ATLANTIC SALMON AT SEA - factors affecting their growth and survival

This project has greatly enhanced the knowledge base on what impacts growth and survival of Atlantic salmon during their ocean feeding migration and redefined paradigms about where, when, and why salmon are spending their time at sea. Factors influencing the growth and survival in the ocean are among the most important and interesting features of the salmon life cycle to study because of the extreme variability, the multitude of factors controlling these variables, and the effect of survival on recruitment. Extensive use of genetic data and satellite tags have helped unravel the oceanic migration of salmon, with key feeding areas in the Norwegian and Barents seas. Surprising results showed that salmon used larger ocean areas and migrated further north than expected, made more use of oceanic fronts between the North Atlantic current and cold polar water, performed deeper dives, and stayed in colder water than was known before. Different distributions of regional stocks indicate fundamental differences in their migration behaviours, and variation in responses to environmental change and marine survival. Our analyses of salmon data improved the ecological understanding of the ocean system far beyond the species level and revealed an ecological regime shift in the Northeast Atlantic Ocean during the last twenty years. Scientists have been puzzled by a drastic reduction in the marine growth of salmon around the year 2005. We uncovered that the drop happened because of a reduced supply of nutrient-rich Arctic water into the Norwegian Sea. We showed that the reduced growth and reduction of fish larvae in the salmon stomachs over large geographic areas may be the result of a bottom-up driven process in the ocean ecosystem, through reduced nutrient-rich Arctic water masses and reduced zooplankton abundance. Climate change may be the cause, but what controls the inflow of Arctic water is not fully known. Several of our studies have shown the importance of fish larvae in the marine diet of young salmon, and how reduced prey ability during the first months at sea has contributed to the decline in salmon. In periods of poor feeding opportunities, there was a clear reduction in how much food was found in salmon stomachs, and the young salmon were slimmer and had been eating less energy-rich food such as fish larvae and Amphipoda crustaceans. This is likely caused by ocean warming, decreased primary productivity, and reduced recruitment of important fish larvae. There has been a dramatic warming of the ocean feeding areas over the last decades. We found that ocean warming may have limited direct effects on energy budgets and food consumption needed to maintain growth in adult Atlantic salmon. The most pronounced climate effects will likely be due to altered ecosystems, prey quality and abundance. Energy content of prey was the most important factor determining food consumption required to maintain growth. Young salmon start feeding soon after leaving the rivers. Feeding conditions during the first weeks within Norwegian fjords seemed not impacted by the North Atlantic regime shift. During the return migration from the ocean, salmon stop feeding before they reach the fjord areas. The frequent diving seen in coastal areas near the rivers is likely for navigation purposes. Herring and mackerel outnumber salmon in the sea by several orders of magnitude. There is no evidence that the lower marine survival for salmon in recent years was caused by competition with herring or mackerel. Competition between the species may have a negative impact on salmon, but it is not expected that a decreased abundance of pelagic fish would imply a large increase in salmon survival. Predation on adult Atlantic salmon by bluefin tuna, porbeagle, other large fish and toothed whales was shown. In a review paper, we found little evidence that predation alone has been the cause for driving healthy salmon populations below conservation limits. Remarkable studies in this project have also demonstrated human activities as drivers of evolution. Both direct and indirect effects of fishing and hydropower development can induce rapid evolution in salmon life-history in terms of how long they stay at sea, and thereby how large they are when returning to the rivers to spawn. Importantly, it has also been shown how poor ocean conditions may worsen the effects of other human impacts on wild salmon, like reduced growth caused by salmon lice from aquaculture. The pink salmon invasion in the Atlantic basin is an emerging threat and ultimately a massive ecological experiment. Pink salmon ocean distribution and diet overlap with Atlantic salmon. At present, the pink salmon abundance is too low to expect negative impacts on other species in the ocean ecosystem, but local impacts of pink salmon in coastal ecosystems cannot be ruled out. The project has resulted in almost 30 international scientific publications.

