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Brown trout is a partially migratory salmonid that makes use of diverse habitats to maximise growth and fitness. One of the most substantial threats to brown trout is infection with pathogens from open net‐pen fish farming, which creates hotspots for pathogen reproduction and transmission. Western Norway is a global hotspot for both fish farming and wild salmonids, which generates conflicts due to the impacts of the farms on the behaviour, survival, and fitness of salmonids that overlap with farming activities. In this study, we tagged adult brown trout (>35 cm) at two spatiotemporal intervals that corresponded to two different life history stages: springtime in the river when trout were completing overwintering and summer in the fjord when trout were in their marine feeding phase. The tagging revealed three different behaviours, fish that remained in freshwater, fish that migrated between freshwater and the fjord, and fish that remained in the estuary. Although some trout moved >100 km to the outer fjord areas, most trout remained relatively close to the river. Depth sensor transmitters in a subset of trout also revealed that the trout remained in the upper water column. Most of the horizontal and vertical movements therefore resulted in spatial overlap with fish farming for the migratory trout, but not for resident trout that remained in the estuary or in freshwater. Findings reveal the challenges of managing a fish with such behavioural plasticity but the urgency of recognising how important inner fjord habitats are for migratory brown trout. [ABSTRACT FROM AUTHOR]

Despite the preponderance of exorheic lakes in rivers home to anadromous salmonids, little research has focused on how salmon, trout, and char use lakes as part of their anadromous life histories. The literature on this subject has so far revealed that some parr move into lakes to feed and grow before smoltification but that smolts moving through lakes tend to have high mortality in disproportion to what is observed in other habitats they migrate in or through. Adults have been observed using lakes for behavioural thermoregulation prior to spawning, and kelts of iteroparous species often exploit lakes to overwinter before returning to sea to recondition. We summarized knowledge on lakes as salmonid habitat and identified knowledge gaps about the use of lakes by anadromous salmonids related to whether lakes are barriers that structure genetics of populations, whether mortality in lakes is compensatory or additive, and whether systems with lakes have higher rates of repeat spawning among iteroparous salmonids. Human activities that alter lakes require further study to understand how changes in temperature, oxygen, ice, or circulation affect navigation and fate. Malgre une preponderance de lacs exoreiques le long de rivieres ou salmonides anadromes sont resident, peu de recherche se sont penches sur l'utilisation faite des lacs par les saumons, truites et ombles durant leurs cycles biologiques anadromes. La documentation a ce sujet a, a ce jour, revele que certains tacons entrent dans des lacs pour se nourrir et croitre avant la smoltification, mais les saumoneaux traversant des lacs tendent a presenter une forte mortalite comparativement a ce qui est observe dans d'autres habitats par ou dans lesquels ils migrent. L'utilisation de lacs par les adultes a des fins de thermoregulation avant le frai a ete observee, et les saumons vides d'especes iteropares exploitent souvent les lacs pour hiverner avant de retourner en mer pour se reconditionner. Nous presentons un resume des connaissances sur les lacs comme habitats de salmonides et cernons des lacunes dans les connaissances sur l'utilisation des lacs par les salmonides anadromes, concernant un role eventuel des lacs comme barrieres qui structurent la genetique de populations, le caractere compensatoire ou additif de la mortalite dans les lacs et une eventuelle frequence accrue de frais repetes de salmonides iteropares dans les reseaux comptant des lacs. Les activites humaines qui modifient les lacs doivent faire l'objet de plus d'etudes afin de comprendre l'influence des modifications de la temperature, des concentrations d'oxygene, de la glace ou de la circulation sur la navigation et le destin des poissons. [Traduit par la Redaction], Introduction Salmonids (Subfamily Salmoninae) are distributed throughout the Holarctic in rivers, creeks, streams, ponds, and lakes. Species in this family are exceptional colonizers and have great capacity to move into [...]

Smolt migration through lakes is hazardous, as the predation pressure can be extreme and the hydrology a great contrast to that of a riverine area. However, the mechanisms yielding these challenges have been scarcely investigated. We conducted an acoustic telemetry field study in Lake Evangervatnet, Voss, Norway, utilising Vemco V5 predation tags. Atlantic salmon (Salmo salar) smolts (N = 20) were tagged with the novel predation sensor tag to investigate mortality, the lacustrine migration behaviour of smolts, and the applicability of these tags for smolt studies. A total of 60% of tagged Atlantic salmon (Salmo salar) smolts perished in the lake. Half of the mortalities (30% of tagged fish) were directly attributed to predation by brown trout (Salmo trutta) based on predation sensors. The surviving smolts were slow to traverse the 6.5 km lake, with progression rate between lake inlet and outlet on average 0.016 m/s over a mean of 7.9 ± 6.2 (SD) days. Acoustic detections revealed a consistent pattern of nocturnal migration and multidirectional movements within the lake. By running a series of correlated random walks under varying parameters and comparing the simulated travel times to the observed travel time used by the tagged smolts, we emulated the observed behaviour of the smolts. These simulations suggested that smolts lacked the ability to efficiently navigate through the lake, instead swimming in random directions until they reached the lake outlet. Predation sensors can offer improved resolution when tracking the behaviour and fate of smolts and can facilitate better mitigation efforts by identifying survival bottlenecks and separating predation from non‐predatory mortality. [ABSTRACT FROM AUTHOR]