The productivity of floodplain rivers is directly dependent on the connectivity of the main channel and its backwaters and on periodic flooding. Backwaters and inundation areas are important as spawning places and nurseries for many river fish. Over tha last 100 years, most of the lowland European rivers were highly modified. The 0+ fish assemblages was investigated in the River Morava (a tributary of the Danube), the main channel of which has been isolated by channelisation from its flood plain and regulated by weirs 25 year ago 6,13. Young of the year fish (YOY), collected since 1991, were used as an indicator of reproduction potential of ichthyocenoses. Species richness and the structure of 0+ fish assemblages depend mainly on the spawning places and potential nurseries available. In the absence of areas with lentic backwaters or side-arms with aquatic vegetation in the channelized river, the fish could only use the stabilized banks of stony rip-rap or rare shallow-slope gravel shorelines. Bitterling Rhodeus sericeus, gudgeon Gobio gobio, chub Leuciscus cephalus, bleak Alburnus alburnus and roach Rutilus rutilus were the most abundant species in the modified main channel. However, YOY of phytophilous species had almost disappeared, and a decline in density was also found for rheophils, such as vimba Vimba vimba, barbel Barbus barbus and nase Chondrostoma nasus, previously dominant species in the river 16,30,40.

This research was funded by the Czech Academy of Sciences and the Project Morava. Current research was extended on less modified downstream Slovak stretch of the Morava River and its tributary Dyje River. Study is conducted in cooperation with CHKO Záhorie.

top

Managed flooding and natural fish reproduction

Fish diversity and abundance were investigated in six man-made borrow pits in the flood plain of the River Dyje (Danube basin) in 2001. All the borrow pits had uniform habitats without shelters, with limited spawning and nursery areas. Spawning and nursery habitat conditions in three borrow pits were experimentally improved by managed flooding during the spring and summer whilst the other three borrow pits were not flooded. Adult (ł 1 year) fish were surveyed in spring and autumn by beach seining and 0+ fish were monitored monthly by dipnetting and fry beach seine nets. Flooded borrow pits had slightly higher adult species richness, considerably higher adult fish abundance and considerably higher 0+ fish species richness and abundance. The seasonal decline in 0+ fish abundance varied between flooded and non-flooded borrow pits, with a faster decline in 0+ abundance in non-flooded sites. Abundance of 0+ fish was approximately 60 times greater in flooded than in non-flooded sites at the end of the study season 34, 44.

The reproductive success of fish was affected also by the larval stages, metacercariae, of the trematode Posthodiplostomum cuticola, which cause "black spot disease" 42, 49, 50. Managed flooding improved spawning and nursery conditions, however it also enhanced the abundance of intermediate and definitive hosts of the parasite. Abundance of aquatic snails, Planorbidae, the first intermediate host, and frequency of visits of piscivorous birds, especially Ciconiiformes, the definitive host, were significantly higher in the flooded than non-flooded borrow pits and thereby parasite transmission increased. Consequently, intensity of infection of fish (the second intermediate host) was higher in flooded borrow pits.

The managed flooding of borrow pits could improve conditions for spawning and 0+ fish survival during the first few months, and thereby enhance fish reproduction in these alternative man-made habitats. The quantity of 0+ fish is sufficient to suppress parasite infection 51.

This research was funded by the Grant Agency of Czech Republic. Study was conducted in cooperation with the Milan Gelnar (Department of Zoology and Ecology Masaryk University Brno)

top