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Havørrederne fra vandløbene i Mariager Fjord strejfer så meget, at man ikke kan se forskel genetisk. Nye analyser af den kemiske sammensætning af fiskens øresten afslører, at der er forskelle, som kan vise, hvilke vandløb ørrederne kommer fra.

Salmonids are well known for their natal homing behaviour, meaning they return to breed in the same area where they originated. However, not all individuals return to their natal breeding grounds—a behavioural trait known as straying. The prevalence of straying is difficult to explore and therefore quantitative estimates for straying are seldom reported. In this study, otolith microchemistry and genetics were combined to investigate patterns of straying over ecological and evolutionary time, respectively, between neighbouring rivers flowing into Mariager fjord, Denmark. Otolith microchemistry was used to determine the river of origin for sea trout (Salmo trutta) upon their return to freshwater and 288 SNP markers were used to determine genetic structure among the rivers in the fjord. In this system, where the distance between rivers is short, otolith microchemistry achieved 80% accuracy in assigning juvenile brown trout to their natal river, thus allowing us to determine that approximately 43% of the adult sea trout had returned to non-natal rivers to spawn, with a similar proportion of strayers and natal homers in all of the rivers. Genetic analysis further supported that there was substantial gene flow among individuals originating from different rivers, indicating that sea trout in Mariager fjord make up one population. The findings obtained from otolith microchemistry and genetics complement each other and provide further evidence that sea trout in this system migrate to non-natal rivers and spawn there, which consequently affects the genetic structure of the population.

Salmonids are a group of iconic species with high cultural and socio-economic significance, making them one of the most studied group of fish. Salmonids are out-standing for their incredible life-histories, which involve significant physiological and morphological transformations, and migrations that can span over hundreds or even thousands of kilometres. One of the most amazing aspects of salmonid life-histories is their ability to return and spawn in the same area where they were born - a life-history strategy referred to as natal philopatry or natal homing. However, not all salmonids return to their natal river, meaning there is often a proportion of individuals that return to non-natal rivers - a behavioural trait referred to as straying. While straying has important implications for different population parameters, its prevalence, connection to other life-history characteristics and spatio-temporal patterns characterising it remain largely unexplored. To address some of these unknowns, this thesis aimed to investigate various aspects of straying behaviour among anadromous brown trout, also referred to as sea trout, originating from rivers Villestrup, Kastbjerg, Valsgaard and Maren Møllebæk, of which all flow into Mariager fjord, Denmark. The primary objective of this thesis was to determine the prevalence of straying in Mariager fjord over the course of an individual’s life as well as across evolutionary time. This was achieved by PIT tagging juvenile brown trout in river Villestrup during their out-migration in the spring and following their movements from river Villestrup to all of the studied rivers (MS I, II); or by PIT tagging juvenile brown trout in the summer in all of the rivers (MS III) and following their out-migration in the autumn or spring and tracking their return to any of the rivers. Additionally, as PIT telemetry is not applicable in all systems, suitability of otolith microchemistry in quantifying the level of straying was te

Skarver æder også voksne havørreder.

The increase in abundance of great cormorants (Phalacrocorax carbo sinensis) over the last decades has caused concern for the sustainability of fish populations. Cormorants are opportunistic piscivorous birds that eat fish from a wide range of species and sizes, in marine, lacustrine and riverine habitats. It has been documented that juvenile salmonids are under significant predation pressure by great cormorants, although knowledge on the predation of adult salmonids by cormorants is limited. In this study, adult anadromous brown trout (Salmo trutta) were tagged over multiple years with PIT tags during their spawning migration in a Danish lowland river. Two nearby cormorant colonies were subsequently scanned for PIT tags to estimate predation rate. It was estimated that by minimum 15.4% of tagged adult sea trout were predated by cormorants. The majority of the individuals were predated in the river, and females were more likely to be predated than males. Moreover, length had a significant effect on predation probability: individuals between 35 and 43 cm had the highest likelihood to be predated, while smaller and larger individuals were less likely to be predated. Our findings challenge the assumption that cormorants do not prey on reproductively mature salmonids. Furthermore, predation of mature individuals may have implications for the recruitment and sustainability of the population.

Havørreder vandrer på forskellige tidspunkter i deres livscyklus. Selvom de kan vandre flere hundrede kilometer ude i havet, er de kendte for med høj præcision at vende tilbage til deres oprindelsesvandløb for at gyde. DTU Aquas seneste undersøgelser viser dog et andet billede fra tilløb til Mariager Fjord.

Coexistence of ecotypes, genetically divergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod (Gadus morhua) occur as two such coexisting ecotypes. We applied a combination of acoustic telemetry, genotyping, and stable isotope analysis to 72 individuals to investigate movement ecology and food niche of putative local "Fjord" and putative oceanic "North Sea" ecotypes-thus named based on previous molecular studies. Genotyping and individual origin assignment suggested 41 individuals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype individuals left the study system during the study while 42% of North Sea individuals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among individuals. Fjord ecotype cod had significantly higher δ13C and δ15N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.