Among-river pattern in relative abundance of two salmonid fishes reflects temperature-dependent competition

Temperature is a key driver in determining species distribution and abundance across climatic and geographical gradients. This occurs through direct physiological effects of temperature on performance, but may also result from temperature effects on species interactions. In the current study we experimentally test for temperature-dependent competition between juvenile anadromous Atlantic salmon and brown trout and evaluate the role of temperature in shaping the relative abundance of these species across 88 Norwegian rivers. When the two species were reared in allopatry their somatic growth rates were similar across a range of temperatures (4–16°C). When reared in sympatry, the growth of salmon was greatly reduced at the coldest temperature relative to when reared in allopatry, whereas this was not the case for trout. The effect of interspecific competition on growth was more similar for the two species at warmer temperatures. Thus, interspecific competition effects were strongly asymmetric only at the lowest temperature, with trout outcompeting salmon. The results from the experiment were reflected in qualitative patterns of relative abundance of salmon and trout in natural populations, which shifted from being trout dominated in rivers with low summer temperatures to being salmon dominated in warmer rivers. These results provide an empirical example of how temperature effects on the relative abundance of species can only be understood in light of information about temperature-dependent species interactions.publishedVersio

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios

Given the limited information on prey use during the marine residency period for Atlantic salmon, scales were collected from salmon at return to the River Namsen (Norway) for spawning after 1 year at sea, and scale material from the first and second summer marine feeding periods was analysed using stable isotope methods to understand dynamics of their trophic ecology. As the salmon increased in size from the first to second summer, they reduced their feeding niche and specialised more (narrowed the δ13C range) and increased their dependency on higher tropic level (δ15N) prey, likely fish. Changes in δ13C indicated a consistent pattern of movement towards the north and west between summer feeding periods. Hence, salmon during their first year at sea may have a migration route roughly resembling that of previous spawners, as inferred from earlier tagging studies. Feeding conditions and nutrient composition during the last summer at sea, i.e. in the months before returning to the river for spawning, impacted final body size and within-season timing of return. Fish undergoing the largest trophic niche shift (δ13C and δ15N combined) between summer feeding periods, returned earliest. The earliest returning fish had the fastest specific growth rates at sea. Hence, salmon encountering abundant high-quality fish food during the marine migration, particularly during the last months, may reach a size and energetic state whereby it is better to return early to a safer environment in freshwater than risk being eaten by a big predator at sea. Both trophic status (δ15N), resource use (δ13C) and growth rates were significantly correlated between feeding periods. Nutrient composition during the first summer at sea did not impact the fish body length after the following winter, but growth conditions during the first summer evidenced carry-over effects from the first to the second summer of feeding. diet, energetics, feeding, growth, marine migration, Salmo salar Life history ecologyTemporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratiospublishedVersio

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

Hva om Hunderørret var laks

Storørret defineres som en selvreproduserende stamme med regulær forekomst av store fiskespisende individer, ofte med et nisjeskift og markert vekstomslag ved overgangen til fiskediett. Den mest kjente storørretstammen her i landet er den storvokste Hunderørreten som gyter og vokser opp i nedre del av Gudbrandsdalslågen og bruker Mjøsa som matfat. Til tross for likhetstrekkene med laks, er forvaltningen av Hunderørreten svært forskjellig fra moderne lakseforvaltning. Vi foreslår derfor at prinsippet med forvaltning etter gytebestandsmål også brukes for Hunderørreten. Det vandrer inntil 700 storørret forbi fisketrappa i Hunderfossen kraftverk årlig. Strekningen ovenfor Hunderfossen kraftverk og opp til Harpefossen representerer et stort potensial for naturlig rekruttering. Ved å øke oppvandringen forbi Hunderfossen ved hjelp av fangst og transport av gytefisk, er det mulig å øke bestanden betydelig basert på naturlig rekruttering. Vi mener det er realistisk å øke oppvandringen til 2000 fisk årlig, noe som vil sikre bevaring av bestanden og øke avkastningen i fremtiden.publishedVersio

Influence of hydraulics on the dowsntream migratory route of Atlantic salmon (Salmo salar)

