Making the impossible possible – Tracking under water in the dark

Making the impossible possible – Tracking under water in the dark

Posted by Olga Krips on Mon 21 Oct. 2013

European eels (Anguilla anguilla) have a life cycle in which both larvae and adults travel incredible distances. The eggs hatch in the Sargasso Sea near North America and the larvae swim all the way to Europe. Near the European estuaries (the coastal areas with brackish water) they transform into the so-called glass eels that are transparent and approximately 8 cm in size. These glass eels enter the estuaries and move upstream in the fresh water, overcoming all sorts of natural challenges and tackling many obstacles to reach the waters where they will grow up. Then after 5 to 20 years, they mature and make the entire difficult journey back to the Sargasso Sea to spawn and die [1,2].

Dramatic population drop

The number of eels that reaches the European estuaries yearly has dropped dramatically during the last decades, with a drop of at least 90% [2]. Most likely this is caused by overfishing, pollution, parasites, and changes in the ocean currents. However, an increased occurrence of obstacles in the estuaries and rivers, such as hydroelectric plants, may be also responsible for the decline. Eels are a popular delicacy and their dramatic population decline has alarmed researchers. This has led to an increased research interest in the migratory behavior of glass eels.

Severe tracking challenges

Delcourt et al. [3] studied the migratory behavior of glass eels in a laboratory situation. Because glass eels show shoaling behavior, they wanted to investigate the swimming behavior of multiple glass eels simultaneously with video tracking. However, because glass eels travel at night also, the authors had to do part of the tracking in the dark. With the realization that the glass eels are transparent and swim under water, it becomes clear that tracking these animals is a severe challenge. Tracking more than one animal simultaneously is even more difficult.

Fluorescent color markers

The authors used Visible Implant Elastomer Tags (VIE-tags) [4] which were originally developed for capture-recapture studies on swimming animals. These tags are available in six colors that show up under UV light. Four of these tags (red, blue, orange and green) were used as distinct color markers for tracking fish in EthoVision. The pink tag was too similar to the red one under UV light and the yellow resembled green. The authors could successfully track four individual glass eels with these VIE-tags in a shoal of 50 conspecifics.









FREE TRIAL: Try EthoVision XT yourself!

Request a free trial and find out what EthoVision XT can do for your zebrafish research!

  • Track zebrafish adult, larvae or embryo
  • Suitable for tracking in any arena
  • Most cited video tracking system


Beating the challenge

The authors showed that many glass eels swim alternatingly with and against the current, following the tidal rhythm. This way they can swim up to 21 km in 8 days in a laboratory setting, which is comparable to what is found in estuaries. The authors hypothesize that tracking more animals simultaneously should be possible if VIE tags become available in additional colors. Furthermore, they give the suggestion to make color tracking with a combination of color markers on one individual possible, or to identify the animal by the intensity of the color marker. With the current method the authors already have beaten the challenge to track multiple animals under extremely difficult circumstances.

References

  1. http://en.wikipedia.org/wiki/Eel_life_history
  2. http://en.wikipedia.org/wiki/European_eel
  3. Delcourt, J.; Ylieff, M.; Bolliet, V.; Poncin, P.; Bardonnet, A. (2011). Video tracking in the extreme: A new possibility for tracking nocturnal underwater transparent animals with fluorescent elastomer tags. Behavior Research Methods, 43 (2), 590-600.
  4. VIE tags, (Northwest Marine Technology Inc., Shaw Island, WA, USA), http://www.nmt.us/products/vie/vie.shtml
Subscribe to the blog
Share this post
Topics
Learn
more
Relevant Blogs
assessing-motor-deficits-traumatic-brain-injury

Assessing motor deficits in mice following traumatic brain injury

Today Marcelle Cline and Donna Cross are so kind to share their insights on testing TBI mice with the CatWalk XT system.
standing-cows

Standing cows

Does it matter how much time a cow spends standing up or lying down? Bert Tolkamp thought that it did matter, and he proved his point by winning an IgNobel prize for his work on this.
seed-production-plants

Caterpillars speed up seed production in plants

Did you know that plants are not as passive as they appear to be at first sight? Although plants cannot run away when they are attacked by plant eating insects, they have several sophisticated ways to defend themselves.