Drift of larval and juvenile fishes: a comparison between small and large lowland rivers

Abstract

Drift of early developmental stages of fishes was studied in two adjacent lowland rivers (Danube River basin, Czech Republic) which differed in size. The River Morava was 60 m wide with an average discharge of 64 m 3. s -1 at the study site, whereas the River Kyjovka was 6 m wide with an average discharge of 1.1 m 3. s -1 . Both rivers have been channelized, however, only the Morava has been regulated by weirs. Drift was sampled at ten day intervals between May and August 1997. A 15 minutes sampling period was conducted every 3 hours during 24 hours using a drift net with an opening of 0.6 m2. As our sampling season coincided with extensive flooding of the River Morava basin in July, we also assessed any effect of this on fish drift. A total of 19 drifting species were recorded. The most abundant species were roach (Rutilus rutilus), bitterling (Rhodeus sericeus) and bleak (Alburnus alburnus) in the River Morava and Japanese minnow (Pseudorasbora parva), bitterling and silver crucian carp (Carrasius auratus) in the River Kyjovka. Seasonal dynamics of drift were similar between both rivers with two peaks in total drift abundance: initially in mid-June corresponding with the reproductive season and the second in early July corresponding with the beginning of flood. A nocturnal pattern of diel periodicity was observed at time of average discharge and disappeared when water transparency decreased during the flood.