The interaction between hydrodynamics and the upstream migration of fish has been investigated in several studies over the past few years, but there has been a relative lack of investigation into the effects of hydraulics on the downstream migration of fish. In Norway, mortality associated with downstream migration is strongly related to hydropower production. However, there is a poor understanding of how fish choose their migratory routes, which limits the development of measures to mitigate the impact of anthropogenic structures on fish migration. The current study aimed to analyze the effect of hydraulics on the choice of downstream migration route of Atlantic salmon smolts (Salmo salar) in the proximity of an intake to a hydropower plant in Norway. We combined computational fluid dynamic modeling with 3-Dimensional acoustic telemetry positioning of a group of tagged smolts to investigate the interaction between fish and the flow dynamics, and to determine the influence of this interaction on fish migratory routes. A Hidden Markov Model was used to predict migratory routes based on the relations found between the hydraulic variables and fish behaviour. Results showed relationships between fish migratory routes and the different components of flow velocity. The knowledge acquired in this study is expected to inform opportunities and decision making in aquatic resources management and engineering design to increase fish survival past hydropower intakes and other anthropogenic structures

Modelling the effect of hydropeaking‐induced stranding mortality on Atlantic salmon population abundance

Studies of hydropeaking‐induced stranding mortality on fish populations have been confined to analysis of empirical data and/or short‐term hydraulic‐habitat modelling of individual events and are thus limited as to how they may be used to infer long‐term effects in fish populations. In this study, the effects of stranding mortality on an Atlantic salmon population were simulated using an individual‐based Atlantic salmon population model with the objective of determining the sensitivity of population dynamics to stranding. It was found that density‐dependent mortality (an alternative source of mortality in juvenile Atlantic salmon) partially compensated for stranding mortality, acting as a negative feedback mechanism that dampened change in population abundance. Stranding caused a perturbation in population dynamics, and effects of individual stranding events persisted in time across the life stages of the population. Effects on population abundance depended on the time of year when stranding was applied, both because of intra‐annual changes in stranding mortality probability and because of intra‐annual changes in the ability of density‐dependent mortality to compensate for stranding mortality. We concluded that empirical measurements of stranding mortality have limited potential for inference of overall effects on the population, and a more dynamic modelling approach, incorporating system feedback, allows for a better modelling of the impact of stranding. Sensitivity analysis showed that population abundance was highly sensitive to density‐dependent mortality, and we suggest that this area should be prioritized for further research when investigating the effects of hydropeaking on rivers.acceptedVersio

Multi-criteria decision analysis in Bayesian networks-Diagnosing ecosystem service trade-offs in a hydropower regulated river

The paper demonstrates the use of Bayesian networks in multicriteria decision analysis (MCDA) of environmental design alternatives for environmental flows (eflows) and physical habitat remediation measures in the Mandalselva River in Norway. We demonstrate how MCDA using multi-attribute value functions can be implemented in a Bayesian network with decision and utility nodes. An object-oriented Bayesian network is used to integrate impacts computed in quantitative sub-models of hydropower revenues and Atlantic salmon smolt production and qualitative judgement models of mesohabitat fishability and riverscape aesthetics. We show how conditional probability tables are useful for modelling uncertainty in value scaling functions, and variance in criteria weights due to different stakeholder preferences. While the paper demonstrates the technical feasibility of MCDA in a BN, we also discuss the challenge

Novel large-scale mapping highlights poor state of sea trout populations

1. The state of sea trout in 1251 Norwegian watercourses was assessed based on a scoring system for human pressures, abundance data, and local knowledge. 2. Over 16,000 km of rivers and lakes were available to sea trout in these watercourses, spanning from the temperate to Arctic regions. 3. Sea trout were classified to be in a good or very good state in fewer than 25% of the watercourses and in a poor or very poor state in almost 40%. Twenty-nine watercourses had lost their sea trout populations. 4. Salmon lice from aquaculture salmon farms had by far the largest adverse effect on sea trout among the human impact factors, both in the number of watercourses (83%) and river area affected (60%), and the total effect on sea trout abundance. 5. Agriculture and hydropower production also had strong adverse impacts (35% and 19% of watercourses), but substantially lower than that caused by salmon lice. Culverts related to road crossings and other habitat alterations also had impacts on sea trout in many watercourses (27%). 6. Exploitation of sea trout has been reduced in Norway in recent years, both in the marine and freshwater fisheries. Yet, the exploitation pressure was moderate or high in almost 14% of the watercourses where the state of sea trout was poor or very poor, suggesting a high potential for overexploitation in these. 7. The state of sea trout was best in the northern sparsely populated areas. However, distribution of watercourses with sea trout in a poor or very poor state was more linked to aquaculture, agriculture, and hydropower production than human population density. 8. The developed approach for large-scale mapping of state and pressures, which is vital for prioritizing management measures, may inspire other nations in their conservation effort for this important species. acidification, anthropogenic pressures, brown trout (Salmo trutta), culverts, exploitation, hazardous substances, salmon lice (Lepeophtheirus salmonis), sewagepublishedVersio

