Wageningen Marine Research (WMR) executes a variety of fish monitoring programs in the Dutch rivers and lakes. These programs are often commissioned by the Ministry of Agriculture, Nature and Food Quality (Ministerie van Landbouw, Natuur en Voedselkwaliteit, LNV) and Rijkswaterstaat (RWS). Some of the activities within the monitoring programs use traps for trend monitoring of diadromous fish which are present in the large lakes and large rivers for a limited amount of time during migration periods (spring and fall). In this project two potential alternative methods are tested and evaluated: 1) camera monitoring and 2) eDNA sampling. These pilot studies will provide first insights in the applicability of both methods as an alternative for conventional trap monitoring.

De 10.400 ton hebben betrekking op de binnenwateren.

This report drafts a monitoring program aiming at determining and optimizing the effectiveness of restoring fish migration at Kornwerderzand with the 'Fish Migration River' (FMR.) For an adaptive management of the operation of the FMR, monitoring and evaluation are key aspects. The main research questions underlying the monitoring program will be outlined. The proposed research and monitoring approach describes which monitoring techniques can be applied, what set-up and schedule of different research components involved.

Habitats fragmentation of freshwater by barriers causes serious problems for fish migration. The Water Frame Directive propagates unhampered fish migration within entire water catchments and free access to sea. To achieve this, different fish passage facilities have been developed and built in the last decennia by water boards in the Netherlands to make barriers like weirs, sluices and pumping stations passable. In this study we evaluate connectivity and spatial use of freshwater fish in the Langbroekerwetering water system, situated in the central part of the Netherlands, that is regulated by six low-head weirs which are facilitated with De Wit Passages.

Evaluatie van het Nederlandse aalbeheerplan: maatregelen hebben in eerste instantie geleid tot een substantiële verbetering van de overleving tussen de perioden 2005-2007 en 2008-2010 gevolgd door een bescheiden verbetering in overleving tussen de perioden 2008-2010 en 2011-2013; positieve effecten op de aalpopulatie kunnen pas na vele jaren zichtbaar worden en blijven onzeker, omdat de aal pas na vele jaren terug zwemt naar zee om zich voort te planten en omdat niet goed bekend is welke de oorzaken zijn van de achteruitgang in de aalpopulatie.

During the descend to sea, silver eel face different hazards causing migration delay or mortality (e.g. dams, hydropower stations (HPS), weirs, ship locks, fisheries, predation). In 2010 fisheries was closed from September – November 30th and from April 1th 2011 the commercial eel fisheries in the river Meuse was closed for a undetermined period, due to dioxin pollution. To compare general mortality percentages with earlier studies (2002 and 2004), additional telemetry studies were carried out in 2010 and 2011. Compared to earlier studies more detection stations were used to estimate the mortality of the migration silver eel. This report attempts to answer two research questions: 1) Did the closure of the fisheries starting in 2010 result in a lower mortality rate for downstream migrating silver eel in the Meuse compared to previous studies in 2002-2006? 2)What is the delayed and direct Hydro power station (HPS) mortality for migrating silver eel in the Meuse during 2010-2012? In 2010 a batch of 150 female silver eel with surgically implanted transponders was released in the river Meuse and they could be followed with an array of detection stations of the Nedap TrailTM System. From the detection data mortality estimations were obtained. The present report is restricted to analysing migration routes and mortality estimations for the batch released in 2010. In accordance with the methodology used in 2002 (16 – 26% mortality) and 2004 (25 – 34% mortality) there was no big difference found in mortality estimations in 2010 (13 – 26% mortality). A comparison between the methodology used in previous studies (13 – 26% mortality) and new mortality estimations using more detection stations (22-25%) suggests that a more precise mortality estimation must be found in the upper limits of the mortality ranges derived from previous studies. Based on telemetry we found a minimum percentage of delayed and direct mortality of at least 20% of the migrating batch (121 eel). This percentage was corrected for other factors causing mortality (e.g. natural mortality, disease, predation etc.).

Connectivity between sea and freshwater inland habitats is heavily obstructed by anthropogenic barriers leading to a high pressure on diadromous fish populations including European eel (Anguilla anguilla).Nevertheless each spring millons of juvenile eels (glass eel) arrive at European coasts to access freshwater habitats. Coastal glass eel migration behaviour highly depends on tidal currents including in estuaries. However, this behaviour is severly hampered in regulated water systems. Therefore a better understanding of glass migration behaviour in highly regulated water systems is needed to take proper mitigation actions. Therefore a mark-recapture experiment was conducted in spring 2022 by releasing 21.424 tagged glass eels along the coast (Haringvliet and Nieuwe Waterweg) and 4.870 glass eels at inshore locations of the Nieuwe Waterweg, Hollandsche Ijssel, Haringvliet and adjecent rivers. In addition 1.097 young yellow eel were tagged at multiple inland locations of the study area.The results showed that (untagged) glass eels were caught throughout the study area up to at the very eastern side of the study area 70km inland. The number of tagged glass eels released along the coast were enough to answer the research questions (abundance and distribution). Multiple recaptures(n=121) were present at the west side of the study area: Rozenburg, Westland and Zaaijer, but also at the very east side of the study area at Krimpenerwaard. Therefore, using the data collected in 2022, an overall abundance estimate can be made and how this relates to inland local abundance estimates.For the Haringvliet multiple recaptures (n=1980) were found at the coastal side of the Haringvlietdam,Zuiderdiep and the Goereese sluis (Stellendam) to estimate the coastal abundance at the Haringvliet. Additionally one tagged glass eel was found in the hinterland at the pumping station Putten.In addition to the eastwards dispersal of glass eels, tagged glass eels showed also dispersal along the coast. Glass eels released at Haringvliet and Nieuwe Waterweg were recaptured at Katwijk (and viceversa). Tagged glass eels showed also dispersal between Nieuwe Waterweg and Haringvliet.Multiple locations showed increased catches of young yellow eels (elvers) during the season. Similar to the local abundance estimates of glass eels using tagged an locally released glass eel, a young yellow eel (elver) local abundance estimate at relevant locations can be made.This report gives an overview of the fieldwork procedures, catches and recaptures. This report is anticipating final results, which will be incorporated in a final report to be published in 2023. The research will be continued in 2023 when multiple fish passages will be studied on efficiency.

Sterfte van vis tijdens het passeren van turbines in waterkrachtcentrales is een wereldwijd probleem, met name voor migrerende vissoorten. In deze studie testen we een nieuw type turbine die is ontwikkeld om visvriendelijk te zijn door Pentair Fairbanks Nijhuis/FishFlow Innovations. In een gecontroleerd experiment zijn groepen Europese paling Anguilla anguilla gedwongen blootgesteld aan passage via deze nieuwe turbine en zijn de directe en vertraagde sterfte van deze groepen vergeleken met controle groepen paling die afgezien van turbine passage een gelijke behandeling hebben gehad.