Glass eel monitoring has been executed since 1938 after closing the Zuiderzee with the Afsluitdijk. At this location 400-600 hauls are collected each season (8 hauls each night) which makes it one of the most extensive monitoring in Europe and important for ICES advice. Since 1990 also other locations along the Dutch coast are monitored by volunteers equipped with small square lift nets. At these locations the effort is dependent on the motivation of the volunteers.
Due to the decline of the eel population many zero-hauls are collected and volunteers are de-motivated. Moreover, many migration measures have been taken to improve migration of glass eel along the coast. These improvements are directly affecting an index value and makes a year to year comparison unclear.
For this reason another method (glasaal detector) was introduced to run alongside the lift net program. This method is a more robust method which ensures equal effort along different locations. Moreover, mark recapture with glass eel allows for abundance and residence estimates. For index values, these estimates are becoming more important when migration measures or other adjustments have been made. Alterations will affect the index value and can be corrected by residence time estimates. However, it is unknown whether these mark methods and estimates are influenced by predation. If predation risk is higher compared to unmarked fish, due to the visible coloured marks, abundance estimates are overestimated and residence time will be underestimated.
Doel van dit project
Merk terugvangst experimenten zijn nodig om aanbod schattingen te maken langs de Nederlandse kust. Daarnaast is verblijftijd een maat voor barrièrewerking van een gemaal of ander type barrière. Het is echter onbekend in hoeverre predatierisico wordt verhoogd door het merken van glasaal. Deze studie beoordeeld of er een verhoogd risico is op de predatie van glasaal door het gebruik van merkmethodes.