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.

Pre‐stocking acclimatisation of brown trout Salmo trutta: effects on growth and capture in a fast‐flowing river

“…Cresswell & Williams (1983) stated that high water velocities resulted in increased downstream migration of acclimated hatchery-reared fish compared with medium water velocities. This observation was confirmed by Thorfve (2002) for in-stream acclimated hatcheryreared European grayling. Fish that were acclimated in fast-flowing parts of the river were more likely to disappear from the study area than those acclimated in slow-flowing pools (Thorfve 2002).…”

“…This observation was confirmed by Thorfve (2002) for in-stream acclimated hatcheryreared European grayling. Fish that were acclimated in fast-flowing parts of the river were more likely to disappear from the study area than those acclimated in slow-flowing pools (Thorfve 2002). As discussed above, flood events and passive drift may also account for the different capture rates observed in the 2 years.…”

Recapture and condition of pond-reared, and hatchery-reared 1 + European grayling stocked in addition to wild conspecifics in a small river

“…However, this cannot be generalized; observations published by Weiss and Schmutz (1999) indicated that abiotic factors in heterogeneous biotopes substantially affected growth of stocked fish. The possible short-term weight loss reported by Thorfve (2002) in stocked grayling was not observed during the 5 months of the present experiment. Although the condition factor of reared grayling was initially lower, it was equal to that of wild fish at recapture, suggesting that those artificially reared fish that remained in the experimental sections adapted to local conditions.…”

Post‐release growth and dispersal of pond and hatchery‐reared European grayling Thymallus thymallus compared with their wild conspecifics in a small stream

“…The period between release and recapture might also be significant. Thorfve (2002) found a decrease of > 10% from pre‐release mass in stocked T. thymallus after 1 week in the watercourse. Weiss & Schmutz (1999) showed that stocked hatchery‐reared S. trutta had lost 5–25% of their body mass 3 months after release in the stream.…”