
Tor Fredrik NæsjeNorwegian Institute for Nature Research | NINA
Tor Fredrik Næsje
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Publications (241)
Objective
Surveillance fishing surveys can be performed to estimate the proportion of farmed salmon represented in the spawning stock of native Atlantic Salmon Salmo salar populations. These surveys take place after the recreational fishing period and therefore closer to the spawning period than the open recreational fishing season. Although catch‐...
In many populations of Atlantic salmon, individuals return to rivers months or even a year prior to spawning. This premature arrival in fresh water by far exceeds the time needed for upstream migrations to spawning areas. Leaving the ocean early means forgoing substantial growth and incurring high metabolic costs of fasting in freshwater. Differenc...
Intra-specific variability in movement behaviour occurs in all major taxonomic groups. Despite its common occurrence and ecological consequences, individual variability is often overlooked. As a result, there is a persistent gap in knowledge about drivers of intra-specific variability in movement and its role in fulfilling life history requirements...
The fishery for the African tigerfish Hydrocynus vittatus in northern Namibia makes a substantial contribution to surrounding food security and stimulates the local economy through subsistence and recreational fishing. However, local fishers suggest that catch rates of H. vittatus have declined and the fish are now smaller. The Namibian Ministry of...
Marine aquaculture of Atlantic salmon (Salmo salar) is a relatively new industry where breeding programs have led to rapid genetic change in the captive populations that were built up alongside conspecific wild individuals. Throughout its 50-years history, marine aquaculture of Atlantic salmon has been associated with escapes, and studies have show...
The fine-scale movement of Mustelus mustelus in relation to environmental conditions was investigated at various temporal scales with acoustic telemetry and temperature–depth transmitters inside a coastal marine protected area in South Africa. Twenty-four sharks were equipped with acoustic tags and logged by an array of 28 acoustic receivers from N...
Determining the mechanisms driving range-wide reductions in Atlantic salmon marine survival is hindered by an insufficient understanding of their oceanic ecology and distribution. We attached 204 pop-up satellite archival tags to post-spawned salmon when they migrated to the ocean from seven European areas and maiden North American salmon captured...
Brown trout (Salmo trutta) display extensive plasticity in marine migratory behaviours, with marine migrations considered to be an adaptive strategy which enables sea trout to maximize growth and reproductive potential. However, marine migrations are not without associated costs, including threats posed by ever‐increasing salmon lice (Lepeophtheiru...
Salmon lice Lepeophtheirus salmonis infestation of sea trout Salmo trutta results in both additional marine mortality and behavioral changes which may contribute to sea trout population decline. For effective management of activities that increase exposure to salmon lice, such as salmon aquaculture, it is necessary to have a full understanding of h...
The viability of wild Atlantic salmon populations is threatened by genetic introgression from escaped farmed salmon. Farmed Atlantic salmon are genetically improved for important commercial traits and a life in captivity but are poorly adapted to the natural environment. The rate of geneflow from escaped farmed to wild salmon depends on their spawn...
Within the last decades, globalization has changed the international seafood trade, allowing low-income countries to access markets in high-income countries and vice versa. Nevertheless, the effects of globalization are controversial and in particular the impacts on small-scale fishers and local fish traders are unclear. This paper examines the eco...
Fouling of externally attached tags is an important consideration in long-term tagging studies as it may affect fish behaviour and well-being. Two externally attached radio transmitters on African tigerfish Hydrocynus vittatus were covered with short green algae, after the fish were recaptured 49 and 64 days after tagging in the Kavango River, Nami...
Knowledge of population sizes of threatened fish species is essential for formulation and implementation of effective conservation and management policies. Visual counts (VC) from two SCUBA diving expeditions during February and May 2019 were used to estimate the population size of the subterranean blind clariid cave-catfish Clarias cavernicola Tre...
Marine growth has strong implications for reproductive potential and ultimate fitness of sea trout. Hence, the effects of anthropogenic factors on marine growth are important when evaluating population responses and implementing management measures. Temporal changes in growth patterns of sea trout from three Norwegian and two Irish watercourses wer...
To gain more knowledge about escaped farmed salmon Salmo salar in the wild, we investigated the proportion of escapees, body length at escape, proportion escaped as smolts/post-smolts (≤300 mm) and number of winters in the wild (winter zones in the growth pattern in the scale samples) in catches on the Norwegian coast and in an adjacent fjord from...
