Who needs the floodplain? Differential floodplain use by adult fish of different Life-history guilds (Volga, Russian Federation)

Floodplains are a key habitat in the life cycle of many riverine fish species for spawning, nursery, foraging or a combination of these. While in Europe and North-America, about 90% of the floodplains are already considered functionally extinct, the vast floodplains of the lower Volga are among the few in Europe that are still ecologically functional. Therefore it serves as an excellent instrument to unravel different strategies of floodplain resources utilization by adult fish of different life-history strategies under different spring flow regimes. We used commercial fisheries data from the main river channel and floodplain lakes as well as our own sampling data from migration surveys at the entrance to, and within, Abstracts 149 the floodplain. Our surveys were performed during the flooding periods of 2006 (extreme low flooding) and 2007 (average flooding). We distinguished three species groups of floodplain habitat use in the lower Volga: 1) resident species (depending on permanent floodplain water bodies, mostly associated with opportunistic life history strategies); 2) migratory species (moving into the floodplain during flooding episodes , mostly associated with moderate periodic life histories); and 3) non-dependent species (mainly species of the mainstream of the river with no significant role of floodplains, mostly associated with periodic life histories). We also observed that flooding conditions strongly affected spring spawning behaviour of the migratory species. In contrast to high/ long flood during extremely low/short flood peaks fish did not migrate deep into the floodplain but spawned closer to the entrances not utilizing available spawning/ nursery habitats.