How pathogens affect the marine habitat use and migration of sea trout (Salmo trutta) in two Norwegian fjord systems

Wild fish are confronting changing pathogen dynamics arising from anthropogenic disturbance and climate change. Pathogens can influence animal behaviour and life histories, yet there are little such data from fish in the high north where pathogen dynamics may differ from comparatively southern regions. We aimed to compare the pathogen communities of 160 wild anadromous brown trout in two fjords in northern Norway and to determine whether pathogens influenced area use or return to spawn. Application of high‐throughput qPCR detected 11 of the 46 pathogens screened for; most frequently encountered were Ichthyobodo spp., Flavobacterium psychrophilum and Candidatus Branchiomonas cysticola. The rate of returning to freshwater during the spawning season was significantly lower for the Skjerstadfjord fish. Piscichlamydia salmonis and F. psychrophilum were indicator species for the Skjerstadfjord and pathogen communities in the two fjords differed according to perMANOVA. Individual length, Fulton's condition factor and the time between first and last detection of the fish were not related to the presence of pathogens ordinated using non‐metric multidimensional scaling (NMDS). However, there was evidence that pathogen load was correlated with the expression of smoltification genes, which are upregulated by salmonids in freshwater. Correspondingly, percentage of time in freshwater after release was longer for fish with greater pathogen burdens.acceptedVersio

A review of iteroparity in anadromous salmonids: biology, threats and implications

Iteroparity occurs when organisms reproduce more than once, and is seen as a bet-hedging reproductive strategy. Despite a wealth of research on iteroparous Atlantic salmon, steelhead, brown trout, and Arctic charr, the determinants of reproductive investment, the intra- and interspecific differences in the degree of iteroparity, the drivers of repeat spawning, and the contribution of repeat spawners to populations and sustainability remain unclear. In particular, the knowledge base is stronger for Atlantic salmon and brown trout, but relatively weak for Arctic charr and steelhead. While juveniles, maiden spawners and repeat spawners are facing similar challenges, many threats specific to the kelt stage are emerging (e.g., downstream migration passed barriers after spawning). Recent work has quantified the benefits of iteroparity for population resilience, and the potential for iteroparity to increase when anthropogenic stressors are mitigated. This is the first literature review paper synthetizing the growing knowledge base that exists on various aspects of the ecology and biology of repeat spawners in freshwater and at sea, the threats they face, the proximate and ultimate mechanisms underlying iteroparity in salmonids, the importance of iteroparity for population-level processes, as well as highlighting pressing areas of research. Collectively, this work offers a valuable resource to fisheries scientists and managers by shedding light on an important life-history stage that warrants more attention to mitigate these threats and restore healthy wild salmonid populations

How pathogens affect the marine habitat use and migration of sea trout (Salmo trutta) in two Norwegian fjord systems

Wild fish are confronting changing pathogen dynamics arising from anthropogenic disturbance and climate change. Pathogens can influence animal behaviour and life histories, yet there are little such data from fish in the high north where pathogen dynamics may differ from comparatively southern regions. We aimed to compare the pathogen communities of 160 wild anadromous brown trout in two fjords in northern Norway and to determine whether pathogens influenced area use or return to spawn. Application of high‐throughput qPCR detected 11 of the 46 pathogens screened for; most frequently encountered were Ichthyobodo spp., Flavobacterium psychrophilum and Candidatus Branchiomonas cysticola. The rate of returning to freshwater during the spawning season was significantly lower for the Skjerstadfjord fish. Piscichlamydia salmonis and F. psychrophilum were indicator species for the Skjerstadfjord and pathogen communities in the two fjords differed according to perMANOVA. Individual length, Fulton's condition factor and the time between first and last detection of the fish were not related to the presence of pathogens ordinated using non‐metric multidimensional scaling (NMDS). However, there was evidence that pathogen load was correlated with the expression of smoltification genes, which are upregulated by salmonids in freshwater. Correspondingly, percentage of time in freshwater after release was longer for fish with greater pathogen burdens

Marine depth use of sea trout Salmo trutta in fjord areas of Central Norway

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–February by acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day v. night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May–September was 1·7 m (individual means ranged from 0·4 to 6·4 m). Hence, S. trutta were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May–September was deeper in the near-shore habitats with or without steep cliffs (2·0 m and 2·5 m, respectively) than in the pelagic areas (1·2 m). May–September mean swimming depth in all habitats was slightly deeper during day (1·9 m) than at night (1·2 m), confirming that S. trutta conducted small-scale diel vertical movements. During summer, S. trutta residing in near-shore habitat progressively moved deeper over the period May (mean 1·1 m) to August (mean 4·0 m) and then reoccupied shallower areas (mean 2·3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean 1·3 m). The swimming depths of S. trutta coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of S. trutta, this study illustrates how S. trutta utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of S. trutta and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes

Marine migration and habitat use of anadromous brown trout Salmo trutta

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. Overall, fish spent 68% of their marine residence time close to river mouths (< 4 km). Most fish registrations (75%) were in near shore habitats, but pelagic areas were also used. The maximum migration distance of tagged fish was categorized as short (< 4 km from river mouth, 40% of fish), medium (4-~13 km, 18% of fish) or long (> ~13 km, 42% of fish). Long distance migrants had poorer body condition in spring prior to migration, used pelagic areas more often and returned earlier to freshwater than short and medium distance migrants. Marine residence time was 7-183 days, and was positively correlated to body length and smolt age, but negatively correlated to the date of sea entry