Kramer, Hauke (2018) Population characteristics of a Baltic sea trout spawning cohort from a small North German river: Age composition, spawning experience, growth and temporal genetic diversity. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 107 pp.

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Abstract

The sea trout (Salmo trutta fario trutta) is a popular target species for recreational and professional fisheries along the coasts of the Baltic Sea. Especially fishing tourism creates substantial economic value. Therefore, it is essential to maintain an efficient sea trout management system to ensure sustainable use of the resource sea trout and a high population status in the future. To address that, thorough system- and process-knowledge, combined with appropriate management measures and restoration programs are needed. The affiliated research in Schleswig-Holstein waters is conducted by the SMARRT-Project (“Smolt and Parr Produktion in Theorie und Praxis (SMARRT)“ - Projekt zur Optimierung des Meerforellenmanagements in Schleswig-Holstein; C. Petereit et al., GEOMAR). This Master thesis investigated some aspects within the broader SMARRT research framework, addressing specific research questions related to the ecology and other characteristics of adult sea trout spawning cohorts by using classic fish scale reading and state-of-the-art population genetic methods. The investigations were conducted in the Farver Au, a small (~15km) North German river discharging into the Baltic Sea between Kiel and Fehmarn. The Farver Au was place to the last studies about sea trout spawning cohorts in the early 1970s by Gehlhaar. During electro fishing seasons in 2015 to 2017 a total number of 898 adult sea trout were caught of which 367 were aged by scale reading. The sample selection followed several premises with selecting equal shares of both sexes and a manual selection of the ten biggest and smallest sea trout of each year. The smolt age that were found during the investigations differed from 75,9 % to 89,1 % (mean 83,3 %) of one-year old smolts. The remaining smolts were aged two years while no older smolts were found. The results match those of the most recent smolt trapping study in 2016 and 2017 in the comparable sized river Lipping Au (Rathjen, 2017). Six different sea age classes of returning sea trout varied over the sampling period with the most commonly found sea age being A.1+. This age class also represented most of the first-time spawners with a mean length of 48,8 cm. In total 70,5 % to 88,4 % of all scale-read fish were first-time spawners. Spawning marks were found on 76 scales, which equals a proportion of repeated spawners from 11,6 % to 29,5 % during 2015 to 2017. Growth rates were back-calculated and significant differences were observed when it comes to spawning experience. Post-smolt sea trout were able to grow up to 25 cm in an eight-month period during their first summer at sea. Maiden sea trout grew around 10 to 14 cm in each of the following sea years, while growth rates in spawning fish decreased due to reproduction losses. To validate scale reading results, sea trout were tagged with T-bar Tags during the fishing season 2016. Recaptures in the following season allowed to compare observed with back-calculated growth. Only a limited number of recaptures was available, however, in general a very high coincidence among the results could be shown...

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