset of river descent and river discharge and/or water temperature. In all wild smolt groups, the initiation of river descent was largely synchronous and took place during nighttime. River descent started in mid-April for the earliest individuals in Upper Vosso, but ca 80% descended during the 3-12 May period that coincided with a flooding event. Because the migration of smolt from both Upper Vosso and Vosso was severely delayed by the unavoidable lake passage(s) of Vangsvatnet and Evangervatnet, these smolts experienced an up to two week later fjord arrival than smolt from Bolstadelva. The progression rate through the lakes were on average 0.23 body lengths/sec in Vangsvatnet and 0.06 body length per sec in Evangervatnet. Fjord migration rates were more similar among the three groups, ranging between one to three body lengths per sec. The first 25% percentile of tagged smolt arriving at Sørfjorden/Nordhordlandsbrua arrived at May 8 (Bolstadelva smolt), May 15 (Vosso) and May 21 (Upper Vosso). The smolt survival was low in both lakes and in the Straume-Stamnes estuary area. Smolt from Upper Vosso and Vosso tagged with IDT experienced higher mortality than individuals tagged with DST. A transmitter type effect on survival was not found in smolt from Bolstadelva. Survival through Vangsvatnet was 47% and 55% for wild smolt tagged with IDT and DST, respectively. The same numbers were 18% and 51% for Evangervatnet, and 54% and 57% for the Straume-Stamnes estuary area. Apart from the estuary survival, these numbers are high compared to other Atlantic salmon smolt migration studies, however most other studies constitute systems without lakes included in their migration route. Altogether, using DST data only, about 17% of the tagged smolt from Upper Vosso reach the sea (i.e., Stamnes), 25% from Vosso and 50% from Bolstadelva. These numbers are higher than in the 2015 study, and comparable to other lake-holding study systems. Capture-mark-recapture analyses revealed that mortality per km was highest in inlet areas to the lakes and right “downstream” Evanger power station. A backtracking survey conducted in late August documented aggregation of non-moving pinging transmitters in these areas. Interpretations of the migration pattern and depth use indicated that predators ate a large fraction of the dead tagged smolt. Due to lack of predator behavior data from the system, these interpretations have not been verified. However, stomach-content data retrieved from large brown trout (Salmo trutta) and Artic charr (Salvelinus alpinus) caught during the descent period (trap nets and rod-and-reel) showed smolt to be frequently eaten by the larger brown trout, but not by Arctic charr. Smolt with DST displayed diel vertical migration throughout the migration route, with the largest amplitude in the lakes and in Bolstadfjorden (mean 7 m in Vangsvatnet; 12 m Evangervatnet; 8 m Bolstadfjorden). The smolt used deep water during daytime and shallow during night. This diel vertical migration pattern can be attributed to anti-predation behavior, as can other aspects of the descent pattern: synchronous migration (shoaling), nighttime migration and migration during high water discharge conditions. The fjord diel vertical migration amplitude was less pronounced resulting in smolt having little contact with proper seawater before arriving the Nordhordlandsbrua area (~70 km beyond the Bolstadelva river mouth). The differential effect from tag type on Vosso smolt survival (but not Bolstadelva smolt) indicates that the tag may affect the smolt negatively. IDT-tagged individuals had a higher relative tag weight compared to DST-tagged individuals (8.2% and 6.5% of body weight, respectively), resulting from larger individuals being chosen for the DST tags during the implantation process. Because DST-tagged individuals seemed less affected by the tag, we argue that individuals from this group resemble non-tagged individuals’ survival and descent behavior to the largest extent, but even this group provides conservative estimates on the real smolt survival. Due to large inter-individual and spatial variation in survival, the markrecapture analyses reveal little support for overall individual size or relative tag weight effects on survival. Owing to the observed anti-predation behavior (diel vertical migration, nighttime migration, synchronous migration) and responsiveness to environmental cues many days after tagging and release, we argue that the tagging procedure only slightly affects the smolt behavior and survival. The acoustic telemetry method therefore proves relevant for studying patterns and processes affecting Atlantic salmon smolt migration in the Vosso river system, but for future