Abstract
The anadromous allis shad Alosa alosa has suffered dramatic population declines throughout Europe and is currently considered as endangered throughout its entire distribution range. In order to reestablish allis shad in the River Rhine, which formerly housed one of the largest and most important populations, an EU ‐ LIFE Project ‘The re‐introduction of allis shad in the Rhine system’ was started in 2007. In course of the LIFE + Projects, allis shad larvae bred from genitor fish of the Gironde–Garonne–Dordogne population in France were reared in a pilot ex situ stock plant pilot facility in Aßlar, Germany. At an age of 1–2 months, about 100% of these fish developed approximately 0.5‐ to 0.8‐cm large, fluid‐filled, transparent cysts in conjunction with the upper jaw. The performed microbiological, virological, parasitological and histological examinations did not detect any infectious agents. Possible causative agents are discussed with regard to environmental factors and the nutrition of larvae. In conclusion, the observed malformations are considered a sign for a severe health problem and therefore a risk for the successful breeding of allis shad in aquaculture.