Leif Asbjørn Vøllestad

• Austad, Benedikte; Vøllestad, Leif Asbjørn & Foldvik, Anders (2021). Frequency of vateritic otoliths and potential consequences for marine survival in hatchery-reared Atlantic salmon. Journal of Fish Biology. ISSN 0022-1112. 98, p. 1401–1409. doi: 10.1111/jfb.14683. Full text in Research Archive
• Evangelista, Charlotte; Dupeu, Julia; Sandkjenn, Joakim; Pauli, Beatriz Diaz; Herland, Anders & Mériguet, Jacques [Show all 8 contributors for this article] (2021). Ecological ramifications of adaptation to size-selective mortality. Royal Society Open Science. ISSN 2054-5703. 8(10). doi: 10.1098/rsos.210842.
• Nater, Chloe Rebecca; Stubberud, Marlene Wæge; Langangen, Øystein Ole Gahr; Rustadbakken, Atle; Moe, S. Jannicke & Ergon, Torbjørn [Show all 8 contributors for this article] (2021). Towards a future without stocking: harvest and river regulation determine long-term population viability of migratory salmonids. Climate Research (CR). ISSN 0936-577X. doi: 10.3354/cr01644.
• Stubberud, Marlene Wæge; Nater, Chloe Rebecca; Vindenes, Yngvild; Vøllestad, Leif Asbjørn & Langangen, Øystein Ole Gahr (2021). Low impact of first-time spawners on population growth in a brown trout population. Climate Research (CR). ISSN 0936-577X. doi: 10.3354/cr01645.
• Poleo, Antonio; Kjelsberg, Birgitte Marie; Rukke, Nina Alstad & Vøllestad, Leif Asbjørn (2021). Rapid recovery of normal gill morphology and blood physiology in brown trout (Salmo trutta) after short-term exposure to toxic concentrations of aqueous aluminium under non-steady state chemical conditions. Journal of Limnology. ISSN 1129-5767. 80(2). doi: 10.4081/JLIMNOL.2021.2000. Full text in Research Archive
• Evangelista, Charlotte; Diaz Pauli, Beatriz; Vøllestad, Leif Asbjørn & Edeline, Eric (2020). Stoichiometric consequences of size-selective mortality: An experimental test using the Japanese medaka (Oryzias latipes). Science of the Total Environment. ISSN 0048-9697. 724(138193), p. 1–8. doi: 10.1016/j.scitotenv.2020.138193. Full text in Research Archive Show summary

  The determinants of intraspecific stoichiometric variation remain difficult to elucidate due to their multiple ori- gins (e.g. genetic vs. environmental) and potential interactive effects. We evaluated whether two size-selected lines of medaka (Oryzias latipes) with contrasted life-history strategies (small- and large-breeder lines with slow growth and early maturity vs. fast growth and late maturity) differed in their organismal stoichiometry (percentage and ratios of carbon [C], nitrogen [N] and phosphorus [P]) in a mesocosm experiment. We also tested how size-selection interacted with environmental conditions (i.e. two levels of fish density and light intensity), body condition and sex. Results showed that large-breeder fish were significantly N-enriched compared to small-breeders, while the two size-selected lines did not differ in body P composition. Size-selection interacted with density – high density only affected small-breeders leading to decreasing %C and C: N – and with sex – large-breeder females had higher %C and C:N values than large-breeder males. Finally, C:P and N:P ratios in- creased with body condition due to decreasing %P. Overall, our results show that the ecological consequences of size-selective mortality extend to organismal stoichiometry and may, from there, change nutrient cycling and ecosystem functioning.

• Durif, Caroline; Diserud, Ola Håvard; Sandlund, Odd Terje; Thorstad, Eva Bonsak; Poole, Russell & Bergesen, Knut Aanestad [Show all 9 contributors for this article] (2020). Age of European silver eels during a period of declining abundance in Norway. Ecology and Evolution. ISSN 2045-7758. 10(11), p. 4801–4815. doi: 10.1002/ece3.6234. Full text in Research Archive
• Evangelista, Charlotte; Vøllestad, Leif Asbjørn; Diaz Pauli, Beatriz & Edeline, Eric (2020). Density-dependent consequences of size-selective induced life-history changes to population fitness in medaka (Oryzias latipes). Canadian Journal of Fisheries and Aquatic Sciences. ISSN 0706-652X. 77(10), p. 1741–1748. doi: 10.1139/cjfas-2019-0406. Full text in Research Archive
• Pettersen, Ruben Alexander; Junge, Claudia; Østbye, Kjartan; Mo, Tor Atle & Vøllestad, Leif Asbjørn (2020). Genetic population structure of the monogenean parasite Gyrodactylus thymalli and its host European grayling (Thymallus thymallus) in a large Norwegian lake. Hydrobiologia. ISSN 0018-8158. 848, p. 547–561. doi: 10.1007/s10750-020-04431-7. Full text in Research Archive
• Taugbøl, Annette; Quinn, Thomas P.; Østbye, Kjartan & Vøllestad, Leif Asbjørn (2020). Allometric relationships in morphological traits associated with foraging, swimming ability, and predator defense reveal adaptations toward brackish and freshwater environments in the threespine stickleback. Ecology and Evolution. ISSN 2045-7758. 10, p. 13412–13426. doi: 10.1002/ece3.6945. Full text in Research Archive Show summary

  Freshwater colonization by threespine stickleback has led to divergence in morphology between ancestral marine and derived freshwater populations, making them ideal for studying natural selection on phenotypes. In an open brackish–freshwater system, we previously discovered two genetically distinct stickleback populations that also differ in geometric shape: one mainly found in the brackish water lagoon and one throughout the freshwater system. As shape and size are not perfectly correlated, the aim of this study was to identify the morphological trait(s) that separated the populations in geometric shape. We measured 23 phenotypes likely to be important for foraging, swimming capacity, and defense against predation. The lateral plate morphs in freshwater displayed few significant changes in trait sizes, but the low plated expressed feeding traits more associated with benthic habitats. When comparing the completely plated genetically assigned populations, the freshwater, the hybrids, the migrants and the lagoon fish, many of the linear traits had different slopes and intercepts in trait‐size regressions, precluding our ability to directly compare all traits simultaneously, which most likely results from low variation in body length for the lagoon and migrant population. We found the lagoon stickleback population to be more specialized toward the littoral zone, displaying benthic traits such as large, deep bodies with smaller eyes compared to the freshwater completely plated morph. Further, the lagoon and migrant fish had an overall higher body coverage of lateral plates compared to freshwater fish, and the dorsal and pelvic spines were longer. Evolutionary constraints due to allometric scaling relationships could explain the observed, overall restricted, differences in morphology between the sticklebacks in this study, as most traits have diversified in common allometric trajectories. The observed differences in foraging and antipredation traits between the fish with a lagoon and freshwater genetic signature are likely a result of genetic or plastic adaptations toward brackish and freshwater environments.