monitoring of population size and absolute survival levels a less invasive tagging method such as passive integrated transponder (PIT) telemetry (combined with trap net and rotary screw trap) should be used. This latter method will also provide data on sea survival and number of spawning episodes in returning adults. The hatchery-reared smolts, as in 2015, performed poorly during their river descent. This result indicates that the hatchery-reared smolt is not well adapted to the natural conditions and seemed to have low migration motivation. More than 50% of the individuals in this group were not detected at all after release, indicating either high predation rate or no down-stream migration (or both). The ones detected as migrators displayed suboptimal behavior. A large fraction migrated during daytime, they showed no response towards environmental cues and migrated largely asynchronous with other smolts. Most of them died in the Vangsvatnet inlet area. Analyses of hatchery-reared smolt gill enzyme activity revealed low levels Na+-K+- ATPase (NKA) values (<10 μmol ADP/mg protein/hr) throughout the migration period and higher values in early April than during early May. This maladaptive NKA development trajectory occurred despite implementation of a novel light-regime during 2016 aiming at stimulating an earlier descent time than observed in 2015. Analyses of individuals from the same population of hatchery smolt performed by another laboratory showed higher NKAvalues (mean= 12.3 μmol ADP/mg protein/hr) in early May. These latter findings are also consistent with previous years’ results, leaving our NKA-results dubious. The hatchery-reared smolt therefore seem to have adequate seawater tolerance at relevant time of descent but seem to have poorly developed migration motivation. These results indicate that the hatchery smolt may not be relevant as model organisms for understanding processes influencing natural smolt migratory behavior and migration survival in the Vosso river system. Environmental conditions and production protocols at the hatchery ought to be assessed and revised to enable migration synchronicity with wild smolt in the river. The main conclusion from the project is that the Vosso river system smolt is subjected to large predation pressure, most likely from large brown trout, during their descent and that Vangsvatnet, Evangervatnet and Straume-Stamnes area constitute key survival bottlenecks. The environmental conditions in areas directly influenced by the Evanger power plant are not acutely lethal to the smolt. However, the combined effects from altered water current conditions, dissolved gas super saturation and cold-water temperatures imposed by the power plant outlet water may increase the smolt’s lake residence time and thus exposure time to predators. The most relevant way to address this potential complex effect on smolt survival from the power plant would be to stop the power plant during parts of the smolt descent period for detailed mapping of the smolt behavior response during such periods. The project strongly recommends a multi-year study on the predator fish behavior and diet in the Vosso river system to better understand smolt-predator interactions in this system. Such a study will potentially reveal the relative importance of resident vs anadromous predator contribution to the smolt mortality. Furthermore, such data will prove pertinent for addressing if degree of brown trout anadromy is decreasing in the Vosso river system. Increased brown trout freshwater residency may increase salmon smolt predation in freshwater and challenge the ongoing salmon re-establishment program.Vossolaksen gjekk sterkt attende over ein kort periode på slutten av 1980-talet og vart freda frå og med 1992. Det er framleis ikkje kjent kva som gjorde at laksen gjekk sterkt attende, og kva som gjer at laksestamma ikkje byggjer seg opp att, trass i ei rekkje tiltak. Det kan sjå ut som at problemfaktor(ane) ikkje har endra seg fram til i dag. Tidlegare merkestudiar tyder på eit stort tap av utvandrande klekkjerismolt i indre fjordstrok og særleg i Bolstadfjorden. I 2015 vart det gjennomført ein akustisk telemetristudie som fann særs høg smoltdødelegheit i Evangervatnet, og særleg i området ikring Evanger kraftverk. Det vart spekulert i om gassovermetting frå kraftverksvatnet kunne gje auka dødelegheit, anten akutt eller indirekte ved at smolten vert meir utsett for predasjon under påverknad av gassovermetta vatn. Det vart difor gjort framlegg om at vandringsåtferd og overleving skulle kartleggjast i heile ferskvassutvandringsruta i 2016, med særleg detaljert kartlegging i Evangervatnet og alle osområda. Dette prosjektet har kartlagt vandringsmønsteret