Behavior and recruitment biology of lake trout with special emphasis on the effect of cormorant predation on smolt survival (38271)

The scope of this study is to investigate the movement behavior in brown trout, Salmo trutta. Movements between Lake Hald and its two major tributaries and outlet are monitored by the use of passive telemetry. 
Juvenile and adult trout, caught in the tributaries, have been tagged with passive integrated transponder (PIT) tags and subsequently their passages at automated listening stations have been registered. The trout population is per definition landlocked, as barriers allow only for out-migration and prohibit the return of anadromous individuals. The movements in this semi-closed system allowed surveillance of general migration patterns and identification of within-population variations in life history strategies (stream resident, lake resident and migratory).
A number of trout have been caught, sampled and released and use in retrospective evaluation of physiology. Measured variables from blood and gill samples were used to identify physiological differences that had discriminatory power between the three identified life history strategies. Additionally, the movements of lake resident spawners will also be looked upon in this study.

The trout population has been in decline for the last decade. This coincides with the establishment and growth of a cormorant colony on the lake shore. Furthermore, a heron colony close by has likewise grown in this period. Hence, trout which reside in tributaries and lake are subjected to predation from nearby cormorant and gray heron. The accumulation of PIT tags has been monitored with high temporal resolution, revealing the periods of peak predation pressure and the overall annual minimum predation. This, combined with PIT records, will reveal the habitat a trout has been predated in and hereby expose temporal vulnerabilities of lake and tributaries.

The project is coordinated by DTU Aqua.
Research area: Freshwater Fisheries and Ecology