• Nater, Chloe R; Vindenes, Yngvild; Aass, Per; Cole, Diana; Langangen, Øystein & Moe, S. Jannicke [Show all 10 contributors for this article] (2020). Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout. Journal of Animal Ecology. ISSN 0021-8790. 89(9), p. 2122–2133. doi: 10.1111/1365-2656.13269. Full text in Research Archive Show summary

  Evidence‐based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic vs. natural causes. This is the case particularly when individuals vary in their vulnerability to different causes of mortality due to traits, life history stages, or locations. Here, we estimated harvest and background (other cause) mortality of landlocked migratory salmonids over half a century. In doing so, we quantified among‐individual variation in vulnerability to cause‐specific mortality resulting from differences in body size and spawning location relative to a hydropower dam. We constructed a multistate mark–recapture model to estimate harvest and background mortality hazard rates as functions of a discrete state (spawning location) and an individual time‐varying covariate (body size). We further accounted for among‐year variation in mortality and migratory behaviour and fit the model to a unique 50‐year time series of mark–recapture–recovery data on brown trout (Salmo trutta) in Norway. Harvest mortality was highest for intermediate‐sized trout, and outweighed background mortality for most of the observed size range. Background mortality decreased with body size for trout spawning above the dam and increased for those spawning below. All vital rates varied substantially over time, but a trend was evident only in estimates of fishers' reporting rate, which decreased from over 50% to less than 10% throughout the study period. We highlight the importance of body size for cause‐specific mortality and demonstrate how this can be estimated using a novel hazard rate parameterization for mark–recapture models. Our approach allows estimating effects of individual traits and environment on cause‐specific mortality without confounding, and provides an intuitive way to estimate temporal patterns within and correlation among different mortality sources.

• Dauwalter, D. C.; Duchi, Antonio; Epifanio, John; Gandolfi, Andrea; Gresswell, Robert & Juanes, Francis [Show all 18 contributors for this article] (2020). A call for global action to conserve native trout in the 21st century and beyond. Ecology of Freshwater Fish. ISSN 0906-6691. 29, p. 429–432. doi: 10.1111/eff.12538.
• Moe, S. Jannicke; Nater, Chloé R; Rustadbakken, Atle; Vøllestad, L Asbjørn; Lund, Espen & Qvenild, Tore [Show all 8 contributors for this article] (2020). Long-term mark-recapture and growth data for large-sized migratory brown trout (Salmo trutta) from Lake Mjøsa, Norway. Biodiversity Data Journal. ISSN 1314-2836. 8. doi: 10.3897/BDJ.8.e52157. Full text in Research Archive
• Sørdalen, Tonje Knutsen; Halvorsen, Kim Aleksander Tallaksen; Vøllestad, Leif Asbjørn; Moland, Even & Olsen, Esben Moland (2020). Marine protected areas rescue a sexually selected trait in European lobster. Evolutionary Applications. ISSN 1752-4571. 13(9), p. 2222–2233. doi: 10.1111/eva.12992. Full text in Research Archive
• Sävilammi, Tiina; Papakostas, Spiros; Leder, Erica; Vøllestad, Leif Asbjørn; Debes, Paul V. & Primmer, Craig R. (2020). Cytosine methylation patterns suggest a role of methylation in plastic and adaptive responses to temperature in European grayling (Thymallus thymallus) populations. Epigenetics. ISSN 1559-2294. 16, p. 271–288. doi: 10.1080/15592294.2020.1795597. Full text in Research Archive
• Nilsson, Anna L.K.; Reitan, Trond; Skaugen, Thomas; L'Abee-Lund, Jan Henning; Gamelon, Marlène & Jerstad, Kurt [Show all 11 contributors for this article] (2020). Location is everything, but climate gets a share: analyzing small-scale environmental influences on breeding success in the white-throated dipper. Frontiers in Ecology and Evolution. ISSN 2296-701X. 8. doi: 10.3389/fevo.2020.542846. Full text in Research Archive
• Nilsson, Anna L. K.; Skaugen, Thomas; Reitan, Trond; L'Abee-Lund, Jan Henning; Gamelon, Marlène & Jerstad, Kurt [Show all 11 contributors for this article] (2020). Hydrology influences breeding time in the white-throated dipper. BMC Ecology. ISSN 1472-6785. 20(70). doi: 10.1186/s12898-020-00338-y. Full text in Research Archive
• Savilammi, Tiina; Primmer, Craig R.; Varadharajan, Srinidhi; Guyomard, Rene; Guiguen, Yann & Sandve, Simen Rød [Show all 9 contributors for this article] (2019). The chromosome-level genome assembly of european grayling reveals aspects of a unique genome evolution process within salmonids. G3: Genes, Genomes, Genetics. ISSN 2160-1836. 9(5), p. 1283–1294. doi: 10.1534/g3.118.200919. Full text in Research Archive
• Muhlfeld, C.C.; Dauwalter, D. C.; D'Angelo, V. S.; Ferguson, A.; Giersch, J. J. & Impson, D. [Show all 12 contributors for this article] (2019). Global status of trout and char: Conservation challenges in the twenty-first century. In Kershner, J. L.; Williams, J. E.; Gresswell, R. E. & Lobon-Cervia, Javier (Ed.), Trout and Char of the World. American Fisheries Society. ISSN 978-1-934874-54-7. p. 717–760.
• Vøllestad, Leif Asbjørn & Primmer, Craig R (2019). Understanding local adaptation in a freshwater salmonid fish: evolution of a research programme. ICES Journal of Marine Science. ISSN 1054-3139. 76(6), p. 1404–1414. doi: 10.1093/icesjms/fsz037. Full text in Research Archive
• Diaz Pauli, Beatriz; Garric, Sarah; Evangelista, Charlotte; Vøllestad, Leif Asbjørn & Edeline, Eric (2019). Selection for small body size favours contrasting sex-specific life histories, boldness and feeding in medaka, Oryzias latipes. BMC Evolutionary Biology. ISSN 1471-2148. 19(1). doi: 10.1186/s12862-019-1460-x. Full text in Research Archive
• Stubberud, Marlene Wæge; Vindenes, Yngvild; Vøllestad, Asbjørn; Winfield, Ian J.; Stenseth, Nils Christian & Langangen, Øystein (2019). Effects of size‐ and sex‐selective harvesting: An integral projection model approach. Ecology and Evolution. ISSN 2045-7758. 9(22), p. 12556–12570. doi: 10.1002/ece3.5719. Full text in Research Archive
• Varadharajan, Srinidhi; Sandve, Simen Rød; Gillard, Gareth Benjamin; Tørresen, Ole K.; Mulugeta, Teshome Dagne & Hvidsten, Torgeir Rhoden [Show all 11 contributors for this article] (2018). The grayling genome reveals selection on gene expression regulation after whole-genome duplication. Genome Biology and Evolution (GBE). ISSN 1759-6653. 10(10), p. 2785–2800. doi: 10.1093/gbe/evy201. Full text in Research Archive
• Mäkinen, Hannu; Savilammi, Tiina; Papakostas, Spiros; Leder, Erica; Vøllestad, Leif Asbjørn & Primmer, Craig R. (2018). Modularity Facilitates Flexible Tuning of Plastic and Evolutionary Gene Expression Responses during Early Divergence. Genome Biology and Evolution (GBE). ISSN 1759-6653. 10(1), p. 77–93. doi: 10.1093/gbe/evx278. Full text in Research Archive
• Sørdalen, Tonje Knutsen; Halvorsen, Kim Aleksander Tallaksen; Harrison, Hugo B.; Ellis, Charlie D.; Vøllestad, Leif Asbjørn & Knutsen, Halvor [Show all 8 contributors for this article] (2018). Harvesting changes mating behaviour in European lobster. Evolutionary Applications. ISSN 1752-4571. 11(6), p. 963–977. doi: 10.1111/eva.12611. Full text in Research Archive
• Poole, W. Russell; Diserud, Ola Håvard; Thorstad, Eva Bonsak; Durif, Caroline; Dolan, Conor & Sandlund, Odd Terje [Show all 10 contributors for this article] (2018). Long-term variation in numbers and biomass of silver eels being produced in two European river systems. ICES Journal of Marine Science. ISSN 1054-3139. 75(5), p. 1627–1637. doi: 10.1093/icesjms/fsy053. Full text in Research Archive
• Nater, Chloé Rebecca; Rustadbakken, Atle; Ergon, Torbjørn; Langangen, Øystein; Moe, S. Jannicke & Vindenes, Yngvild [Show all 8 contributors for this article] (2018). Individual heterogeneity and early life conditions shape growth in a freshwater top predator. Ecology. ISSN 0012-9658. 99(5), p. 1011–1017. doi: 10.1002/ecy.2178. Full text in Research Archive Show summary