og overlevinga hjå vill- og klekkjeriprodusert laksesmolt frå ulike delar av Vossovassdraget ut til Nordhordlandsbrua i 2016-sesongen. Det vart nytta akustisk telemetri med 89 lyttebøyer i eit lyttegardinoppsett som delte utvandringsruta inn i 20 soner (13 i ferskvatn og 7 i fjorden). Lyttebøyene var operative frå midten av april til månadsskiftet august-september. Området ved Evanger kraftverk vart dekka med eit trianguleringsoppsett for meir detaljerte analysar enn elles i vassdraget. I alt vart 150 ville smolt og 50 klekkjeriprodusert smolt merka med akustiske merke. Klekkjerismolten vart alle sett ut oppstraums Vangsvatnet (Øvre Vosso). Den merka villsmolten kom frå tre ulike seksjonar av Vossovassdraget der 50 individ vart merka i kvart segment. Dei vart fanga inn og sett ut att i Øvre Vosso, Vosso og Bolstadelva, og halvparten (25 stk.) innan kvar gruppe fekk merke med djupnesensor medan den andre helfta vart merka med vanlege id-merke. Det vart i alt registrert meir enn 8 millionar deteksjonar, og 91,4% av villsmolten vart detektert etter at dei vart sett ut, medan 46% av klekkjerismolten vart detektert. All klekkjerifisk døydde før dei kom til Evangervatnet, og dei hadde særs avvikande åtferd både i høve til når dei vandra på døgeret og kva for dato dei vandra, samt at vandringa deira ikkje korrelerte med miljøtilhøva. Villsmolten frå alle dei tre utsettstadane hadde særs tydelege positive korrelasjonar mellom vassføring og/eller vasstemperatur og når dei starta på nedvandringa. Alle gruppene hadde synkron nedvandring, og dei aller fleste vandra om natta. Dei fyrste vandrarane sette i gang vandringa frå midten av april (særleg frå Øvre Vosso), men 80% frå alle dei tre utsettstadane vandra på flaumen som fann stad 3.-12. mai. Sidan nedvandringa hjå villsmolten frå Øvre Vosso og Vosso vart kraftig forseinka gjennom innsjøane (Vangsvatnet og Evangervatnet) vart datoen dei kom ut til Bolstadfjorden ulik for dei tre gruppene, med tyngdepunkt 8. mai for Bolstadelvfisken, 18. mai for Vossofisken og 16. mai for Øvre Vossofisken. I det store og heile var vandringsfarta aukande nedover utvandringsruta, og gjennomgåande særs låg gjennom innsjøane (snitt: 0,23 kroppslengder/sek i Vangsvatnet og 0,06 kroppslengder/sek i Evangervatnet). Den fjordvandrande smolten (utanfor Stamnes) vandra i det store og heile med 1–3 kroppslengder/sek. Dei 25% tidlegaste vandrarane kom til Sørfjorden-Nordhordlandsbruaområdet frå 8. mai (Bolstadelvfisken), 15. mai (Vosso) og 21. mai (Øvre Vosso). Smoltdødelegheita var høg gjennom begge innsjøane og i området Straume-Stamnes. Smolt med id-merke frå Øvre Vosso og Vosso hadde lågare overleving enn tilsvarande djupnemerka fisk, medan merkegruppene hadde lik overleving hjå Bolstadelvsmolten. Mellom 47% (idmerka) og 55% (djupnemerka) overlevde vandringa gjennom Vangsvatnet, og mellom 18% og 51% overlevde gjennom Evangervatnet. For estuarieområdet mellom Straume og Stamnes overlevde mellom 54 og 57% av id-merka og djupnemerka smolt. Med unnatak av området Straume-Stamnes er desse dødelegheitene høgare enn normalt for utvandrande laksesmolt, men i dei fleste andre undersøkingane manglar innsjøar i utvandringsruta. Om ein legg tala for dei djupnemerka individa til grunn, overlever om lag 17 % av den merka villsmolten frå Øvre Vosso, 25% frå Vosso og 50% frå Bolstadelva frå utsettstaden til Stamnes (som kan reknast som yttergrensa til estuariet). Desse overlevingstala (for Vosso og Bolstadelvgruppene) er litt høgare enn for 2015-undersøkjinga, og er godt innanfor kva som elles er rapportert frå andre laksesystem med innsjøar i utvandringsruta. Merke-attfangstanalysar synte at i begge innsjøane var det høg dødelegheit pr. km i områda rett utanfor innosen, men i Evangervatnet var det òg høg dødelegheit pr. km i områda rett «nedstraums» Evanger kraftverk i begge åra. Dette vart verifisert med ettersøk med manuell peilar i september der det vart detektert mange ‘pingande’ merke i desse områda. Tolking av åtferda til den registrerte fisken synte at ein stor del av den daude fisken vart eten av fiskepredatorar, men desse tolkingane kan vere usikre då vi ikkje har data på typisk predatorfiskåtferd frå systemet. Fiske med stong og storruse som vart gjennomført i Evangervatnet under smoltutvandringa synte at aure i systemet ofte hadde smolt i magen, men ikkje røyene. Fisk med djupnemerke viste tydelege vertikale døgervandringar gjennom heile utvandringsruta, med størst døgeramplitydar i innsjøane og i Bolstadfjorden (snitt: 7 m i Vangsvatnet, 12 m i Evangervatnet og 8 m i Bolstadfjorden). Smolten gjekk djupt på dagtid og grunt om natta, noko som er typisk antipredatoråtferd og som difor passar fint inn med smolten si synkrone vandring (stiming), vandring