The reasons for the decline in some Atlantic salmon populations are not well understood at present.

Professor Asbjørn Vøllestad is leading a research project that uses data from a wide

geographical area over long timescales in an effort to address this

Firstly, can you outline the main focus of this investigation?

The project stemmed from the reported decrease in Atlantic salmon production and catch throughout Europe and North America. A large number of scientific papers are produced annually on this topic, but general answers about the main drivers behind this decrease are still lacking. We wanted to use the available long-term catch data, predominately from Norway and Scotland, to try to identify the main environmental factors driving the changes, focusing on a range of temporal and spatial scales. A more practical question we asked was what kind of biological signals are present in the available long-term catch data, and how can they be used.

Are you seeking to answer any specific questions with regard to population decline and maturity of Atlantic salmon?

Crucially, not all salmon populations are declining. Rather than framing our questions in order to discover why certain populations are in decline, we want to understand what drives the variability more generally. Are there any particular environmental factors in

fresh, coastal and marine waters that drive this variability in numbers; and are the same factors important in all rivers, or is there spatial variation? To understand variation in numbers it is important to understand whether it is caused by changes in survival or life history tactics (phenotypic plasticity). The last question can be answered to some degree by investigating how age at first maturation varies.

What have your results revealed about large-scale changes occurring in the northeastern Atlantic pelagic food web and variations in river conditions?

We (as do many others) find that the large-scale changes in ocean temperatures probably drive large-scale changes in salmon productivity and food-web structure. Clearly, changes in ocean conditions impact salmon growth and survival. Local river conditions also have an effect on the salmon catch, both indirectly through smolt quality and directly in terms of catchability. The fishing season in Norway is short (generally two months during summer), and factors such as water flow and temperature directly influence catch.

Can you summarise the other findings made by the study to date?

We have identified strong geographical trends in salmon catches across Norway (from south to north). Some of these trends are explained by known factors such as water quality (some southern rivers were acidified; water quality has now been restored due to reduced emissions of pollutants and through mitigating efforts in the rivers) and impact of aquaculture activities. Within individual rivers, the effect of the development of hydropower is usually – but not always – negative. However, we still find an unexplained negative trend over time, indicating that our analyses are missing some important environmental drivers.

To what extent is the work multidisciplinary? What skills are

represented by the research group?

This kind of work is highly collaborative as we need people who know about the fish and its management, population dynamics, hydrology and statistics. Our group contains researchers with this particular kind of expertise and our investigation would have been impossible without this divergent knowledge.

Have you faced any major challenges over the course of your research?

A major problem with long-term data collected by others is that there may be many errors; you need to be very critical about what to use. We have avoided using data from rivers where the quality of the data is uncertain. This cleaning process takes a lot of time and is difficult, but is absolutely necessary.

How do you intend your results to be implemented? Are you communicating your findings to fishing industry professionals and policy makers?

The results as they are cannot directly be implemented into management practice, but the background knowledge they provide should be used when designing management plans. To that end, I have been invited to be a member of an advisory body that gives annual advice to the Government about useful management actions. In this way there will be a direct transfer of information to the appropriate agencies.

Studying salmonPR

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80 INTERNATIONAL INNOVATION

Fishy foraysPopulations of Atlantic salmon

vary markedly in key areas of Europe and North America. Researchers at

the University of Oslo in Norway are carrying out a uniquely wide-ranging

study to try to understand why

THE ATLANTIC SALMON is an anadromous fish which

migrates to sea as a smolt, where it grows fast before

returning to rivers to spawn. Its spawning habitats are the fresh

waters of countries bordering the northern North Atlantic Ocean

basin. Studies have shown that smolts originating in European

rivers undertake long-distance feeding migrations across most of

the longitudinal range of the northern Atlantic Ocean, and that there are a

large number of discrete local breeding and rearing populations. Furthermore,

spawning and rearing habitats are largely isolated at a range of scales varying from

among-continent to among-river, as well as within-river. This means that the abundance of

local population units has the potential to vary independently of each other.

Atlantic salmon are of great economic and conservational importance, but populations throughout the species’ distributional range appear to be declining in ways that cannot be explained or predicted. Little formal analysis of variations in patterns of population strength has been carried out to establish the generality of the perceived declines, and an improved understanding is urgently needed.

NORWEGIAN AND SCOTTISH CATCH DATA

A group at the Center for Ecological and Evolutionary Synthesis at the University of Oslo in Norway is addressing this knowledge gap by analysing long-term data for the Atlantic salmon, obtained over a wide geographical range and across a gamut of spatial subscales. Led by Professor Asbjørn Vøllestad, the team is using rod catch data from Norwegian and Scottish

rivers and will be proposing hypotheses about population abundance that can be tested in the future. “We want to be able to put forward testable hypotheses as to which factors most strongly impact salmon productivity; we look for correlations between factors and are very clear that correlation is not causation,” Vøllestad highlights. “Our hypotheses have to be rigorously tested using other approaches, but correlational studies such as these can lead the way and point at useful avenues for further work.”

The temporal correlation in catches is very longterm, with trends persisting over several decades. The spatial correlation is relatively short range, indicating strong local-scale effects on catch. Furthermore, Scottish salmon populations have exhibited recent negative trends in contrast to some more positive trends in Norway – especially in the north. The group’s analyses of these correlations will lead to an improved understanding of the factors that drive population abundance in Atlantic salmon.

