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Strontium to calcium (Sr:Ca) and barium to calcium (Ba:Ca) ratios were quantified in 74 otoliths of brackish water-living burbot Lota lota, collected from two regions in the eastern Baltic Sea. Considerable amount of life history plasticity was observed. Ninety-six per cent of analysed burbot were of freshwater origin; only three specimens displayed signs of hatching in low-salinity (1-2) estuarine environment. Most of the juveniles emigrated from freshwater during late winter and spring the next year after birth, while nine individuals did so at the age of 2-3 months. Age and length at maturity (as determined by first freshwater spawning migration) varied from 2-6 years and 189-540 mm, respectively, with most of burbot maturing during the second or third year of life. It is hypothesised that the bulk juvenile downstream migration is triggered by high-flow conditions as evidenced by Ba: Ca peak around the point of freshwater exit. This study provides new information on burbot life history.

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

In the eastern Baltic rivers, anadromous salmonid parr are known to smoltify and migrate to the sea from March until June, depending on latitude, climate and hydrological conditions. In this study, we present the first records of autumn descent of brown trout Salmo trutta and Atlantic salmon Salmo salar from the Baltic Sea Basin. Otolith microchemistry analyses revealed that these individuals hatched in freshwater and had migrated to the brackish water shortly prior to capture. The fish were collected in 2006, 2008, 2009 and 2013 from Eru Bay (surface salinity 4.5-6.5 %), Gulf of Finland. This relatively wide temporal range of observations indicates that the autumn descent of anadromous salmonids is not a random event. These results imply that autumn descent needs more consideration in the context of the effective stock management, assessment and restoration of Baltic salmonid populations and their habitats.

Otolith Sr:Ca and Ba:Ca profiles were used in parallel with age data to investigate the life‐history characteristics of ide, Leuciscus idus (L.) (n = 111), in the Väinameri Sea, West Estonia. Sr:Ca profiles were more variable and useful than Ba:Ca profiles. Flexible life‐history patterns were observed within and among the three study sites. Most of the individuals (72%) hatched in semi‐enclosed bays that are fresh water during spring spawning but are often flooded with brackish water during other seasons. The importance of lotic spawning varied among sites and was the highest (88%) in Matsalu Bay, moderate in Saunja Bay (33%) and lowest (0%) in Käina Bay. Young of the year emigrated from natal sites and entered the sea within the first summer; 95% did so during the first month post‐hatch. Juvenile ide undertook non‐spawning, freshwater migrations in the following spring; however, the reasons behind this phenomenon remain unknown. As the importance of lotic spawning has significantly decreased and multiple historically important ide spawning rivers lack anadromous runs altogether, it is suggested that actions should be taken to aid the recovery of those imperilled spawning stocks.

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.

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