om natta og vandring under høg vassføring. I fjorden var amplitudane mindre og smolten gjekk sjeldan ned i vasslag med sjøvatn. Den negative effekten av id-merket på overleving hjå villsmolt som var merka og sleppt i særleg Vosso (men ikkje i Bolstadelva) peiker på at merket og/eller merkeprosedyren påverkar smolten negativt. Dei id-merka individa hadde i snitt ei høgare merkevekt i forhold til kroppsvekta (8,2%) enn djupnemerka (6,5%) fisk, sidan større fisk vart plukka ut til djupnemerkene for å kompensere for at desse merka er litt større. I og med at den djupnemerka smolten var noko mindre påverka meiner vi at overlevinga og vandringsåtferda til denne gruppa ligg nærast opp mot den umerka villfisken, men det er rimeleg å gå ut frå at overleving hjå denne gruppa truleg òg er lågare enn for den umerka smolten, og at estimata frå denne studien difor utgjer minsteestimat. Fordi det var stor individ- og romleg variasjon i overlevinga var ikkje dødelegheita korrelert med storleik på smolten eller relativ merkevekt. Smolten hadde døgerlege vertikalvandringar, reagerte på miljøstimuli og vandra synkront mange dagar etter merking og utsett. Vi meiner difor at metoden i det store og heile i liten grad påverkar dei underliggjande dødelegheits- og vandringsprosessane som er aktive i dette studiestsystemet. Det vart difor konkludert at AT-metoden fangar opp dei store linene i dødelegheitsmønsteret og vandringane på ein representativ måte i Vossosystemet. For framtidas overvaking av smolten i Vossovassdraget meiner vi at dei mest pålitelege tala for estimering av absolutt populasjonsstorleik og dødelegheit under utvandringa lyt basere seg på PIT-telemetri kombinert med innfanging i smoltskruve/storruser. Om det kjem på plass ein velfungerande, heilårleg PIT-antenneteknologi som gjev tilfredstillande resultat, vil denne òg gje informasjon om sjøoverleving og tal på gyteepisodar hjå den utvandrande merka smolten som kjem attende som kjønnsmogen laks etter sjøopphaldet. Klekkjerismolten sin høge dødelegheit, som òg vart funne i 2015, skuldast truleg at denne smolten ikkje var rett kalibrert til eit opphald i naturleg miljø, og at den ikkje hadde den same motivasjonen til å vandre ut av vassdraget. Over halvparten av klekkjerismoltane vart ikkje detektert etter utsett, noko som kan tyde på at dei anten vart lette bytte for predatorar i Øvre Vosso eller at dei stod att i elva og ikkje vandra nedstraums i det heile teke. Av dei som vandra hadde dei aller fleste ikkje-optimal anti-predatoråtferd (vandra på dagtid, ikkje når det var flaum, ikkje synkront med andre smolt) og dei døydde nesten alle saman i innosområdet av Vangsvatnet. Det er på det reine at klekkjerismolten ikkje presterte optimalt, og NKAmålingar synte òg at dei fleste hadde relativt låge verdiar (<10 μmol ADP/mg protein/time) i fyrste halvdel av mai, og at verdiane var høgare i byrjinga på mars enn i siste del av april. Då målingar frå andre laboratorium på smolt frå dei same klekkjeritankane synte høgare verdiar 3. mai (snitt 12,3 μmol ADP/mg protein/time) er de usikkert i kva grad verdiane faktisk er låge hjå klekkjerismolten. Trass i at ljosregimet vart endra for å sjå om dette kunne stimulere klekkjerismolten til å vandre tidlegare i 2016 enn han gjorde i 2015, såg ikkje dette ut til å gje dei ynskja resultata. Slik stoda er med klekkjerismolten i Vosso no er han ueigna som modellfisk for villsmolten og det kan synest som at det er vandringsvilja og ikkje sjøvasstoleransen som er problemet. Prosjektet konkluderer med at Vossosmolten er utsett for høg predasjon, mest truleg frå stor aure i både Vangsvatnet, Evangervatnet og Straume-Stamnes. Miljøtilhøva ved Evanger kraftverk er ikkje akutt dødelege for smolten, men predasjonen i Evangervatnet kan vere særskilt høg pga mogleg forseinking i utvandringa grunna endra vasskvalitet, temperatur og straumtilhøve ved kraftverket. Den mest effektive måten å finne ut av den eventuelle totale effekten frå kraftverket vil vere å stogge kraftverket i periodar under utvandringa for å sjå om dette påverkar overlevinga, åtferda og framdrifta hjå laksesmolten. Då vi veit særs lite om predatorane i dei to innsjøsystema, særlig kva rolle stasjonær røye og aure, og sjøaure spelar, gjer prosjektet framlegg om at det i nær framtid vert gjennomført ei telemetristudie av desse, kombinert med innfanging med storruse for mageanalysar. Ein slik studie vil kunne gje svar på den relative rolle stasjonær og anadrom aure har på smoltpredasjonen i vassdraget. Denne informasjonen, saman med annan overvakingsinformasjon, vil vere viktig for å vurdere om auren er i ferd med å verte meir stasjonær og difor ein aukande trugsel for reetablingsprogrammet for laks i Vossovassdraget