SLIDE 1 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
SLIDE 2 Highest point: 171 m above sea level No natural barriers in the streams (except on Bornholm) More than 90 % of the streams are regulated
Denmark is a lowland
Bornholm
SLIDE 3
Many fish species migrates between streams and the sea High population densities and growth rates of salmonids
SLIDE 4 Mean stream width (m) Stream length (km)
Most Danish streams are small
< 2,5 m’s wide
SLIDE 5 Some river spawning species
Salmon (Salmo salar) River lamprey (Lampetra fluviatilis) Grayling (Thymallus thymallus) North Sea houting (Coregonus oxyrhynchus) Dace (Leuciscus leuciscus ) Sea- trout (Salmo trutta trutta)
SLIDE 6 Salmonids, lampreys and some other fish spawn on riffles
in the gravel of the stream, where also ”clean-water” invertebrates are found.
SLIDE 7 Spent Salmon from River Skjern make very long migrations as far away as Svalbard
Svalbard River Skjern
SLIDE 8
Populations of salmon and trout are increasing
Spawning run of Salmon to River Skjern 1983 - 2015
Number of spawners
SLIDE 9 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 m2
Streams >= 2m
Numbers/100 m
> 130 (high status) 80-130 (good status) 40-79 1-39 > 250 (high status) 150-250 (good status) 100-149 1-99
www.kort.fiskepleje.dk
SLIDE 10 Good or high ecological status in 29 % of the trout streams
according to the Danish ”Trout” Index DFFVø
www.kort.fiskepleje.dk
SLIDE 11
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.
SLIDE 12 THE LIFE CYCLE of salmon and sea trout
Migration Sea residence Downstream migration
Spawning
Hatching Spawning migration
SLIDE 13
Spawning and rearing areas
SLIDE 14 Further, gravel composition and low water is important
Natural riffle with a self-sustaining trout population
Natural slope is the key to succesfull restoration
SLIDE 15
Recommended spawning gravel size
based on natural composition
Diameter 16-32 mm 33-64 mm Small streams (< 3 m) 85 % 15 % Larger streams and rivers (> 3 m) 75 % 25 %
SLIDE 16 Local involvement in restoration
SLIDE 17
71 habitat improving projects
Density of juvenile brown trout almost tripled after restoration
SLIDE 18 Man m ade bypass stream w ith spaw ning areas 1 9 9 1 Now allways high ecological status (1,3 yearlings/ m 2)
High ecological status
SLIDE 19
Remeandering of River Vejle at Vingsted
2013 2015
SLIDE 20
How to avoid losses of migrating fish ?
SLIDE 21
Many problems at culverts
SLIDE 22 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
SLIDE 23 Effects of weirs
- Barriers for fish migrations (up- and downstream direction)
- Destruction of natural spawning and nursery areas in the flooded zone
2008
Flooded zone
SLIDE 24
Heavy losses of up- and downstream migrating fish at weirs
Hydropower Fish farming Water Mill
SLIDE 25 Dam s, w eirs and lakes can be deadly for dow nstream m igrating fish
Type Average % sm olt-loss Mill ponds 30 Traditional fish farms 42 Hydropower stations 82
SLIDE 26 2008
Reestablishm ent of a brow n trout population
The River Gudenå was reestablished In the flooded zone
1,5 km of flooded zone
2008
SLIDE 27 Now allways high ecological status for brown trout
- n the natural spawning areas 1,5 km upstream the weir
High ecological status
SLIDE 28 Also a large increase of the trout population downstream the weir
High ecological status
SLIDE 29
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
SLIDE 30
6 mm grids necessary to stop smolts
SLIDE 31 Migrating fish follow the water
They often fail to find small fish passages
Small fish ladder Small bypass stream
Main flow Main flow
SLIDE 32
Year Smolt loss (%) 2003 35 2004 58
2005 0 River Kongeaa Fish Farm No smolt loss after reconstruction to a modern fish farm (decreased water intake, removal of weir)
SLIDE 33 Barriers for the spawning run of salmon and trout at weirs and small fish ladders
Video: Bo Skelmose and Jørn Chemnitz
SLIDE 34 After removal of the weir
(still water intake to fish farm, 1 mm grid)
Fish farm Removed weir
SLIDE 35
Now the salmon population in River Varde is increasing
SLIDE 36 Improvement of the sea trout population in River Villestrup
Spring: 22 meters over sea level Length: 20 kilometers
Villestrup Gods
SLIDE 37 River V r Villestru rup 1 1999 999:
- 7 fish farms
- Very few seatrout
Malfunctioning fish ladder Fish Farm River
SLIDE 38 Data: DTU Aqua and Thorsten Møller Olesen
Significant increase of smolt numbers
following removal of the 7 weirs at fish farms in 2008-2010
Yearly run of sea trout smolts
Year
SLIDE 39 Stream length: 14 km
- 250 seatrout/km or
- 1 seatrout/4 m
SLIDE 40 Still many man made barriers in Denmark
The waterplans point out a number of problems to be solved
SLIDE 41
Wetlands
Often established at streams with the purpose of removing nutrients from the water
SLIDE 42
River Om m e: I ncreasing population of salm on, trout and grayling The wetland was established as a remeandered river with spawning riffles and periodic ”natural” flooding of the valley River Omme
SLIDE 43
W etland at River Von: I ncreasing populations of salm on and trout Remeandered river with spawning riffles and periodic ”natural” flooding River Von
SLIDE 44 Lakes
Delays, loss of orientation and high mortality
SLIDE 45
Establishm ent of w etlands as lakes directly in rivers can introduce heavy losses of smolts
SLIDE 46 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)
SLIDE 47 Smolt loss is 51-72 % in Årslev Engsø
- etablished as a wetland in 2003
River Aarhus
SLIDE 48 Inlet Outlet
Vejle Å flows south of the dike. (Only 8 % of mean min flow goes through the lake = 230l)
26 ha lake
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
Data from Vejle Kommune and Vejle Amt Limited water intake to the lake Main river flow
SLIDE 49
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.
Conclusions Fauna passage improvement projects for riverine species