  Body size can have profound impacts on survival, movement, and reproductive schedules shaping individual fitness, making growth a central process in ecological and evolutionary dynamics. Realized growth is the result of a complex interplay between life history schedules, individual variation, and environmental influences. Integrating all of these aspects into growth models is methodologically difficult, depends on the availability of repeated measurements of identifiable individuals, and consequently represents a major challenge in particular for natural populations. Using a unique 30‐yr time series of individual length measurements inferred from scale year rings of wild brown trout, we develop a Bayesian hierarchical model to estimate individual growth trajectories in temporally and spatially varying environments. We reveal a gradual decrease in average juvenile growth, which has carried over to adult life and contributed to decreasing sizes observed at the population level. Commonly studied environmental drivers like temperature and water flow did not explain much of this trend and overall persistent and among‐year individual variation dwarfed temporal variation in growth patterns. Our model and results are relevant to a wide range of questions in ecology and evolution requiring a detailed understanding of growth patterns, including conservation and management of many size‐structured populations.

• Haugen, Thrond Oddvar & Vøllestad, Leif Asbjørn (2018). Pike Population Size and Structure: Influence of Density-Dependent and Density-Independent Factors. In Skov, Christian & Nilsson, Anders (Ed.), Biology and Ecology of Pike. CRC Press. ISSN 9781482262902. p. 123–163. doi: 10.1201/9781315119076-8.
• Vøllestad, Leif Asbjørn; Skurdal, Jostein & L'Abee-Lund, Jan Henning (2018). 10 års erfaring med nasjonale laksevassdrag - virker systemet? Vann. ISSN 0042-2592. 2018(1), p. 102–117.
• van Leeuwen, Casper; Dalen, Kristine; Museth, Jon; Junge, Claudia & Vøllestad, Leif Asbjørn (2018). Habitat fragmentation has interactive effects on the population genetic diversity and individual behaviour of a freshwater salmonid fish. Rivers Research and Applications: an international journal devoted to river research and management. ISSN 1535-1459. 34(1), p. 60–68. doi: 10.1002/rra.3226. Full text in Research Archive Show summary

  Sufficient genetic diversity can aid populations to persist in dynamic and fragmented environments. Understanding which mechanisms regulate genetic diversity of riverine fish can therefore advance current conservation strategies. The aim of this study was to investigate how habitat fragmentation interacted with population genetic diversity and individual behaviour of freshwater fish in large river systems. We studied a population of the long‐distance migratory, iteroparous freshwater salmonid European grayling (Thymallus thymallus) in south‐eastern Norway. Genotyping (n = 527) and radio‐tracking (n = 54) of adult fish throughout a 169‐km river section revealed three major migration barriers limiting gene flow and depleting genetic diversity upstream. Individuals from upstream areas that had dispersed downstream of barriers showed different movement behaviour than local genotypes. No natal philopatry was found in a large unfragmented river section, in contrast to strong fidelity to spawning tributaries known for individuals overwintering in lakes. We conclude that (a) upstream sub‐populations in fragmented rivers show less genetic variation, making it less likely for them to adapt to environmental changes; (b) fish with distinct genotypes in the same habitat can differ in their behaviour; (c) spawning site selection (natal philopatry) can differ between fish of the same species living in different habitats. Together this implies that habitat loss and fragmentation may differently affect individual fish of the same species if they live in different types or sections of habitat. Studying behaviour and genetic diversity of fish can unravel their complex ecology and help minimize human impact. barriers, dams and weirs, natal philopatry, radiotelemetry, spawning site fidelity, Thymallus thymallus

