Feeding, growth and survival of European grayling in culture and after stocking

The aim of the work described in this thesis has been to evaluate factors of importance for commercial production and stocking of European grayling (Thymallus thymallus (L.)). Studies on rearing technique were performed in a commercial hatchery (Papers I-III), and the post-stocking performance of the reared fish was studied in natural lakes (paper IV) and in a semi-natural stream (papers V-VI). The main results of the studies are as follows: good growth and high survival were obtained during the initial two weeks of feeding by supplying grayling with either natural or artificial dry starter diets. The growth and survival of one-year-old grayling were not affected by varying the stocking density over the range of 5-50 kg (m3)-1. The main factor limiting grayling production was found to be an ulcerative disease caused by the bacteria Aeromonas salmonicida subsp. achromogenes (ASA), which can cause 100% mortality among non-treated grayling under rearing conditions. Both during rearing and after stocking in natural waters, grayling with ASA infection fed less frequently than healthy fish and showed a negative growth rate. Post-stocking recapture and feeding after stocking in a barren lake were shown to be higher for fish reared in a natural pond or in a net pen compared with fish reared in tanks. By contrast, when released in a lake containing predatory perch (Perca fluviatilis (L.)), survival rates were higher among the large fish reared in tanks than among fish reared in the natural pond or net pen, which were smaller. When both brown trout (Salmo trutta (L.)) and grayling were released in a semi-natural stream, significantly more trout than grayling stayed within the release area. Furthermore, the number of stocked trout remaining in the upper part of the relase area was negatively affected by the presence of grayling at the time of stocking. Both species started feeding within a day after release into the stream. The post-stocking dispersal of two lacustrine and one fluvial grayling population released into the semi-natural stream resulted in a large proportion of the stocked fish moving out of the stream into a pond irrespective of their origin. More fish of riverine origin stayed in the upper region of the stream compared with fish of lacustrine origin. Most of the fish that re-entered the rapids of the stream from the pond were of riverine origin.