Fouling of externally attached tags is an important consideration in long term tagging studies as it may affect fish behaviour and well-being. Two externally attached radio transmitters on African tigerfish Hydrocynus vittatus were covered with short grown green algae, after the fish were recaptured 49 and 64 days after tagging in the Kavango River...
African tigerfish Hydrocynus vittatus (n = 35) were tagged with external radio‐transmitters in the Kavango River, Namibia, to determine whether freshwater protected areas could be an effective tool for the management and conservation of this species. They were manually tracked in the core study area of 33 km every c. 12 days from July–October 2016...
Norway is the world's largest producer of farmed Atlantic salmon and is home to 400 rivers containing wild salmon populations. Farmed escapees, a reoccurring challenge of all cage-based marine aquaculture, pose a threat to the genetic integrity, productivity, and evolutionary trajectories of wild populations. Escapees have been monitored in Norwegi...
Undersøkelser av innslag av oppdrettslaks i Altaelva og Repparfjordelva i 2018. Klassifisering av laks med tanke på opphav er basert på en kombinasjon av ytre kjennetegn og analyser av skjellkarakterer. I 2018 ble det sendt inn 392 skjellprøver fra sportsfisket i Altaelva, som ut fra skjellanalyser ble klassifisert som 383 villaks og ni laks med us...
The return migration of adult Atlantic salmon was investigated by analysing recaptures of individuals tagged and released as smolts in the River Altaelva and the River Halselva using a catch per unit effort approach. Although the salmon were recaptured over a large area along the coastline (from >1100 km south to > 500 km northeast of their home ri...
Undersøkelsene i Altaelva og Repparfjordelva har skjedd årlig siden 1989. Klassifisering av laks med tanke på opphav er basert på en kombinasjon av ytre kjennetegn og analyser av skjellkarakterer. I skjellmaterialet fra laks fanget under sportsfiske i Altaelva var det ifølge skjellanalyser 673 villaks og én oppdrettslaks. Skjellmaterialet fra sport...
Understanding movement behaviour is essential for effective management of fishery species. Dart tags were used to study coastal movement patterns of white steenbras Lithognathus lithognathus, an overexploited seabream (Sparidae) in South Africa. In total, 6 962 fish (190–1 080 mm fork length) were tagged throughout the species’ distributional range...
From 2011-2013, anadromous brown trout Salmo trutta L. (213-730 mm) were collected during or shortly after their marine feeding migration at seven different localities in Central Norway. The mean volume of stomach content (%) of marine fish prey eaten by S. trutta captured in marine waters varied from 34 % to 89 %. There was a high prevalence (67-1...
The vertical behaviour of 44 veteran sea trout Salmo trutta (275-580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May to February by using acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day vs night), season, seawater temperatur...
The diving behaviour of adult Atlantic salmon Salmo salar L. post-spawners in the Norwegian and Barents Seas was monitored with pop-up satellite archival tags (PSATs) and data storage tags (DSTs). Salmon from the 3 studied populations showed similar depth use patterns: tagged specimens spent most of their time near the surface (mean of 82% of the t...
Interbreeding between domesticated and wild animals occurs in several species. This gene flow has long been anticipated to induce genetic changes in life-history traits of wild populations, thereby influencing population dynamics and viability. Here, we show that individuals with high levels of introgression (domesticated ancestry) have altered age...
White stumpnose Rhabdosargus globiceps is the main target of the linefishery in Saldanha Bay. Increased fishing pressure over the last three decades, particularly by the recreational sector, has led to concerns regarding sustainability of the local white stumpnose stock. The fishery was exceptionally productive between 2006 and 2008, with an estima...
Understanding the level of connectivity between estuarine and coastal waters is essential for appropriate management of estuarine-associated taxa. Most studies have focused on the role of a single estuary, while limited research exists on the importance of multiple estuaries to individuals of estuarine-associated species. This study used acoustic t...
Long-term home-range and movement dynamics of spotted grunter Pomadasys commersonnii and the distribution of their dominant prey, the sandprawn Callichirus kraussi, were investigated in the intermittently open East Kleinemonde Estuary, South Africa. In addition, the effects of fish length, diel cycle, season and other abiotic factors on home range...
Within-river movements and distribution of wild and escaped farmed Atlantic salmon before and during spawning were compared. Seventy-four wild and 43 escaped farmed Atlantic salmon were captured in the sea, tagged with radio transmitters and released. Based on examination of farmed and wild Atlantic salmon most, if not all, fish entering the River...
