How it works

The natural power of a lure current


ELFI is an innovative method to monitor the presence of elvers using a lure current. In contrast to dip net monitoring, ELFI monitors continuously and in a standardized manner, which means that very accurate data can be collected on elver migration at the research sites.

Researcher effort is limited to actual monitoring and, if necessary, the cleaning of the detectors at the same time.



The ELFI functions by creating an artificial lure current. A floating collection tray is filled with water, which then flows into the waterway to be investigated via a gutter lined with a climbing substrate (usually canvas). Elvers are attracted into the lure current and crawl up through the canvas, after which they fall into the tray and are collected.

The tray is emptied at least once a week at which time the elvers are counted. Because ELFI is in constant operation, it is possible to gain insight into the developments of elver migration over a longer period (up to a few months).


ELFI is used for the following types of studies:

  • Elver migration: determining numbers and timing at a specific research site.
  • Attraction effect of research locations: which waters are most attractive to elvers? This knowledge can also be used to steer elvers towards a fish passage, or to make provision for locations that have an undesirable tempting effect, such as waste-water treatment plants.
  • Barrier effects: mark-recapture provides insights into the time elvers spend in an area, giving a measure of the difficulty of migrating further.
  • Functioning of the fish migration facility: through a supply and return study, ELFI can demonstrate whether elvers are able to migrate via a fish migration facility.
  • Migration route and speed: how to elvers spread within an area and how quickly do they do this? Mark-recapture provides insights into the routes taken by elvers and their possible preference for locations.