Hooking mortality of small brown trout and grayling in Finnish rivers catch and release fisheries

Experiments to evaluate post-capture hooking mortality caused by widely used flies and other angling-gear types for brown trout (Salmo trutta L.) and grayling (Thymallus thymallus (L.)) in catch and release fishing were conducted in four typical Finnish rivers. Most of the fish caught were smaller than the legal minimum size. The average total body length (± 95% confidence limit) of brown trout (n = 380) was 19.3 ± 0.4 cm and 30.4 ± 1.3 cm, respectively, in the two main research areas. The average length of grayling was 26.6 ± 1.1 cm (n = 60). Mortality was estimated by returning released fish into wire-cages that were held in the river. The duration of caging was usually 3 days. Only three (i.e. 1%) brown trout died. No mortality occurred among grayling. The most visible injury in dying brown trout was bleeding in the gills caused by hooking. Because hooking-mortality for these species was almost non-existent, no relation could be established between mortality and fish size, gear type, and water temperature. Potential factors for the low mortality rate in our study are the relative short playing times and careful handling of released fish. The low hooking mortality of brown trout and grayling supports the concept of catch and release fishing.