Nocturnal tidal barrier management improves glass eel migration in times of drought and salinization risk

Abstract

Adjusted tidal barrier management (ATBM, setting tidal doors ajar during tidal rise) is currently applied in Belgium to improve glass eel passage through tidal gates. However, salt intrusion in the receiving waters due to upwelling and accumulation of saline groundwater as a result of intensive drought put severe pressure on the unrestrained use of this cost‐efficient mitigation measure. We evaluated the efficacy of a very restricted ATBM at a tidal barrier on a small canalized waterway, located 3.5 km from the sea: one out of seven gate doors was left 20 cm ajar for ca. 30 min when the water levels on both sides of the tidal barrier were approximately equal. During 20 inflow events differing in tidal flow (i.e., flood or ebb tide) and daytime (i.e., day or night) in March/April 2019, migrating glass eels were caught with a fyke net fixed on the temporally opened gate. In total, 12,853 glass eels were captured, almost all during flood tides at night (96%), with a maximum catch of 3,827 individuals per inflow event. These results show that even small windows of opportunity can help glass eels pass tidal barriers.

Glass eel migration in an urbanized catchment: an integral bottleneck assessment using mark-recapture

“…Migration successes for glass eel highly depend on nocturnal opportunities [ 46 , 47 ]. As seen in Belgium, nocturnal glass eel migration near tidal barriers can already be successful using short lasting windows when the water levels directly upstream and downstream of the barrier are equal [ 48 ]. Although not intentionally managed for this purpose, migration through the ship locks at IJmuiden may function in a similar way as adjusted tidal barrier management, as presented in studies carried out in Belgium [ 48 – 50 ].…”

“…As seen in Belgium, nocturnal glass eel migration near tidal barriers can already be successful using short lasting windows when the water levels directly upstream and downstream of the barrier are equal [ 48 ]. Although not intentionally managed for this purpose, migration through the ship locks at IJmuiden may function in a similar way as adjusted tidal barrier management, as presented in studies carried out in Belgium [ 48 – 50 ]. A clearly noticeable salinity gradient accompanied by a clearly noticeable but discontinuous local attraction flow and large volumes of water going back and forth through the locks apparently efficiently facilitates glass eel migration.…”

Wetenschappelijke onderbouwing en ondersteuning van het visserijbeleid en het visstandbeheer

“…Daarnaast zijn er langs de Vlaamse kust ook nog enkele kleinere potentiële intrekroutes ter hoogte van enkele uitwateringsconstructies. In eerder onderzoek werd glasaalmigratie geanalyseerd en werden mogelijke mitigerende alternatieven geëvalueerd ter hoogte van de spuiconstructies in de Ganzepoot (IJzermonding, Mouton et al 2010, Verhelst et al 2023a, Verhelst et al 2023b, het Sas Slijkens (KGO, Buysse et al 2012), het Maertensas (Noordede, Vandamme et al 2020, Van Wichelen et al 2022, het Caemerlinckxcomplex en het uitwateringscomplex van Zeebrugge (Buysse et al 2015a). Het op een kier zetten van de sluisdeur gedurende het opkomend tij (= aangepast spuibeheer) bleek een kostenefficiënte en effectieve manier te zijn om stroomopwaartse glasaalmigratie te verbeteren, zonder risico op verzilting van de polders door de instroom van zeewater (Mouton et al 2013).…”