Impacts of in-stream acclimatization in post-stocking behaviour of European grayling in a Swedish stream

Abstract The post-stocking behaviour of hatchery-reared juvenile European grayling, Thymallus thymallus (L.), with in-stream acclimatization in a pool for 4, 8, and 12 days before release was compared with fish released directly into a 370-m long seminatural stream during 1995 and 1996. The behaviour of these groups was compared with fish acclimatized for 16 h in a holding pen (0.7 m³) in a more fast-flowing part of the stream. All fish were released in the experimental stream at the same time. Initially groups of 30 individuals were compared, but this was reduced because of the mortality during acclimatization. Temporal and spatial distribution, feed intake in relation to sex and size of fish were followed for 7 days after release. Between 33 and 59% of all groups of fish released into the pool left the experimental area and were recaptured in a trap at the downstream end of the experimental stream. The acclimatization period had no significant influence on the percentage of fish moving downstream. Significantly more grayling left the stream from the holding pen (76.7%) than fish released in the pool (47.2%). Of the recaptured fish, 77% had been feeding. Fish recaptured within a week had lost more than 10% of their initial weight measured at the hatchery. Fish recaptured more than 2 weeks after release had lost a significantly smaller percentage of their initial weight compared with fish recaptured in the first 2 weeks. No differences were found in spatial distribution or feeding activity between sexes.