Otolith Sr:Ca profiles demonstrated that juvenile anadromous brown trout (ABT) Salmo trutta may descend to the brackish waters of the Baltic Sea from their natal streams as parr or fry and then migrate into non-natal streams before transforming into smolt. To our knowledge, no such published documentation exists for ABT. The ecological significance of this life-history strategy is presently not clear. However, stream shifting through the marine environment should to be considered with regard to stock management and the assessment and restoration of salmonid populations and their potential habitats.
K E Y W O R D Slife history, otolith microchemistry, parr migration, salmonids
Brown trout Salmo trutta L. parr were sampled from 21 Estonian and three Finnish streams to investigate whether national sea trout parr surveys sample the progeny of anadromous or resident maternal parents. Otolith Sr:Ca core values were used and validated as a tool for distinguishing between the progeny of the two forms. In Estonia (n = 283), 92% of the parr were the progeny of anadromous maternal parents, and 8% were the progeny of resident maternal parents, whereas in Finland (n = 24), the respective proportions were 79% and 21%. Variation in the maximum otolith Sr:Ca core values among progeny of anadromous maternal parents indicated that some adult females may enter fresh waters several months before spawning. It was concluded that easily accessible locations situated up to 30 km from the sea largely contain progeny of sea trout, whereas sites with poor connectivity with the sea can be dominated by progeny of resident trout. This study demonstrated that the method applied provided an effective means to distinguish between the progeny of sea trout and resident brown trout.
Mixed‐stock fishery of sea trout (Salmo trutta L.) in the Baltic Sea targets wild and stocked individuals, which could lead to over‐exploitation of wild stocks. Studying the contribution of different spawning streams and hatcheries to the fishery would aid in more effective management and conservation of S. trutta. The aim of the present study was to explore whether and to what extent parr from different S. trutta spawning streams and hatcheries can be differentiated, using water chemistry and otolith microchemistry. Water chemistry samples demonstrated significant spatial variation between different study regions (Estonia and Finland); Sr, Mg, Fe, Zn, Ca and Cu contributed most to the separation. Parr collected from different sites showed a significant difference in otolith element:Ca and 87Sr:86Sr ratios on a smaller scale, including only Estonian samples (n = 361), and on a larger scale incorporating parr from Estonia, Latvia and Finland (n = 564). The reclassification rate of S. trutta parr was on average 73%, being highest in Finnish streams and hatcheries, followed by Latvia and Estonia. Otolith 87Sr:86Sr and Sr:Ca ratios were the most important in discriminating fish from different sampling sites. The present study demonstrates the potential of separating S. trutta on various spatial scales including different countries and neighbouring streams, making this the most extensive otolith microchemistry study to date in terms of streams and regions studied. The established reclassification rules form a basis for future work investigating the natal origin of adult S. trutta caught in the mixed‐stock fishery in the sea.
For the first time, an overlooked aspect of partial migration was quantified using otolith microchemistry and brown trout, Salmo trutta, as a model species. Relative contributions of freshwater resident and anadromous female brown trout to mixed-stock sea trout populations in the Baltic Sea were estimated. Out of 236 confirmed wild sea trout sampled around the coast of Estonia 88% were of anadromous maternal origin and 12% were of resident maternal origin. This novel finding underscores the importance of the resident contingent in maintaining the persistence and resilience of the migratory contingent.
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