Project
Glass eel Year-Round
Since 2019 so called ELFI’s (www.elverfinder.com) are used to run alongside the traditional glass eel monitoring lift net program including a large mark recapture program. The ELFI’s are installed to ensure equal effort of monitoring along the Dutch coast, to estimate glass eel index calculations and to estimate abundance. The monitoring season starts around 15 March and stops around 15 June and was assumed to cover the start and peak of the migration season which is between 15 April and 15 May. This period was mainly based on data collected by the traditional lift net monitoring (1 March – 30 May) which indicated the first catches beginning of March, a peak around mid-April and the last catches end of May. At some locations however, catches in the ELFI’s are still roughly as high as in the middle of the season (beginning of May) when the monitoring stops (mid-June). In some cases, catches are at highest at the end of the monitoring period. This contradicts to the assumptions of the monitoring period and suggests that either the glass eel migration season is longer than expected or that glass eel becomes more active at the end of the season resulting in a higher catch efficiency. Both circumstances would mean that the glass eel monitoring would miss a considerable part of the glass eels and consequently an underestimation of the glass eel recruitment and abundance estimate would be calculated.
In addition to this, results also show that at the end of the season either pigmented glass eel (new recruits) or young yellow eel (elvers, 1+ year old) are caught in various length classes. In many cases it is unclear whether these fish are young yellow eel from previous years or that those are glass eels but are already pigmented and have already been grown to similar lengths as small yellow eel. Some of them were tagged one year ago and recaptured as elver.
Sinds 2019 is het glasaal monitoringsprogramma uitgebreid met ELFI’s (www.elverfinder.com). Het Deze glasaalmonitoringseizoen begint ieder jaar half maart en eindigt half juni. Uit de resultaten blijkt echter dat, in tegenstelling tot het kruisnetprogramma, de vangsten in de ELFI’s op verschillende locaties aan het einde van het veronderstelde migratieseizoen nog steeds hoog zijn. Bovendien worden aan het einde van het monitoringseizoen ook oudere jonge aal gevangen. Hier is het niet altijd duidelijk of het dan om gepigmenteerde glasaal van het lopende jaar gaat of om kleinere, maar oudere aal gaat van voorgaande jaren. Om het glasaalmonitoringprogramma te optimaliseren en te onderzoeken of het glasaalmonitoringprogramma een aanzienlijk deel van de glasaal mist met misrekeningen tot gevolg, beoogt dit project (bijna) jaarrond te monitoren (februari – november). Daarnaast worden gewichts- en lengtemetingen gedaan en wordt het pigmentatie stadium geregistreerd.
Dit project heeft tot doel de glasaalmigratie verder te bestuderen in het licht van migratieactiviteit en de ELFI-monitoring en onderzoekt het habitatgebruik van jonge rode aal in kustwateren en brakke zones.