• Nilsson, Anna; L'Abee-Lund, Jan Henning; Vøllestad, Leif Asbjørn; Jerstad, Kurt; Larsen, Bjørn Mejdell & Røstad, Ole Wiggo [Show all 10 contributors for this article] (2018). The potential influence of Atlantic salmon Salmo salar and brown trout Salmo trutta on density and breeding of the white-throated dipper Cinclus cinclus. Ecology and Evolution. ISSN 2045-7758. 8(8), p. 4065–4073. doi: 10.1002/ece3.3958. Full text in Research Archive Show summary

  Interactions between birds and fish are often overlooked in aquatic ecosystems. We studied the influence of Atlantic salmon and brown trout on the breeding population size and reproductive output of the white-throated dipper in a Norwegian river. Acidic precipitation led to the extinction of salmon, but salmon recolonized after liming was initiated in 1991. We compared the dipper population size and reproductive output before (1978–1992) and after (1993–2014) salmon recolonization. Despite a rapid and substantial increase in juvenile salmon, the breeding dipper population size and reproductive output were not influenced by juvenile salmon, trout, or total salmonid density. This might be due to different feeding strategies in salmonids and dippers, where salmonids are mainly feeding on drift, while the dipper is a benthic feeder. The correlation between the size of the dipper population upstream and downstream of a salmonid migratory barrier was similar before and after recolonization, indicating that the downstream territories were not less attractive after the recolonization of salmon. Upstream dipper breeding success rates declined before the recolonization event and increased after, indicating improved water quality due to liming, and increasing invertebrate prey abundances and biodiversity. Surprisingly, upstream the migratory barrier, juvenile trout had a weak positive effect on the dipper population size, indicating that dippers may prey upon small trout. It is possible that wider downstream reaches might have higher abundances of alternative food, rending juvenile trout unimportant as prey. Abiotic factors such as winter temperatures and acidic precipitation with subsequent liming, potentially mediated by preyabundance, seem to play the most important role in the life history of the dipper. birds, breeding, predators, river, salmonids

• Østbye, Kjartan; Taugbøl, Annette; Ravinet, Mark; Harrod, Chris; Pettersen, Ruben Alexander & Bernatchez, Louis [Show all 7 contributors for this article] (2018). Ongoing niche differentiation under high gene flow in a polymorphic brackish water threespine stickleback (Gasterosteus aculeatus) population. BMC Evolutionary Biology. ISSN 1471-2148. 18(14), p. 1–18. doi: 10.1186/s12862-018-1128-y. Full text in Research Archive Show summary

  Background: Marine threespine sticklebacks colonized and adapted to brackish and freshwater environments since the last Pleistocene glacial. Throughout the Holarctic, three lateral plate morphs are observed; the low, partial and completely plated morph. We test if the three plate morphs in the brackish water Lake Engervann, Norway, differ in body size, trophic morphology (gill raker number and length), niche (stable isotopes; δ15N, δ13C, and parasites (Theristina gasterostei, Trematoda spp.)), genetic structure (microsatellites) and the lateral-plate encoding Stn382 (Ectodysplasin) gene. We examine differences temporally (autumn 2006/spring 2007) and spatially (upper/lower sections of the lake – reflecting low versus high salinity). Results: All morphs belonged to one gene pool. The complete morph was larger than the low plated, with the partial morph intermediate. The number of lateral plates ranged 8–71, with means of 64.2 for complete, 40.3 for partial, and 14.9 for low plated morph. Stickleback δ15N was higher in the lower lake section, while δ13C was higher in the upper section. Stickleback isotopic values were greater in autumn. The low plated morph had larger variances in δ15N and δ13C than the other morphs. Sticklebacks in the upper section had more T. gasterostei than in the lower section which had more Trematoda spp. Sticklebacks had less T. gasterostei, but more Trematoda spp. in autumn than spring. Sticklebacks with few and short rakers had more T. gasterostei, while sticklebacks with longer rakers had more Trematoda. spp. Stickleback with higher δ15N values had more T. gasterostei, while sticklebacks with higher δ15N and δ13C values had more Trematoda spp. The low plated morph had fewer Trematoda spp. than other morphs. Conclusions: Trait-ecology associations may imply that the three lateral plate morphs in the brackish water lagoon of Lake Engervann are experiencing ongoing divergent selection for niche and migratory life history strategies under high gene flow. As such, the brackish water zone may generally act as a generator of genomic diversity to be selected upon in the different environments where threespine sticklebacks can live. Adaptation, Ectodysplasin, Evolution, Gill raker, Natural selection, Panmixia, Stable isotope analyses, Stn382, Theristina gasterostei, Trematoda spp

• Villar, Jaime Otero; L'Abee-Lund, Jan Henning & Vøllestad, Leif Asbjørn (2017). Temporal and spatial variation in recreational catches of anadromous brown trout, Salmo trutta, in Norwegian rivers. Hydrobiologia. ISSN 0018-8158. 797(1), p. 199–213. doi: 10.1007/s10750-017-3176-5.
• Halvorsen, Kim Aleksander Tallaksen; Sørdalen, Tonje Knutsen; Vøllestad, Leif Asbjørn; Skiftesvik, Anne Berit; Espeland, Sigurd Heiberg & Olsen, Esben Moland (2017). Sex-and size-selective harvesting of corkwing wrasse (Symphodus melops)-a cleaner fish used in salmonid aquaculture. ICES Journal of Marine Science. ISSN 1054-3139. 74(3), p. 660–669. doi: 10.1093/icesjms/fsw221.
• Sandlund, Odd Terje; Diserud, Ola Håvard; Poole, Russell; Bergesen, Knut Aanestad; Dillane, Mary & Rogan, Gerard [Show all 9 contributors for this article] (2017). Timing and pattern of annual silver eel migration in two European watersheds are determined by similar cues. Ecology and Evolution. ISSN 2045-7758. 7(15), p. 5956–5966. doi: 10.1002/ece3.3099. Full text in Research Archive Show summary

  Many animals perform long-distance migrations in order to maximize lifetime reproductive success. The European eel migrates several thousand kilometers between their feeding habitats in continental waters (fresh-,brackish, and sea water) and their spawning area in the Sargasso Sea. Eels residing in freshwaters usually initiate their spawning migration as silver eels during autumn, triggered by diverse environmental cues. We analyzed the time series of silver eel downstream migration in Burrishoole, Ireland (1971–2015), and Imsa, Norway (1975–2015), to examine factors regulating the silver eel migration from freshwater to the sea. The migration season (90% of the run) generally lasted from 1 August to 30 November. Environmental factors acting in the months before migration impacted timing and duration of migration, likely through influencing the internal processes preparing the fish for migration. Once the migration had started, environmental factors impacted the day-to-day variation in number of migrants, apparently stimulating migration among those eels ready for migration. Both the day-to-day variation in the number of migrants and the onset of migration were described by nearly identical models in the two rivers. Variables explaining day-to-day variation were all associated with conditions that may minimize predation risk; number of migrants was reduced under a strong moon and short nights and increased during high and increasing water levels. Presence of other migrants stimulated migration, which further indicates that silver eel migration has evolved to minimize predation risk. The onset of migration was explained mainly by water levels in August. The models for duration of the migration season were less similar between the sites. Thus, the overall migration season seems governed by the need to reach the spawning areas in a synchronized manner, while during the actual seaward migration, antipredator behavior seems of overriding importance. Anguilla anguilla, daily variation, environmental variables, freshwater migration, migration onset, silver eel

