title: The physiological basis of the migration continuum in brown trout (Salmo trutta).
abstract: Partial migration is common in many animal taxa; however, the physiological variation underpinning migration strategies remains poorly understood. Among salmonid fishes, brown trout (Salmo trutta) is one of the species that exhibits the most complex variation in sympatric migration strategies, expressed as a migration continuum, ranging from residency to anadromy. In looking at brown trout, our objective with this study was to test the hypothesis that variation in migration strategies is underpinned by physiological variation. Prior to migration, physiological samples were taken from fish in the stream and then released at the capture site. Using telemetry, we subsequently classified fish as resident, short-distance migrants (potamodromous), or long-distance migrants (potentially anadromous). Our results revealed that fish belonging to the resident strategy differed from those exhibiting any of the two migratory strategies. Gill Na,K-ATPase activity, condition factor, and indicators of nutritional status suggested that trout from the two migratory strategies were smoltified and energetically depleted before leaving the stream, compared to those in the resident strategy. The trout belonging to the two migratory strategies were generally similar; however, lower triacylglycerides levels in the short-distance migrants indicated that they were more lipid depleted prior to migration compared with the long-distance migrants. In the context of migration cost, we suggest that additional lipid depletion makes migrants more inclined to terminate migration at the first given feeding opportunity, whereas individuals that are less lipid depleted will migrate farther. Collectively, our data suggest that the energetic state of individual fish provides a possible mechanism underpinning the migration continuum in brown trout.
authors.affiliation: Technical University of Denmark, Section for Freshwater Fisheries Ecology, Vejlsøvej 39, 8600 Silkeborg, Denmark; 2Aarhus University, Department of Animal Science, Integrative Physiology, Blichers Allé 20, 8830 Tjele, Denmark; 3University of Southern Denmark, Institute of Biology, Campusvej 55, 5230 Odense M, Denmark; 4Aarhus University, Department of Bioscience, Zoophysiologi, C.F. Møllers Allé 3, Building 1131, Room 224, 8000 Aarhus, Denmark; 5Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal.
authors.forename: Mikkel
authors.initials: M
authors.lastname: Boel
authors.forename: Kim
authors.initials: K
authors.lastname: Aarestrup
authors.forename: Henrik
authors.initials: H
authors.lastname: Baktoft
authors.forename: Torben
authors.initials: T
authors.lastname: Larsen
authors.forename: Steffen
authors.initials: S
authors.lastname: Søndergaard Madsen
authors.forename: Hans
authors.initials: H
authors.lastname: Malte
authors.forename: Christian
authors.initials: C
authors.lastname: Skov
authors.forename: Jon C
authors.initials: JC
authors.lastname: Svendsen
authors.forename: Anders
authors.initials: A
authors.lastname: Koed
mesh_terms: D025041:Animal Migration / Q000502:physiology*; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001823:Body Composition; D003718:Denmark; D004734:Energy Metabolism; D009752:Nutritional Status; D045483:Rivers; D014337:Trout / Q000502:physiology*