For Norwegian salmon populations, Vøllestad is using data from Statistics Norway. The datasets consist of catch records from Norwegian rivers covering the period 1876-2006; however due to changes in reporting practice most analyses cover the last four to five decades only. The rivers are distributed along the entire Norwegian coast, and this large spatial scale is expected to capture substantial variation in freshwater, coastal and ocean conditions. Potential interfering factors such as management regimes and salmon farming are evaluated using appropriate statistical methods.

For Scottish Atlantic salmon, the researchers are studying annual catch data from a set of 48 rivers covering the period from 1952 to the present day, together with data from associated coastal and

WWW.RESEARCHMEDIA.EU 81

PROFESSOR ASBJØRN VØLLESTAD

Understanding how within-river

and between-river factors affect the

dynamics of salmon populations will

vastly improve the ability to manage

wild salmon populations

DECLINING LONG-TERM TRENDS IN ATLANTIC SALMON ABUNDANCE: INFERRING MECHANISMS USING CATCH DATA FOR NORWAY AND SCOTLAND

OBJECTIVES

To assess which factors are responsible for observed temporal changes in catch in order to propose hypotheses that can be tested in future international collaborations such as the SALSEA project (see www.nasco.int/sas/salsea.htm).

PARTNERS

Norwegian Water Resources and Energy Directorate, Oslo, Norway

Norwegian Institute for Nature Research, Trondheim, Norway

Department of Mathematics, University of Oslo, Norway

Fisheries Research Services Freshwater Laboratory, Pitlochry, Scotland

FUNDING

Research Council of Norway

CONTACT

Professor Asbjørn Vøllestad Project Coordinator

Center for Ecological and Evolutionary Synthesis Department of Bioscience University of Oslo PO Box 1066 Blindern 0316 Oslo Norway

T +47 22 85 46 40 E avollest@bio.uio.no

www.mn.uio.no/bio/english

ASBJØRN VØLLESTAD is a Professor at the Department of Bioscience, Center for Ecological and Evolutionary Synthesis at the University of Oslo, Norway. He is the editor of Ecology of Freshwater Fish and a contributing editor for Aquatic Biology. His research focuses on understanding variation in traits such as growth, age at maturation, fecundity and egg size – focusing on freshwater fish, including salmon.

estuary fisheries. These rivers are grouped into three regions: those entering the North Sea, the Atlantic Ocean or the Irish Sea. Unusually, salmon return to Scottish rivers during most of the year; earlier return is associated with higher altitude spawning locations.

COLLABORATING ON MODELLING SALMON POPULATIONS

Collaboration across national scales is central to the success of the project, and international experts in salmon biology, population dynamics and statistical modelling have been brought together to work on the hypotheses the group is developing. For example, Vøllestad explains that to use the data they have, good local knowledge is needed to identify important local factors. With this in mind, the group is collaborating closely with the Norwegian Water Resources and Energy Directorate (NVE) and the Norwegian Institute for Nature Research, among others. Researchers at NVE were able to estimate daily water flow for all Norwegian rivers for a 40-year period. “These are not measurements, but model estimates based on precipitation and temperature measurements and information on topography and soil characteristics. These estimates are and will be very useful, and can also be down-scaled to sub-catchment scales,” Vøllestad adds.

The team is using linear mixed-effects models to conduct their assessments, which allows them to use all possible information in one global analysis: “This means that we need to account for the fact that we use time series data from multiple rivers. Each river is unique, with unique unmeasured factors influencing catch, Vøllestad observes. “In such a setting, the incorporation of random factors is very useful and necessary. The development of flexible statistical tools and powerful computers has really made these kinds of analyses possible.”

TRANSFERABLE FINDINGS

Understanding how within-river and between-river factors affect the dynamics of salmon populations will vastly improve the ability to manage wild salmon populations. The group’s results will be communicated to the scientific community as well as fishery managers, landowners, fishers and the interested public,

through oral presentations and non-technical articles in relevant magazines.

Vøllestad’s study is different because of its broad approach, which has meant that the group’s analyses have been met with great interest. “Earlier studies have focused on local issues, without searching for the large-scale drivers of change. Our studies have shown that work at many scales at the same time is required; if the approach is too localised important patterns may easily be missed,” he reflects.

The data covers a large geographical range, suggesting that the findings may represent general processes and thus be transferable to other areas – the local effects may differ however. Moreover, Atlantic salmon from Norwegian rivers intermingle in the northern Atlantic Ocean with Atlantic salmon from other European countries. The findings for the Norwegian populations should therefore be applicable to other populations that use the same area.

Finally, Vøllestad believes that there are many more questions about the Atlantic salmon that his research could answer. His group, together with researchers from various Norwegian research institutions, is currently working towards securing funding for a collaborative effort to model the complete life cycle of the salmon, using hierarchical Bayesian models. In this effort the group plans to use all available local, regional and global information on ecosystem processes and climate: “We could not include all this information in our linear mixed model framework, and especially relevant datasets from the coastal zone and the marine environment were not readily available to us. Our hope is to collect and organise this kind of data through a larger collaborative effort,” Vøllestad enthuses.

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