• Forseth, Torbjørn; Barlaup, Bjørn Torgeir; Finstad, Bengt; Fiske, Peder; Gjøsæter, Harald & Falkegård, Morten [Show all 12 contributors for this article] (2017). The major threats to Atlantic salmon in Norway. ICES Journal of Marine Science. ISSN 1054-3139. 74(6), p. 1495–1513. doi: 10.1093/icesjms/fsx020. Full text in Research Archive Show summary

  Atlantic salmon (Salmo salar) is an economically and culturally important species. Norway has more than 400 watercourses with Atlantic salmon and supports a large proportion of the world’s wild Atlantic salmon. Atlantic salmon are structured into numerous genetically differentiated populations, and are therefore managed at the population level. Long-distance migrations between freshwater and ocean habitats expose Atlantic salmon to multiple threats, and a number of anthropogenic factors have contributed to the decline of Atlantic salmon during the last decades. Knowledge on the relative importance of the different anthropogenic factors is vital for prioritizing management measures. We developed a semi-quantitative 2D classification system to rank the different anthropogenic factors and used this to assess the major threats to Norwegian Atlantic salmon. Escaped farmed salmon and salmon lice from fish farms were identified as expanding population threats, with escaped farmed salmon being the largest current threat. These two factors affect populations to the extent that they may be critically endangered or lost, with a large likelihood of causing further reductions and losses in the future. The introduced parasite Gyrodactylus salaris, freshwater acidification, hydropower regulation and other habitat alterations were identified as stabilized population threats, which have contributed to populations becoming critically endangered or lost, but with a low likelihood of causing further loss. Other impacts were identified as less influential, either as stabilized or expanding factors that cause loss in terms of number of returning adults, but not to the extent that populations become threatened. Management based on population specific reference points (conservation limits) has reduced exploitation in Norway, and overexploitation was therefore no longer regarded an important impact factor. The classification system may be used as a template for ranking of anthropogenic impact factors in other countries and as a support for national and international conservation efforts. classification system, management advice, Salmo salar, threat assessment.

• Carim, Kellie; Vindenes, Yngvild; Eby, Lisa A.; Barfoot, Craig & Vøllestad, Leif Asbjørn (2017). Life history, population viability, and the potential for local adaptation in isolated trout populations. Global Ecology and Conservation. ISSN 2351-9894. 10, p. 93–102. doi: 10.1016/j.gecco.2017.02.001. Full text in Research Archive
• Halvorsen, Kim Aleksander Tallaksen; Larsen, Torkel; Sørdalen, Tonje Knutsen; Vøllestad, Leif Asbjørn; Knutsen, Halvor & Olsen, Esben Moland (2017). Impact of harvesting cleaner fish for salmonid aquaculture assessed from replicated coastal marine protected areas. Marine Biology Research. ISSN 1745-1000. 13(4), p. 359–369. doi: 10.1080/17451000.2016.1262042. Full text in Research Archive
• Anastasiadou, Chryssa; Daliri, Moslem; Oikonomou, Anthi & Vøllestad, Leif Asbjørn (2017). Length weight relationships, relative weight and relative condition factor of three freshwater shrimps from Greece. International Journal of Fisheries and Aquatic Studies. ISSN 2394-0506. 5(1), p. 403–406. Full text in Research Archive
• L'Abee-Lund, Jan Henning & Vøllestad, Leif Asbjørn (2017). Life-history plasticity in anadromous brown trout: a Norwegian perspective. In Lobon-Cervia, Javier & Sanz, Nuria (Ed.), Brown-Trout: Biology, ecology and management. John Wiley & Sons. ISSN 978-1-119-26831-4. p. 251–265. doi: 10.1002/9781119268352.ch11.
• Vøllestad, Leif Asbjørn (2017). Understanding brown trout population genetic structure: A northern-European perspective. In Lobon-Cervia, Javier & Sanz, Nuria (Ed.), Brown-Trout: Biology, ecology and management. John Wiley & Sons. ISSN 978-1-119-26831-4. p. 127–144. doi: 10.1002/9781119268352.ch5.
• Halvorsen, Kim Aleksander Tallaksen; Sørdalen, Tonje Knutsen; Durif, Caroline; Knutsen, Halvor; Olsen, Esben Moland & Skiftesvik, Anne Berit [Show all 9 contributors for this article] (2016). Male biased sexual size dimorphism in the nest building corkwing wrasse (Symphodus melops): implications for a size regulated fishery. ICES Journal of Marine Science. ISSN 1054-3139. 73(10), p. 2586–2594. doi: 10.1093/icesjms/fsw135.
• Østbye, Kjartan; Harrod, Chris; Gregersen, Finn; Klepaker, Tom Olav; Schulz, Michael & Schluter, Dolph [Show all 7 contributors for this article] (2016). The temporal window of ecological adaptation in postglacial lakes: a comparison of head morphology, trophic position and habitat use in Norwegian threespine stickleback populations. BMC Evolutionary Biology. ISSN 1471-2148. 16(102), p. 1–16. doi: 10.1186/s12862-016-0676-2.
• Hansson, Truls Hveem; Fischer, Barbara; Mazzarella, Anna; Voje, Kjetil L. & Vøllestad, Leif Asbjørn (2016). Lateral plate number in low-plated threespine stickleback: a study of plasticity and heritability. Ecology and Evolution. ISSN 2045-7758. 6(10), p. 3154–3160. doi: 10.1002/ece3.2020. Full text in Research Archive
• Wiig, Elisabeth; Reseland, Janne Elin; Østbye, Kjartan; Haugen, Håvard Jostein & Vøllestad, Leif Asbjørn (2016). Variation in Lateral Plate Quality in Threespine Stickleback from Fresh, Brackish and Marine Water: A Micro-Computed Tomography Study. PLOS ONE. ISSN 1932-6203. 11(10). doi: 10.1371/journal.pone.0164578.
• Bærum, Kim Magnus; Vøllestad, Leif Asbjørn; Kiffney, Peter; Remy, Alice Melisande Jacynthe & Haugen, Thrond Oddvar (2016). Population-level variation in juvenile brown trout growth from different climatic regions of Norway to an experimental thermal gradient. Environmental Biology of Fishes. ISSN 0378-1909. 99, p. 1009–1018. doi: 10.1007/s10641-016-0533-6. Show summary

