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Twaite shad

The twaite shad (Alosa fallax (Lacepède, 1803)) is an anadromous species of the herring family (Clupeidae) which spawns in freshwater, but grows at sea. The habitat needs to be of high quality for the species to thrive. Therefore, the species is included in Appendix III of the Bern Convention and Annexes II and V of the EC Habitats Directive.



Left: The twaite shad occurs throughout Europe in the North Sea, Atlantic coast and the Mediterranean (distribution data are obtained by IUCN). - Middle: A twaite shad in a tray ready for surgery (©Pieterjan Verhelst). - Right: A twaite shad with an externally attached data storage tag (@INBO).


After an absence of almost 100 years, a spawning population has returned to the Scheldt Estuary and each spring around the end of April and the beginning of May, spawning twaite shads can be observed in the evening from the river banks from Temse to Zele. However, due to migration barriers, canalization and water pollution twaite shad populations have declined halfway through the 20th century across Europe.

To aid management, knowledge on the spawning migration and habitat use is crucial. We therefore tag twaite shads with both acoustic telemetry and data storage tags. The acoustic transmitters can be detected by the permanent Belgian acoustic receiver network and gives us detailed information on when the shads enter and leave the Scheldt Estuary for spawning. They can also be picked up by the receivers in the lower estuary, Belgian part of the North Sea and even receiver networks of adjacent countries to gain information on their foraging habitats. Since we are dependent on the presence of acoustic receivers to gain such knowledge, we also apply data storage tags. These devices log temperature and depth data experienced by the fish and do not require the presence of receivers. After some time, they come off the fish, wash ashore and are hopefully found by the wide public. When retrieved we can download the data and model the fish’s trajectory.     

To this day, we tagged 157 shads with acoustic transmitters and 60 with data storage tags.