Session A3: Ljubljanica Connects - Enabling Fish Migration

Abstract: The heavily degraded area of the Ljubljanica River corridor upstream and downstream of the Ljubljana urban area is an important habitat for the fragmented and heavily endangered populations of Danube Salmon (Hucho hucho), Danube Roach (Rutilus pigus) and Striped Chub (Leuciscus souffia). Nowadays the water level upstream of the weir on the Ljubljanica River is too low therefore during low flow conditions the main Ljubljanica River channel is not connected to its tributaries. This represents a great obstacle for the habitat connectivity along the river reaches which is worsened by the improperly working fish passes. To enable fish migration along the river concrete restoration measures were performed. The reconstructions of sill and two fish passes on the Ljubljanica River have been implemented and barrier’s lifting system on the weir was modernized. During the entire project fish monitoring is being performed in different ways. It helps us to evaluate if concrete restoration actions (restoration of sill, two fish passes and a barrier) were successful and if fish migration is improved. We follow fish migration with marking the fish with Visible Implant Elastomer (VIE) tags. Regarding the location of catch we implant tags beneath transparent or translucent tissue combining different tag locations (dorsal fin, post ocular tissue) and colours (red and yellow). The success of restoration actions is monitored with the use of on line connected cameras installed on the fish passes. Records from the camera are analysed to evaluate number and type of fish using the fish pass. The data gathered from the fish monitoring will contribute to a better understanding of fish migration along the Ljubljanica and Sava River and will help to evaluate the project success