  Climate-change scenarios predict increasing temperatures and more precipitation at high latitudes. Ectothermic species are highly affected by these environmental variables and due to few dispersal opportunities many populations will need to adapt to these environmental changes. Understanding if, where, and how such adaptation processes occur is important for our understanding of the possible impacts of a changing climate. Individual growth, a key life-history trait influencing population-level parameters is directly affected by temperature especially in ectotherms. Thermal adaptations that optimize growth are therefore expected in such organisms. However, knowledge about how ectothermic animals modify growth rate in the face of climate change is poor at best for many species especially at the local population level. Here, we present a common-garden experiment exploring variations in growth reaction norms for three populations of Salmo trutta (a temperate freshwater fish) over three discrete temperatures. The populations originated from different climatic regions of Norway that vary in temperature and precipitation. Thermal growth reaction norms varied among populations, however we found no convincing evidence for either local thermal adaptations or countergradient adaptations. Rather, the population variation tended to correlate with a variable indicating east vs west climate region, that is strongly associated with a gradient in precipitation in Norway. This results suggests precipitation levels with corresponding flow regimes to have a stronger selection potential for early juvenile growth compared to temperature in these systems. Keywords Climate change . Temperature . Precipitation . Adaptation . Fish . Salmonids

• Edeline, Eric; Groth, Andreas; Cazelles, Bernard; Claessen, David; Winfield, Ian J. & Ohlberger, Jan Philipp [Show all 9 contributors for this article] (2016). Pathogens trigger top-down climate forcing on ecosystem dynamics. Oecologia. ISSN 0029-8549. 181, p. 519–532. doi: 10.1007/s00442-016-3575-8.
• Vindenes, Yngvild; Langangen, Øystein; Winfield, Ian J. & Vøllestad, Leif Asbjørn (2016). Fitness consequences of early life conditions and maternal size effects in a freshwater top predator. Journal of Animal Ecology. ISSN 0021-8790. 85(3), p. 692–704. doi: 10.1111/1365-2656.12489.
• Pettersen, Ruben Alexander; Østbye, Kjartan; Holmen, Johannes; Vøllestad, Leif Asbjørn & Mo, Tor Atle (2016). Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for "the enemy release" hypothesis. Parasites & Vectors. ISSN 1756-3305. 9(51). doi: 10.1186/s13071-016-1306-y.
• van Leeuwen, Casper; Museth, Jon; Sandlund, Odd Terje; Qvenild, Tore & Vøllestad, Leif Asbjørn (2016). Mismatch between fishway operation and timing of fish movements: A risk for cascading effects in partial migration systems. Ecology and Evolution. ISSN 2045-7758. 6(8), p. 2414–2425. doi: 10.1002/ece3.1937. Full text in Research Archive Show summary

  Habitat fragmentation is a growing problem worldwide. Particularly in river systems, numerous dams and weirs hamper the movement of a wide variety of species. With the aim to preserve connectivity for fish, many barriers in river systems are equipped with fishways (also called fish passages or fish ladders). However, few fishways provide full connectivity. Here we hypothesized that restricted seasonal opening times of fishways can importantly reduce their effectiveness by interfering with the timing of fish migration, for both spring- and autumn-spawning species. We empirically tested our hypothesis, and discuss the possible eco-evolutionary consequences of affected migration timing. We analyzed movements of two salmonid fishes, spring-spawning European grayling (Thymallus thymallus) and autumn-spawning brown trout (Salmo trutta), in Norway’s two largest river systems. We compared their timing of upstream passage through four fishways collected over 28 years with the timing of fish movements in unfragmented river sections as monitored by radiotelemetry. Confirming our hypothesis, late opening of fishways delayed the migration of European grayling in spring, and early closure of fishways blocked migration for brown trout on their way to spawning locations during late autumn. We show in a theoretical framework how restricted opening times of fishways can induce shifts from migratory to resident behavior in potamodromous partial migration systems, and propose that this can induce density-dependent effects among fish accumulating in lower regions of rivers. Hence, fragmentation may not only directly affect the migratory individuals in the population, but may also have effects that cascade downstream and alter circumstances for resident fish. Fishway functionality is inadequate if there is a mismatch between natural fish movements and fishway opening times in the same river system, with ecological and possibly evolutionary consequences for fish populations. Brown trout (Salmo trutta), eco-evolutionary consequences, European grayling (Thymallus thymallus), fish passage functionality, life history strategy, salmonids.

• Mäkinen, Hannu; Papakostas, Spiros; Vøllestad, Leif Asbjørn; Leder, Erica H & Primmer, Craig R (2016). Plastic and evolutionary gene expression responses are correlated in European grayling (Thymallus thymallus) subpopulations adapted to different thermal environments. Journal of Heredity. ISSN 0022-1503. 107(1), p. 82–89. doi: 10.1093/jhered/esv069.
• Mazzarella, Anna; Boessenkool, Sanne; Østbye, Kjartan; Vøllestad, Leif Asbjørn & Trucchi, Emiliano (2016). Genomic signatures of the plateless phenotype in the threespine stickleback. Ecology and Evolution. ISSN 2045-7758. 6(10), p. 3161–3173. doi: 10.1002/ece3.2072. Full text in Research Archive
• Tsiamis, Konstantinos; Gervasini, Eugenio; D'Amico, Fabio; Deriu, Ivan; Katsanevakis, Stelios & Crocetta, Fabio [Show all 33 contributors for this article] (2016). The EASIN Editorial Board: Quality assurance, exchange and sharing of alien species information in Europe. Management of Biological Invasions. ISSN 1989-8649. 7(4), p. 321–328. doi: 10.3391/mbi.2016.7.4.02.
• Lennox, Robert J.; Falkegård, Morten; Vøllestad, Leif Asbjørn; Cooke, Steven J. & Thorstad, Eva Bonsak (2016). Influence of harvest restrictions on angler release behaviour and size selection in a recreational fishery. Journal of Environmental Management. ISSN 0301-4797. 176, p. 139–148. doi: 10.1016/j.jenvman.2016.03.031.
• Haddeland, Peter Jørgen Tønnessen; Junge, Claudia; Serbezov, Dimitar & Vøllestad, Leif Asbjørn (2015). Genetic Parentage Analysis Confirms a Polygynandrous Breeding System in the European Grayling (Thymallus thymallus). PLOS ONE. ISSN 1932-6203. 10(3). doi: 10.1371/journal.pone.0122032. Full text in Research Archive