Predation of Atlantic salmon across ontogenetic stages and impacts on populations

Managers and stakeholders increasingly ask whether predation is a driving force behind the poor status of many species, and whether predator control is likely to be a successful management action to intervene. We review existing literature on Atlantic salmon Salmo salar predation and predator control, as well as general ecological theory on the role of predation in the life cycle of this iconic fish. Many bird, mammal, and fish predators target salmon at different life stages. In healthy salmon populations, predation is likely compensated for by reduced intra-specific competition during the freshwater stage. There is little evidence that predation alone has been an underlying mechanism for driving salmon populations below conservation limits. However, depending on the predator’s response to salmon abundance, predation may keep decimated populations from recovering, even when the actual causes of decline have been removed. Under such a scenario, predation control may contribute to recovery, but there are no strong examples that clearly demonstrate the efficacy of managing predators to recover threatened salmon populations, challenging further applications.publishedVersio

Rapport om hvordan Trafikklyssystemet påvirker arbeidet med å oppnå målene satt i Kvalitetsnorm for villaks

Sammenliknet med midten av 1980-årene er mengden villaks som vender tilbake fra havet halvert. Som et ledd i bevaringen av atlantisk laks i Norge ble kvalitetsnormen for villaks vedtatt i 2013. Formålet med Kvalitetsnormen er å «bidra til at viltlevende bestander av atlantisk laks ivaretas og gjenoppbygges til en størrelse og sammensetning som sikrer mangfold innenfor arten og utnytter laksens produksjons- og høstingsmuligheter». Ca. 80 % av villaksbestandene når ikke normen sitt mål om god kvalitet eller bedre, og i 2021 ble villaksen for første gang rødlistet i Norge. Ifølge Vitenskapelig råd for lakseforvaltning er lakselus den største trusselen mot villaks i Norge og står for over en tredel av den totale negative påvirkningen. Kvalitetsnormen for villaks har hjemmel i Naturmangfoldloven og er retningsgivende for myndighetenes forvaltning av villaks i Norge. Klassifisering etter Kvalitetsnormen er en helhetsvurdering av kvaliteten på villaksbestander basert på bestandenes oppnåelse av gytebestandsmål, høstingspotensial og genetisk integritet. For dette oppdraget er det gytebestandsmål og høstingspotensial som er relevant, fordi det er disse to lakselusa påvirker. Høstingspotensialet er det som i størst grad er styrende for måloppnåelsen i Kvalitetsnormen og vurderes i forhold til det som anses å være et estimert «normalt høstingsnivå» for en bestand. Med «normalt høstingsnivå» menes det høstingsnivået bestanden skal kunne tåle ett gitt år på bakgrunn av kunnskap om naturlig sjøoverlevelse, samtidig som bestanden når gytebestandsmålet. Systemet for kapasitetsjustering i oppdrettsnæringen omtales som Trafikklyssystemet, og er forankret i Meld. St. 16 (2014-2015) og Innst. 361 S (2014-2015). I Trafikklyssystemet er norskekysten delt inn i 13 produksjonsområder. Avgrensingen av produksjonsområdene er basert på at det skal være minst mulig spredning av lakselus mellom dem. I hvert av produksjonsområdene skal næringens miljøpåvirkning vurderes ved bruk av miljøindikatorer. Trafikklyssystemet vurderes i dag hovedsakelig ut fra én miljøindikator som er estimert dødelighet hos utvandrende laksesmolt (postsmolt) som skyldes lakselus. I henhold til produksjonsområdeforskriften §§ 9-11 kan produksjonen økes, fryses eller senkes dersom miljøpåvirkningen er henholdsvis akseptabel, moderat eller