Documented impacts of barriers and wetlands on salmonids and how to solve problems Jan Nielsen Technical University of Denmark Section for Freshwater Fisheries and Ecology
Denmark is a lowland Bornholm Highest point: 171 m above sea level No natural barriers in the streams (except on Bornholm) More than 90 % of the streams are regulated
Many fish species migrates between streams and the sea High population densities and growth rates of salmonids
Most Danish streams are small < 2,5 m s wide Stream length (km) Mean stream width (m)
Some river spawning species North Sea houting (Coregonus oxyrhynchus) Sea- trout (Salmo trutta trutta) River lamprey (Lampetra fluviatilis) Dace (Leuciscus leuciscus ) Grayling (Thymallus thymallus) Salmon (Salmo salar)
Salmonids, lampreys and some other fish spawn on riffles in the gravel of the stream, where also clean-water invertebrates are found.
Spent Salmon from River Skjern make very long migrations as far away as Svalbard Svalbard River Skjern
Number of spawners Populations of salmon and trout are increasing Spawning run of Salmon to River Skjern 1983-2015
Brown trout yearlings from spawning Colours indicate the ecological status according to the Waterframe Directive and the Danish Trout Index DFFVø Brooks < 2m Numbers/100 m 2 Streams >= 2m Numbers/100 m > 130 (high status) 80-130 (good status) 40-79 1-39 0 > 250 (high status) 150-250 (good status) 100-149 1-99 0 www.kort.fiskepleje.dk
Good or high ecological status in 29 % of the trout streams according to the Danish Trout Index DFFVø www.kort.fiskepleje.dk
Anglers harvest a big seatrout for every 4-6 meters of trout streams with healthy populations - each year. 85 % of the seatrout are caught in salt water.
THE LIFE CYCLE of salmon and sea trout - avoid «bottlenecks» Spawning migration Migration Sea residence Spawning Hatching Downstream migration
Spawning and rearing areas
Natural slope is the key to succesfull restoration Further, gravel composition and low water is important Natural riffle with a self-sustaining trout population
Recommended spawning gravel size based on natural composition Diameter 16-32 mm 33-64 mm Small streams (< 3 m) Larger streams and rivers (> 3 m) 85 % 15 % 75 % 25 %
Local involvement in restoration of spawning areas
71 habitat improving projects Density of juvenile brown trout almost tripled after restoration
Man made bypass stream with spawning areas 1991 Now allways high ecological status (1,3 yearlings/m2) High ecological status
Remeandering of River Vejle at Vingsted 2013 2015
How to avoid losses of migrating fish?
Many problems at culverts
If removal of the culvert is impossible Water level in the culvert and improvement of upstream passage may be raised by establishment of spawning areas Culvert Water level raised 15 cm Spawning area
Effects of weirs Barriers for fish migrations (up- and downstream direction) Destruction of natural spawning and nursery areas in the flooded zone Flooded zone 2008
Heavy losses of up- and downstream migrating fish at weirs Water Mill Fish farming Hydropower
Dams, weirs and lakes can be deadly for downstream migrating fish Type Average % smolt-loss Mill ponds 30 Traditional fish farms 42 Hydropower stations 82
Reestablishment of a brown trout population - the natural solution 2008 2008 1,5 km of flooded zone The River Gudenå was reestablished In the flooded zone
Now allways high ecological status for brown trout on the natural spawning areas 1,5 km upstream the weir High ecological status
Also a large increase of the trout population downstream the weir High ecological status
Smolt (Oxford Dic): A young salmon (or trout) after the parr stage, when it becomes silvery and migrates to the sea for the first time
6 mm grids necessary to stop smolts
Migrating fish follow the water They often fail to find small fish passages Main flow Main flow Small fish ladder Small bypass stream
River Kongeaa Fish Farm No smolt loss after reconstruction to a modern fish farm (decreased water intake, removal of weir) Year Smolt loss (%) 2003 35 2004 58 2005 0
Barriers for the spawning run of salmon and trout at weirs and small fish ladders Video: Bo Skelmose and Jørn Chemnitz
After removal of the weir (still water intake to fish farm, 1 mm grid) Removed weir Fish farm
Now the salmon population in River Varde is increasing
Improvement of the sea trout population in River Villestrup Spring: 22 meters over sea level Length: 20 kilometers Villestrup Gods
River Villestrup 1999: 7 fish farms Very few seatrout Fish Farm River Malfunctioning fish ladder
Significant increase of smolt numbers following removal of the 7 weirs at fish farms in 2008-2010 Yearly run of sea trout smolts Year Data: DTU Aqua and Thorsten Møller Olesen
Stream length: 14 km 250 seatrout/km or 1 seatrout/4 m
Still many man made barriers in Denmark The waterplans point out a number of problems to be solved
Wetlands Often established at streams with the purpose of removing nutrients from the water
River Omme: Increasing population of salmon, trout and grayling The wetland was established as a remeandered river with spawning riffles and periodic natural flooding of the valley River Omme
Wetland at River Von: Increasing populations of salmon and trout Remeandered river with spawning riffles and periodic natural flooding River Von
Lakes Delays, loss of orientation and high mortality of migrating smolts
Establishment of wetlands as lakes directly in rivers can introduce heavy losses of smolts
Egå Engsø established as a wetland in October 2006 Spring 2005 and 2006 No loss of smolts Now 74 % smolt loss (8 years of monitoring since 2007) Data from Schwinn (2016)
Smolt loss is 51-72 % in Årslev Engsø - etablished as a wetland in 2003 River Aarhus
Smolts can survive if a lake is separated from the river with limited water intake to the lake River Vejle wetland at Skibet (established 2004) Only 19 smolts (0,1 %) moved into the lake in 2005 Main river flow Limited water intake to the lake 26 ha lake Inlet Outlet Vejle Å flows south of the dike. (Only 8 % of mean min flow goes through the lake = 230l) Data from Vejle Kommune and Vejle Amt
Conclusions Fauna passage improvement projects for riverine species The best projects involves natural solutions: When the dam, weir or road culvert are completely removed and the watercourse restored to the original gradient and flow without diversion of water from the river. Heavy losses of migrating riverine species can be expected if the fish have to pass lakes.