Critical problems for migratory fish in the North Sea region from a Danish perspective By Kim Aarestrup, Anders Koed & Niels Jepsen Technical University of Denmark
Overview: Management setup in Denmark A fundamental aspect Overall problems Weirs How to obtain information on moving animals (you can’t see) Examples of studies and consequences of result Does recommendations work?
“The Management Model for Diadromous Fish populations in Denmark” Goal: Self-sustaining populations Management system - Habitat restoration - Population restoration Research and Monitoring Legislation - Ministry of Fisheries - Ministry of the Environment Stakeholders - Anglers - Fishermen
Gross 1987
Migrating Sea residence Downstream migration Spawning Hatching Spawning migration Lifecycle – a fundamental aspect
Causes of the decline Sewage and waste-water caused habitat loss and habitat degradation. Dredging and canalization caused habitat loss and habitat degradation. Over fishing
Establishment of Mill ponds (1000 – 1850), Hydropower development (1920 – 1970) and Fish farming (1894 – 1975). Caused habitat loss, habitat degradation and impaired migration.
A Significant problem
Otterstrøm (1938) and Statistic yearbooks, the Ministery of Fisheries Number of salmon caught in Ringkøbing Fjord
Number of rivers and streams with trout populations in Denmark - retrospective
Investigating the migration of fish – methods? TELEMETRY Limitations: Low numbers Tagging and handling effects Animal welfare/ethics Cost and labour intensive Advantages: Only way to get this type of information Clear results Easy to understand Easy to explain
Release 1 Release 2 Weir Trap Loss calculated as the relative difference in recapture. Delay Calculated as the difference in 50 % fractile Loss can be related to e.g.. Flow released for fish passage
Smolt loss at barriers Aarestrup et al. In prep
TANGE RESERVIOR
N=8 N=10 N=4
Loss is cumulative! Number of weirs to pass Survival in percent
River Villestrup Å, Total 22 m, app. 9 m i 6 weirs (app. 40 %) Weir A Water surface Weir B Distance from source Height above sea-level Stream bed Another serious aspect
..migration barriers have been removed and habitats restored
Successful restoration of salmon populations is based on knowledge obtained from research and monitoring Salmon spawning-run - River Skjern
Population development sea trout – River Villestrup