Occurrence and impact of an epizootic disease (Myxobolus sp.,Protozoa, Myxosporidia) on a population of the cyprinid fish Barbus Barbus (L.) in the River Ourthe (Belgium), during the summer of 1993. Poster

In late June 1993, the first symptoms of an epizoocy ("Beulenkrankreit des Barben) (1) caused by Myxobolus sp. (Protozoa , Myxosporidia) were detected in a population of Barbus barbus (Pisces, Cyprinidae) from the mid River Ourthe (River Meuse basin, southern Belgium). The infestation dynamic was studied throughout summer by electrofishing (EPMC, 2,4 KVA) in the station where the epizoocy was first documented and completed by surveys in various stations upstream and downstream (longitudinal extension = 30 km). The infestation dynamics suggest a single infestation center, with more than 50 % of the barbel population nearby infested by the protozoans. All age classes above 2+ (15 cm, fork length) were affected by the disease and specially the large barbels (> 35 cm FL, mainly females ; +/_ 90 %). No trace of infestation was detected in the alevins and juveniles. The upstream extension of the infested stretch ( +/_ 1,5 km) is consistent with the home range behavior and weak summer mobility demonstrated in adult barbels (2). Census of abundance and composition of shoals of barbel suggest that 95 % of the infested fishes die from the epizootic disease or leave the stretch. The downstream movements of these fishes spread the disease far beyond the distance expected from home range behavior, extending the infested area on 20 km and potentially threatening the populations of the lower part of the River Ourthe. The impact of the epizootic disease on barbel stock is estimated at +/- 0,54-0,67 ton/km within the infested stretch. Besides, since many large females were infested, the population dynamics could be strongly influenced affected within the next 5 years. Although not clearly evident, it is believed that this epizootic disease emerged from the conjunction of exceptionally dry and hot weather during spring in the most densely populated stretch of the river. 1) B. Hofer (1904). Handbuch der Fische Krank Heiten, Schweizerbaret, Stuttgart 2) E. Baras (1993) Cah. Etol. 13