uakseptabel. Dette innebærer at dersom det er sannsynlig at under 10 % av laksesmolten dør som følge av lakselus (akseptabel, grønt lys) kan kapasiteten økes med inntil 6 %. Er det sannsynlig at mellom 10 % og 30 % av laksesmolten dør som følge av lakselus (moderat, gult lys) kan kapasiteten fryses. Er det sannsynlig at over 30 % av laksesmolten dør som følge av lakselus (uakseptabel, rødt lys) kan kapasiteten senkes med 6 %. Fra og med 2022 er kunnskapsgrunnlaget for vurderingene i Trafikklyssystemet utvidet ved at Styringsgruppens Ekspertgruppe også gjennomfører heterogenitetsanalyser for å belyse om sårbare og viktige laksebestander har en avvikende høyere gjennomsnittlig luseindusert dødelighet av utvandrende smolt enn gjennomsnittet for produksjonsområdet. Bakgrunn for heterogenitetsanalysene er utredningene «Vurdering av kriterier for å vekte laksebestander i Trafikklyssystemet» (Thorstad mfl. 2022), «Vurdering av heterogenitet i lakselusindusert villfiskdødelighet innen produksjonsområder i 2020 og 2021» (Stige mfl. 2022) og Styringruppens råd til NFD i 2022. Formålet med Trafikklyssystemet er forutsigbar vekst i oppdrettsnæringen. I praksis er det likevel også Trafikklyssystemet som setter grensen for hvor mye villaks som i snitt tillates å dø i et produksjonsområde pga. lakselus. Lakselus er imidlertid ikke det eneste som påvirker villaksen, og ved å isolere en enkelt påvirkningsfaktor tas det heller ikke høyde for eventuelle synergieffekter. Grenseverdiene for lakseluspåvirkning beskrevet ovenfor er de samme uavhengige av hvilke tiltak som rettes mot enkeltbestander. Klima- og miljødepartementet og Nærings- og fiskeridepartementet har gitt Styringsgruppen for Trafikklyssystemet i oppdrag å nedsette en arbeidsgruppe som skal utrede hvordan Trafikklyssystemet påvirker arbeidet med å oppnå målene satt i kvalitetsnorm for villaks. Gruppen skal bestå av medlemmer med relevant kompetanse på Trafikklyssystemet og kvalitetsnorm for villaks. Gruppen skal, hvis nødvendig, også kunne innhente annen kompetanse fra relevante fagmiljøer. Styringsgruppen skal følge opp arbeidsgruppens arbeid og gjennomgå og evaluere arbeidsgruppens rapport, og levere en oppsummering av denne med relevante faglige vurderinger til Klima- og miljødepartementet og Nærings- og fiskeridepartementet. Formål Det overordnede formålet med arbeidsgruppens oppdrag er å utrede hvordan innretningen på Trafikklyssystemet for kapasitetsjustering i oppdrettsnæringen påvirker muligheten for at målene i kvalitetsnormen for villaks nås, og å foreslå eventuelle endringer i innretningen på trafikklyssystemet og/eller andre tiltak som kan gjøres for å nå normens mål. Mandat Arbeidsgruppen skal gjennomføre en nærmere utredning av om innretningen på Trafikklyssystemet, herunder grenseverdiene for lakselusindikatoren, påvirker mulighetene for at målene i kvalitetsnormen for villaks nås. Oppdraget gjelder kun villaks, og det skal ikke vurderes endringer i kvalitetsnormen. Arbeidsgruppen skal påpeke eventuelle faglige barrierer og behov for ytterligere kunnskap for å kunne utrede mulige tiltak. Arbeidsgruppes arbeid og vurderinger skal skje uavhengig av Styringsgruppen. Det vil bli avhold et oppstartsmøte mellom arbeidsgruppen og Styringsgruppen før arbeidet igangsettes. Arbeidsgruppen skal informere Styringsgruppen om progresjonen i det løpende arbeidet og kontakte Styringsgruppen om det oppstår uklarheter om mandatet og arbeidet. For å avklare eventuelle uklarheter i sluttrapporten ber Styringsgruppen om at det avholdes et informasjonsmøte mellom Styringsgruppen og arbeidsgruppen når et utkast til rapport foreligger. Den endelige rapporten skal oversendes Styringsgruppen senest 1. mars 2024