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• Vøllestad, Leif Asbjørn (2021). Fisken i Haldensvassdraget.
• Vøllestad, Leif Asbjørn (2021). Hva har vi lært (om laksen) i senere tid?
• Hesthagen, Trygve H.; Vøllestad, Leif Asbjørn; Brabrand, Åge; Enerud, Jørn; Fjellheim, Arne & Larsen, Bjørn Mejdell [Show all 10 contributors for this article] (2021). Utbredelse av bekkeniøye i norske vassdrag. Fauna. ISSN 0014-8881. 74(1-2), p. 48–67.
• Hesthagen, Trygve H.; Vøllestad, Leif Asbjørn & Økland, Finn (2020). Forekomsten av havniøye i norske elver . Fauna. ISSN 0014-8881. 73, p. 66–81.
• Vøllestad, Leif Asbjørn (2020). Et stille rop om havet. Biolog. ISSN 0801-0722. 38(3), p. 53–54.
• Vøllestad, Leif Asbjørn (2019). Kalkingens historie er skrevet. Biolog. ISSN 0801-0722. 1-2019, p. 48–50.
• Vøllestad, Leif Asbjørn (2019). Drivers of variation in end-of-season size of 0+ brown trout Salmo trutta.
• Vøllestad, Leif Asbjørn (2019). Oppdrett - ein økologisk katastrofe? Debatt.
• Halvorsen, Andrine E; Vøllestad, Leif Asbjørn; Eldøy, Sindre Håvarstein; Sjursen, Aslak Darre; Rønning, Lars & Davidsen, Jan Grimsrud (2019). Marine migratory behavior of anadromous brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus) in a Norwegian fjord system.
• Vøllestad, Leif Asbjørn (2019). To bøker om ål; fortellinger om en verden i endring. Biolog. ISSN 0801-0722. 2019(3-4), p. 63–65.
• Austad, Benedikte; Foldvik, Anders & Vøllestad, Leif Asbjørn (2019). Forekomst av unormale otolitter hos laksesmolt: påvirker det overlevelsen?
• Heins, David; Budy, Phaedra; Sanz, Nuria & Vøllestad, Leif Asbjørn (2018). 25 years of EFF?a thank you to the co-founding editor Javier Lobon-Cervia. Ecology of Freshwater Fish. ISSN 0906-6691. 27(1), p. 517–518. doi: 10.1111/eff.12343.
• Vøllestad, Leif Asbjørn (2018). Hva er det med laksen?
• Vøllestad, Leif Asbjørn (2018). Norges største ørret blir mindre og mindre. Titan.uio.no.
• Vøllestad, Leif Asbjørn (2018). I hodet på en fisk. Biolog. ISSN 0801-0722. 2018(2), p. 49–50.
• Avset, Liv Mildrid Melkild; Moe, S. Jannicke; Vøllestad, Leif Asbjørn & Rustadbakken, Atle (2018). Norges største ørret blir stadig mindre. [Internet]. forskning.no.
• Vøllestad, Leif Asbjørn (2018). Hvorfor er det så mye styr med laksen? Aftenposten Viten. ISSN 2464-3033. Show summary

  Laksen er ikonisk. Et symbol på det rene, vakre, sterke. Likevel er det ingen annen fisk i Norge som skaper større konflikter og debatter.

• Vøllestad, Leif Asbjørn (2018). Røyekveld. . Show summary

  Den kaldtvannselskende røya var den første fisken som koloniserte norske ferskvannsområder etter siste istid. I dag finnes den både høyt og lavt obver hele landet; i dype lavlandsjøer, mellomstore sjøer, mørke skogsvann og grunne fjellvann. Det finnes mange unntak, men de fleste populasjonene er preget av overbefolkning og dominert av magre småfisker. De siste årene hevdes det at røya er på tilbakegang i Sør-Norge. Endret klima og sur nedbør får skylda, men hva vet vi egentlig? Professor Asbjørn Vøllestad ved Instituttet for biovitenskap ved Universitetet i Oslo vil kåsere rundt temaet. Han arbeider primært med å forstå dynamikken i fiskebestander, hovedsakelig i ferskvann.

