Efforts to aid downstream migrating brown trout (Salmo trutta L.) kelts and smolts passing a hydroelectric dam and a spillway

The autumn and spring descent of 41 brown trout kelts (spent trout) (average total length L-t 75.9 cm) and the spring descent of 27 hatchery-reared smolts (average L-t 26.2 cm) were studied by radiotelemetry in 1993/1994 (kelts) and 1997/1998 (kelts and smolts) at the Hunderfossen dam and power plant, south-east Norway. In 1999 we studied spring descent of 48 untagged kelts by visual observations and video-monitoring at one spillway. In autumn 1993 and 1997, 62.4% and 44.0% of the tagged kelts migrated downstream to the dam at water temperatures between 0.1 and 0.8 degrees C. During release of spillwater, the kelts gathered along the dam with limited movements. Neither smolts nor kelts used the 2 m submerged turbine shafts as a pathway to migrate downstream Hunderfossen dam. Nor did release of large amounts of deep water through spillways provide downstream migration possibilities for kelts and possibly for smolts. The majority of both smolts and kelts migrated downstream at short periods of surface water release through the spillways, indicating high importance of surface water release. The threshold value of descend of kelts at surface release was between 1 and 4 m(3) s(-1) which correspond to a water column between 12 and 36 cm. These findings are highly relevant regarding hydroelectric development in river systems containing iteroparous salmonid species.