docIFM Eel workshop 2015_Johan Coeck
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IFM Eel workshop 2015_Johan Coeck
Optimisation of glass eel migration at tidal barriers in Belgium Johan Coeck, Ans Mouton, David Buysse, Jeroen Van Wichelen 1914-1918 German barriers 2014-2018 Tidal barriers Problem definition Study area: Mouth of River IJzer Problem definition Problem definition Boundary conditions • Historical monument • Flood protection • High runoff • Saltwater inflow to be limited Objective Identify innovative solutions to restore glass eel migration © Nico De Regge Objective Innovative solution: adjusted barrier management Sea River Sea River Sea River Sea River As is To be Low tide High tide Objective • Adjusted barrier management: key hypotheses • H1: Adjusted barrier management allows glass eel to migrate • H2: One widely opened barrier is more effective than several barriers with a smaller opening • H3: Adjusted barrier management does lead to increasing conductivity upstream of the barriers Methods Study glass eel behavior North Sea Lift net sampling point Stow net sampling Methods • Study glass eel behavior • Test efficiency of adjusted barrier management Fyke net Results and discussion • Adjusted barrier management: key hypotheses • H1: Adjusted barrier management allows glass eel to migrate • H2: One widely opened barrier is more effective than several barriers with a smaller opening • H3: Adjusted barrier management does lead to increasing conductivity upstream of the barriers Results and discussion • Fyke samples • closed barrier: 3 glass eel per tidal cycle (on average) • slightly opened barrier (max. 10 cm): 632 glass eel per tidal cycle (on average) => adjusted barrier management can make a significant difference Results and discussion • Lift net samples at night (SVM-analysis) • strong variance in glass eel densities at barrier • strong impact of freshwater flow leaking through closed barriers and attracting glass eel => Instream solution needed! Results and discussion • based on stow net sampling: significant difference in glass eel density at day/at night 0.6 Glasaaldensiteit (m -3) 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 Bemonstering 7 8 9 Results and discussion • based on stow net sampling: significant difference in glass eel density at day/at night • based on fyke net sampling: no difference in glass eel density at day/at night 0.6 Glasaaldensiteit (m -3) 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 Bemonstering 7 8 9 Results and discussion • based on stow net sampling: significant difference in glass eel density at day/at night • based on fyke net sampling: no difference in glass eel density at day/at night 0.6 Glasaaldensiteit (m -3) 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 7 8 Bemonstering negative fototaxis but glass eel is present all day! 9 Results and discussion • Adjusted barrier management: key hypotheses • H1: Adjusted barrier management allows glass eel to migrate • H2: One widely opened barrier is more effective than several barriers with a smaller opening • H3: Adjusted barrier management does lead to increasing conductivity upstream of the barriers Results and discussion • Fyke samples • number of barriers opened • ~ number of glass eel entering • no impact on glass eel density • size of the barrier opening • ~ number of glass eel entering • ~ glass eel density in front of barrier => glass eel attracted to barrier Results and discussion Results and discussion Results and discussion • Fyke samples • number of barriers opened • ~ number of glass eel entering • no impact on glass eel density • size of the barrier opening • ~ number of glass eel entering • ~ glass eel density in front of barrier => glass eel attracted to barrier => increased barrier opening is preferable Results and discussion • Adjusted barrier management: key hypotheses • H1: Adjusted barrier management allows glass eel to migrate • H2: One widely opened barrier is more effective than several barriers with a smaller opening • H3: Adjusted barrier management does lead to increasing conductivity upstream of the barriers Results and discussion • Conductivity measurements: 0.1, 1 and 4.5 km u.s. Results and discussion Results and discussion • Conductivity measurements • increasing conductivity observed • however, even after long period of drought, conductivity reduced to base level Results and discussion • Impact of adjusted barrier management on eel populations in the Yser basin • Population model (Dekker et al., 2008) • Eel regulation: silver eel density target = 10 kg.ha-1 • Total water body surface within Yser basin: 932.2 ha 9322 kg silver eel needed 639545 glass eel needed Results and discussion • Impact of adjusted barrier management on eel populations in the Yser basin 639545 glass eel needed (Dekker et al., 2008) 300 glass eel.(tidal cylce)-1 x 2 tidal cycles.day-1 x 90 days.(glass eel migration period)-1 = 54000 glass eel 8.4 % of total number of glass eel needed Results and discussion • Can we further improve glass eel migration? Objectives • H4: Adjusted barrier management can reach a “glass eel saturation point” • H5: “Saturated” adjusted barrier management does not lead to increased conductivity © Nico De Regge Methods 5 fyke nets Methods Dag Aantal spuien geopend Maandag 1 Dinsdag 5 Woensdag 3 Donderdag 4 Vrijdag 2 2 3 4 5 Rechteroever 1 Linkeroever Aantal geopende spuien Results Saturation at 3 opened barriers Results No impact on conductivity However, a higher volume of sea water entered the river Yser Results Convex flow hypothesis? Sea River Fresh Salt 35 Resultaten Conclusions • Adjusted barrier management cost-efficient and effective solution immediately applicable automatic control feasible only a limited instream of salt water Conclusions • The more, the merrier: attraction of glass eel towards larger opening better to enlarge one opening than to increase number of openings © Nico De Regge Conclusions • Yes we can (reach 40 % silver eel escapement target) Yser basin : 639545 glass eel needed 300 glass eel.(tidal cylce)-1 x 2 (20 cm opening) x 3 (open gates) x 2 tidal cycles.day-1 x 90 days.(glass eel migration period)-1 = 324.000 glass eel = 50% 8.4 % of total number of glass eel needed attraction of glass eel towards larger opening better to enlarge one opening than to increase number of openings © Nico De Regge Conclusions • Further research: convex flow hypothesis? other species / periods? migration of pelagic fish species? tidal flaps? full opening? 41 Emilie Gelaude Karen Robberechts Raf Baeyens Yves Jacobs Seth Martens Maarten Stevens Tom Van den Neucker Thank you! 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