Floods and fish : recruitment and distribution of fish in the Volga River floodplain

Natural river floodplains are among the most diverse and productive ecosystems on Earth and provide key habitats for foraging, spawning and as a nursery for many riverine fish species. Periodic flooding plays a principal role in the ecological processes in floodplain systems resulting in high productivity and diversity, as formulated in the Flood Pulse concept (FPC, a major conceptual framework for understanding the hydro–ecological processes operating in river-floodplain systems). However, little progress has been made in exploring the FPC over larger spatial scales due to lack of empirical data. The Volga-Akhtuba floodplain (Russian Federation) is still relatively undisturbed, while in Europe and North-America about 90% of floodplains have effectively been lost. This thesis provides a quantitative assessment of the FPC by (1) analyzing flood pulse dynamics in response to changes in river flow regime on various spatial and temporal scales and (2) relating these to recruitment success of riverine fish species that use the floodplain; and (3) identifying the hydro-geomorphic variables that are most involved shaping fish abundance and distribution in the floodplain. The results obtained demonstrate that the flood pulse magnitude in the lower Volga has noticeably decreased due to damming of the Volga upstream from the Volga-Akhtuba floodplain. Still, in spite of this hydrological control, considerable year-to-year variation in flood magnitude and timing has remained. Therefore fish populations in the floodplain still depend on variations in the year-to-year flow regime. Moreover, spring temperature and its match with flooding control the recruitment of young fish at the end of the growing season. However, the main sources of spawning stocks in the large-scale Volga-Akhtuba floodplain originate from local floodplain populations and not from the main river channels. Spatial heterogeneity of hydro-geomorphic attributes of the floodplain water bodies is highly significant for structuring fish abundance and distribution in the floodplain. Therefore, even under changing environmental and social circumstances, it is essential to preserve flood dynamics, which is fundamental not only for fish recruitment success but also governs fish distribution and diversity in the floodplain