Norge har mange av verdens viktigste laksebestander, og laksen er viktig for fiske og turisme. Laksen er redusert og rødlistet på grunn av menneskelig aktivitet i elver og langs kysten og endret havmiljø. Norske myndigheter bruker store ressurser på forvaltning av laks, siden dette er en viktig art som det er knyttet store verdier til, og Norge har et spesielt internasjonalt ansvar for å ta vare på arten. Gjennom prosjektet har vi fått omfattende ny kunnskap om laksens vandringer i havet, hvilke områder de bruker og hva som påvirker deres vekst og overlevelse. Dette er viktig kunnskap for lakseforvaltningen, og er et viktig grunnlag for å vurdere effekter og tiltak ved ny og utvidet bruk av havområdene til akvakultur og produksjon av fornybar energi. Norsk lakseforvaltning er betydelig kunnskapsbasert, men mangel på kunnskap om havfasen har redusert muligheten for å basere forvaltningen på prognoser for lakseinnsiget i kommende år. Gjennom omfattende forskning i mer enn 100 år vet vi mye om laksens liv i elvene, men langt mindre om livet i havet. Laksungene er bare 10-15 cm lange når de forlater elvene. Oppholdet i havet er en viktig del av livet deres, fordi det er der de vokser og blir store, og dødelighet i denne fasen bestemmer i stor grad hvor mange voksne laks som kommer tilbake til elvene. Forvaltningen har måttet basere seg på en antakelse om at neste år blir som de fire siste årene, noe som ofte kan være feil, og det må gjøres evalueringer underveis i sesongen for å justere fisket i forhold til det faktiske lakseinnsiget fra havet. En betydelig økt forståelse av hva som påvirker laksen i havet gjennom dette prosjektet har medført en mye bedre forståelse av hvorfor laksebestander varierer, inkludert hvorfor det er regionale forskjeller og likheter, og forskerne er nærmere å kunne gi prognoser for hvor mye laks som hvert år kommer tilbake fra havet. Påvirkninger i havfasen er imidlertid mange og kompliserte, og innebærer datainnsamling fra store geografiske områder som gjennomgår svært raske klimaendringer. Det er nødvendig med videreføring av forskningsprogram på laksen i havet for å kunne gi forvaltningen mer presise råd ved regulering av fiske og utvikling av akvakultur og energiproduksjon, utarbeide prognoser for lakseinnsiget, og tilpasse havforvaltningen til de raske klimaendringene. Prosjektet har gjennom et svært godt forskersamarbeid styrket samarbeidet mellom store norske forskningsinstitusjoner og sikret optimal utnyttelse av store datasett. Dette gjelder også samarbeid med forskere fra Storbritannia, Irland og Canada. Dette har gitt et solid grunnlag for videre samarbeid og bruk av dataserier. Et stort antall vitenskapelige publikasjoner i anerkjente internasjonale tidsskrift har bidratt til å styrke kompetansen og internasjonal anerkjennelse av norske forskere. Prosjektet har i stor grad bidratt til rekruttering og utvikling av kompetanse hos unge forskere, både gjennom PhD, post doc og involvering av andre unge forskere.

The marine mortality of Atlantic salmon has increased during the past decades. Although Atlantic salmon is a well-studied fish, there is still lack of understanding on why the mortality has increased. In this project, we will examine sources of temporal and spatial variation in marine mortality. This is a complicated task, requiring extensive research collaborations between Atlantic salmon scientists, oceanographers and marine biologists. We will establish a long-term inter-institutional collaboration platform, gathering leading research institutions working on Atlantic salmon and marine biology in Norway for a concerted action through present and future projects, and strengthened collaborative use of data. International collaboration is an important part of this effort, and scientists from England, Canada, Ireland and Scotland participate in the project. The project consists of the following components: (1) Mapping and modelling the spatial and temporal variation in abiotic and biotic oceanographic data on conditions potentially influencing growth and mortality of salmon. (2) Mapping distribution and migration routes of Atlantic salmon at sea, which is a prerequisite for analyzing factors affecting them. (3) Analyzing variations in marine growth, survival and population sizes over time and geographical areas. (4) Combining data to identify the environmental and biological factors affecting marine survival. Utilising existing data and activities will be an important element to accomplish this work - including salmon collected at sea, genetic material, archival scale samples, survival data, population size data, migration data, and data series on other marine species and the ocean ecosystem. We will also use new genetic, stable isotope and electronic tagging technologies and modelling to provide novel results.