Males and females share many traits that have a common genetic basis, however selection on these traits often differs between the sexes leading to sexual conflict. Under such sexual antagonism, theory predicts the evolution of genetic architectures that resolve this sexual conflict. Yet, despite intense theoretical and empirical interest, the speci...
Habitat connectivity is a fundamental component of estuary-associated fish behaviour. Understanding this is critical to understanding population dynamics and the nursery role of estuarine habitats. This study aimed to elucidate the intra-population structure of juvenile Argyrosomus japonicus and the role of estuarine and coastal habitats for this w...
Males and females share many traits that have a common genetic basis, however selection on these traits often differs between the sexes leading to sexual conflict. Under such sexual antagonism, theory predicts the evolution of genetic architectures that resolve this sexual conflict. Yet, despite intense theoretical and empirical interest, the speci...
The biology and ecology of anadromous brown trout (Salmo trutta) at sea is poorly understood. This study provided information on spatial and temporal distribution of sea trout in the ocean. The behaviour of 115 individuals (veteran migrants, 270–700 mm) was tracked by using acoustic telemetry in a fjord system during April–September in 2012–2013. O...
To reproduce, Atlantic Salmon Salmo salar return to freshwater rivers and migrate upriver to spawning areas. This migration is the basis for recreational fisheries, which for conservation reasons are increasingly characterized by catch-and-release angling. The effectiveness of catch and release for Atlantic Salmon conservation is contingent on the...
This case study of a fish market near the border of Namibia and Zambia examines how economic and environmental changes have affected the marketing channels associated with a fishery in southern Africa. We monitored the volume and origin of fish entering the market in Katima Mulilo, Namibia, between 2007 and 2012, and conducted semi-structured inter...
Marine migration and habitat use of sea trout Salmo trutta from the watercourse Søa and the river Snilldalselva in Sør-Trøndelag in Central Norway were studied during summer in 2012 and 2013. The sea trout were tagged with acoustic transmitters and their movements and behaviour recorded by automatic listening stations.
There was no difference in j...
Migrations between different habitats are key events in the lives of many organisms. Such movements involve annually recurring travel over long distances usually triggered by seasonal changes in the environment. Often, the migration is associated with travel to or from reproduction areas to regions of growth. Young anadromous Atlantic salmon (Salmo...
Atlantic salmon (Salmo salar) often survive spawning and migrate back to the sea to feed, either shortly after spawning in autumn or the following spring. We conducted a 4-year observational field study using telemetry to evaluate the determinants of migration timing in Atlantic salmon postspawners (kelts). We found that individuals with low energy...
The negative relationship between body mass and population abundance was documented decades ago and forms one of the most fundamental scaling-laws in ecology. However, current theory fails to capture observed variations and the subject continues to raise controversy. Here we unify empirically observed size-abundance relationships with theory, by in...
Little is known about Atlantic salmon behaviour during the last phase of the marine homing migration and subsequent river entry. In this study, 56 adult Atlantic salmon in the Alta Fjord in northern Norway were equipped with acoustic transmitters. Salmon generally followed the coastline, but their horizontal distribution was also affected by wind-i...
The potential of the gill maggot Salmincola salmoneus for use as an indicator of repeat spawning in Atlantic salmon Salmo salar was studied in adult fish captured during their return migration to the River Alta (n = 659) and River Namsen (n = 540) in Norway. Eighty-eight and 49% of previous spawners identified by scale readings were infected with S...
Changes in snow and ice conditions are some of the most distinctive impacts of global warming in cold temperate and Arctic regions, altering the environment during a critical period for survival for most animals. Laboratories studies have suggested that reduced ice cover may reduce the survival of stream dwelling fishes in Northern environments. Th...
As a consequence of a positive phototaxic response, the findings of this study suggest that artificial light often associated with man‐made structures has the potential to alter fish communities within urban estuarine ecosystems by creating optimal conditions for predators. Future coastal developments should consider the ecological implications of...
a b s t r a c t A workshop of over 100 participants, balanced between fisheries management and biodiversity conservation backgrounds, reviewed and synthesised experiences regarding policy and institutional frameworks for use of MPAs in the contexts of fisheries management and conservation of biological diversity. The workshop concluded that althoug...
The movement behaviour of juvenile dusky kob Argyrosomus japonicus (n = 25) was studied in the 12 km long Great Fish Estuary, South Africa, by collecting telemetry data in relation to tidal conditions over 6.5 mo. Two behavioural patterns were observed. Tagged fish either moved in circatidal rhythms over long stretches up and down the estuary, corr...
