Rathjen, Jan-Philipp (2017) Age composition and genetical affiliation of a sea trout (Salmo trutta f. trutta) smolt population in a stream in Schleswig-Holstein. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 64 pp.

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Abstract

Juvenile sea trout (Salmo trutta f. trutta) spend their first years of life in freshwater habitats of little streams and rivers. Here they go through diverse developmental stages before they finally migrate to marine waters. The developmental stage during the first seaward migration is called “smolt”. In this study, a smolt population was trapped during a spring migration in 2016. The smolt trap was erected in the Lipping Au, a little freshwater stream in northern Germany in Schleswig-Holstein discharging into Flensburg Fjord (Baltic Sea), in which natural sea trout reproduction occurs. Three kilometres before the estuary after the union of all tributary streams, the trap captured 2167 smolts during spring seaward migration in a period of two month. Among others scale samples for age determination and tissue samples (fin clip) for parent-offspring assignment tests were taken. In the Baltic Sea area of Schleswig-Holstein, there is a lack of information concerning age structures of smolt populations and the only study dates back decades. There is also little information about the success of stocking programs of early sea trout life stages that are part of the sea trout management in Schleswig-Holstein for more than 30 years. On the one hand, this study determined the age structure of a subsample (n=834) of the spring smolt migration via scale reading. On the other hand, a subsample of 951 smolts was genotyped in the laboratory. The DNA of smolts and adult fish used for stocking programs in 2013 and 2014 (n=255) was extracted from fin clips. A PCR amplified 12 established microsatellite primers before capillary electrophoresis and analysing the output with the programs GeneMarker v1.91 and Colony v2.0.6.1. to identify parent-offspring matches and according to this a stocking background. Scale reading showed that 85.85 % of the down migrating smolts were aged 1+. Remaining 118 smolts aged 2+ and no smolts older than this were found in the subsample. Main migration activity happened during three periods with rising water level. Inside these events older smolts migrated significantly earlier than younger smolts. Microsatellite analyses assigned 22 smolts with a stocking background (parent pair assignment to male and female of the parental stocking pool). That is a proportion of 2.31% of the subsample and a survival rate of 0.02 % of 120,000 stocked fry. An effective population size of 3040 individuals was estimated for the study system Lipping Au combining the two years. This study provides a valuable pilot study for estimating the age structure of a smolt population and the survival success from stocked fry to migrating smolts in a little stream in Schleswig-Holstein. However, a replication of this study is necessary to compare the results among different years with different environmental conditions.

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