• Vøllestad, Leif Asbjørn (2018). Hva er det med laksen?
• Vøllestad, Leif Asbjørn (2017). Population differentiation and life history diversification: rapid and small scale?
• Vøllestad, Leif Asbjørn (2017). Understanding contemporary evolution in grayling Thymallus thymallus. A long-term study.
• Vøllestad, Leif Asbjørn (2017). Hva er det med laksen?
• Sandve, Simen Rød; Hvidsten, Torgeir Rhoden; Lien, Sigbjørn; Rori, Rohlfs; Jakobsen, Kjetill Sigurd & Nome, Torfinn [Show all 9 contributors for this article] (2017). Evolution after whole genome duplication - insights from the salmonids.
• Vøllestad, Leif Asbjørn (2017). Havboka. Biolog. ISSN 0801-0722. 35(3-4), p. 14–15.
• Bærum, Kim Magnus; Vøllestad, Leif Asbjørn; Kiffney, Peter; Remy, Alice & Haugen, Thrond Oddvar (2016). Erratum to: Population-level variation in juvenile brown trout growth from different climatic regions of Norway to an experimental thermal gradient (Environ Biol Fish, 10.1007/s10641-016-0533-6). Environmental Biology of Fishes. ISSN 0378-1909. 99(12). doi: 10.1007/s10641-016-0553-2.
• Vøllestad, Leif Asbjørn (2016). Is it possible to give highly effective lectures?
• Vindenes, Yngvild; Langangen, Øystein; Winfield, Ian J. & Vøllestad, Leif Asbjørn (2016). The relative impacts of early life conditions and maternal size effects in determining average fitness and population growth of pike .
• Høydalsvik, Maria N. & Vøllestad, Leif Asbjørn (2016). Population dynamics in lobster: Effects of marina protected areas.
• Vøllestad, Leif Asbjørn (2016). Fiskeanatomi. Biolog. ISSN 0801-0722. 34(3), p. 37–37.
• Vøllestad, Leif Asbjørn (2016). Dyreriket - en zoologisk reise. Biolog. ISSN 0801-0722. 34(2), p. 46–47.
• Vøllestad, Leif Asbjørn (2016). Dødelighet i kystsonen og i havet. Hvor blir laksen av.
• Vøllestad, Leif Asbjørn (2016). Økologiske konsekvenser av kultivering?
• Vøllestad, Leif Asbjørn (2016). Understanding contemporary evolution in grayling Thymallus thymallus.
• Vøllestad, Leif Asbjørn (2016). Population differentiation and life history diversification: rapid and small scale?
• Mellerud, Ida Kristin & Vøllestad, Leif Asbjørn (2015). Connectivity of Atlantic cod at the Norwegian Skagerrak coast .
• Fuchs, Daniela & Vøllestad, Leif Asbjørn (2015). Personality traits in fish – the consistency of behavior in sea trout (Salmo trutta) of Southern Norway .
• Høydalsvik, Maria N. & Vøllestad, Leif Asbjørn (2015). Population dynamics in lobster: Effects of marine protected areas.
• Juliussen, Elisabeth H. & Vøllestad, Leif Asbjørn (2015). Adaption to temperature in Atlantic cod (Gadus morhua) on a micro-geographic scale.
• Vindenes, Yngvild; Langangen, Øystein; Winfield, Ian J. & Vøllestad, Leif Asbjørn (2015). Relative fitness importance of a size-mediated maternal effect – an integral projection model approach .
• Boessenkool, Sanne; Star, Bastiaan; Trucchi, Emiliano; Merilä, Juha & Vøllestad, Leif Asbjørn (2015). Whole genome analyses of threespine stickleback populations from the Mediterranean to Northern Europe.
• Taugbøl, Annette; Arntsen, Tina; Østbye, Kjartan & Vøllestad, Leif Asbjørn (2015). Small changes in gene expression of targeted osmoregulatory genes when exposing marine and freshwater Threespine Stickleback (Gasterosteus aculeatus) to abrupt salinity transfers.
• Hesthagen, Trygve H.; Vøllestad, Leif Asbjørn & Økland, Finn (2021). Forekomsten av havniøye i norske elver. Norsk institutt for naturforskning (NINA). ISSN 978-82-426-4744-3.
• Halvorsen, Andrine E; Vøllestad, Leif Asbjørn & Davidsen, Jan Grimsrud (2019). Marine migratory behavior of anadromous brown trout and Arctic char in a Norwegian fjord system. Universitetet i Oslo.
• Hindar, Kjetil; Mo, Tor Atle; Eken, Morten; Hagen, Anders Gjørwad; Hytterød, Sigurd & Sandodden, Roar [Show all 8 contributors for this article] (2018). Kan Gyrodactylus salaris utryddes fra Drammensregionen? Sluttrapport fra arbeidsgruppen for Drammensregionen. Norsk institutt for naturforskning. ISSN 978-82-426-3187-9. 1456(1456). Show summary

  Lakseparasitten Gyrodactylus salaris er en av de alvorligste truslene mot villaksen i Norge. Bekjempelse av parasitten har derfor høy prioritet. Målet er å bli kvitt parasitten der dette er mulig, samtidig som risikoen for smittespredning til nye elver og regioner reduseres. Kjemisk behandling i kombinasjon med fiskesperrer, har de seneste årene dannet grunnlaget for bekjempelsesarbeidet, som har ført til færre infiserte vassdrag og redusert spredningsrisiko. I handlingsplanen mot G. salaris 2014-2016, utarbeidet av Miljødirektoratet og Mattilsynet, heter det at det er utarbeidet planer for utryddelse av G. salaris fra alle infiserte vassdrag med unntak av de tre smittede vassdragene i Drammensregionen. Der var forslaget å foreta nødvendige utredninger og undersøkelser med hensyn på muligheten for å utrydde parasitten. En arbeidsgruppe ble oppnevnt av Miljødirektoratet i 2015 og denne rapporten redegjør for arbeidsgruppens vurderinger. Arbeidsgruppen fikk som mandat å gå gjennom alle tilgjengelige alternativer og muligheter for å utrydde parasitten fra Drammensregionen, beskrive hvilke strategier og metoder som kan benyttes, vurdere sannsynligheten for å lykkes med bekjempelse av G. salaris i smitteregionen og beregne kostnadene ved de ulike strategiene/metodene. I 2017 presiserte Miljødirektoratet at arbeidsgruppen i tillegg til å vurdere rotenonmetoden og aluminiumsmetoden, som er de to metodene som har vært brukt mot G. salaris, også skulle foreta en vurdering av klormetoden som ennå er på forsøksstadiet. Arbeidsgruppen har vurdert Drammensfjorden med sitt ferskvannslag og forekomst av infiserte laksunger langt fra nærmeste elvemunning som den største utfordringen med tanke på å utrydde G. salaris fra regionen. En annen stor utfordring er det artsrike fiskesamfunnet i regionen med ferskvannsfiskearter med til dels dårlig kjent biologi som må bevares under bekjempelse av parasitten. De andre utfordringene som er identifisert i regionen, mener arbeidsgruppen kan løses gjennom en detaljplanlegging tilsvarende de som har gått forut for tidligere aksjoner mot G. salaris. På denne bakgrunn mener arbeidsgruppen at det er sannsynlig at G. salaris kan utryddes fra Drammensregionen med kjent metodikk, men at det forut for en kjemisk behandling må skje en kunnskapsinnhenting som gir trygghet for at de store utfordringene som arbeidsgruppen har identifisert, kan håndteres på en sikker måte. Blant viktige strategier og tiltak som forvaltningen bør vurdere, fremhever arbeidsgruppen flere gjentatte behandlinger enn det som hittil har vært vanlig, utprøving av ny metodikk som kan redusere forekomsten av parasitten, tiltak som reduserer behandlingsområdet, og tiltak som reduserer antallet laksunger i området som skal behandles. NØKKELORD : Drammensregionen (Buskerud og Vestfold), Drammenselva, Lierelva, Sandeelva, Drammensfjorden, Gyrodactylus salaris, laks, Salmo salar, utryddelse, kjemisk behandling

• Dalen, Kristine; Vøllestad, Leif Asbjørn & van Leuween, Casper (2016). Effect of fragmentation and lack of precise homing on population structure in European grayling (Thymallus thymallus) within a large Norwegian river system. Institutt for biovitenskap, Universitetet i Oslo.

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