One hundred and twenty-three years (1885-2007) of catch statistics are available from the whitefish (Coregonus lavaretus) seine-fishery in the River Tufsinga, an inlet to Lake Femund, Norway. We describe the fishery and discuss factors that might infl uence the variation in annual catches. The fishermen developed their own locally adapted seine met...
A workshop of over 100 participants, balanced between fisheries management and biodiversity
conservation backgrounds, reviewed and synthesised experiences regarding policy and institutional
frameworks for use of MPAs in the contexts of fisheries management and conservation of biological
diversity. The workshop concluded that although fisheries mana...
The importance of shallow littoral habitat is a long standing paradigm of estuarine ecology, particularly in facilitating the nursery role they play for many fish. Diel cycles have been linked to the redistribution of fish between habitats and may also influence the movement of various sized fish within littoral zone of estuaries. Fine scale tempor...
The fish populations within the East Kleinemonde Estuary, South Africa, were studied using Dual frequency IDentification SONar (DIDSON) during March 2010 which allowed for direct comparisons of the abundance, distribution and behaviour of a wide size range of fishes along the length of the system. Small schooling fishes <100 mm in total length (L(T...
Biotelemetry is an increasingly popular and effective tool to study fish movements. Interpretation of the results relies on the assumption that fish behaviour and physiology is not affected by the presence of the tag. Although the long-term effects of tagging should be evaluated before field telemetry studies, this has rarely been quantified. As a...
The spatial dynamics and home range sizes of early juvenile white steenbras (Lithognathus lithognathus Cuvier, 1829) in the permanently open Great Fish Estuary, South Africa, were assessed using acoustic telemetry. Seven individuals (154–184 mm fork length) tagged with acoustic transmitters spent the majority of their time in the mesohaline to olig...
The migration pattern of adult spawners of Atlantic salmon (Salmo salar) in two Norwegian streams was analysed by ultrasonic and mechanical tagging to investigate within-river migration. Salmon were tagged in the estuary as they approached the river before spawning. They displayed a systematic and directional upstream pattern of movement in the riv...
By comparing the population frequency distributions for specific somatic energy between samplings using quantilequantile (QQ) plots, we tested for energy-related mortality of juvenile (2- and 3-year-old) Atlantic salmon (Salmo salar) sampled at monthly intervals throughout three consecutive winters in a Norwegian river located at 70°N. Between sev...
The lumpsucker Cyclopterus lumpus is distributed throughout the North Atlantic Ocean and migrates considerable distances between offshore feeding areas and shallow inshore spawning grounds. The number of the lumpsucker has declined since the mid 1980s, probably as a result of overexploitation. The lumpsucker is the preferred host of the sea louse C...
The spatial dynamics and home range sizes of early juvenile white steenbras (Lithognathus lithognathus Cuvier, 1829) in the permanently open Great Fish Estuary, South Africa, were assessed using acoustic telemetry. Seven individuals (154–184 mm fork length) tagged with acoustic transmitters spent the majority of their time in the mesohaline to olig...
More synchronous growth was observed between close, than more distantly separated populations of Atlantic salmon (Salmo salar), during both the first and the second year at sea. The marine growth of seven Norwegian populations, located between 60N and 70N, were correlated with sea surface temperatures (SSTs) in the Barents Sea, the Norwegian Sea, a...
Electronic tracking studies have recently focused on the potential negative effects of catch-and-release (C&R) angling in Atlantic salmon (Salmo salar L.). Common for these studies is that the fish were tagged between C&R, and the effects of C&R can thus not be separated from the extra handling effects associated with the tagging procedure. In addi...
Acoustic telemetry was used to study patterns of habitat use and movements of Pomatomus saltatrix L. (common name elf/shad/bluefish/tailor) within the Saldanha Bay with Langebaan Lagoon coastal embayment on the west coast of South Africa. Thirty six mature P. saltatrix were tagged with acoustic transmitters and released within the lagoon in May 200...
Stable coexistence of Arctic charr and whitefish does occur in a number of native lake fish communities in Scandinavia. Even
so, whitefish introductions into Arctic charr lakes have resulted in serious decline and possibly local extinction of Arctic
charr. In this article, we analyze the habitat use and diet of the two species in five Norwegian lak...
Kelts of Atlantic salmon (spent Salmo salar L.) are often caught and released (C&R) by anglers before and during their seaward migration in the river, but the extent of captures and the biological and management implications have not been previously evaluated. In this study, we investigate the impact of C&R on behavior and mortality of kelts by usi...