Importance of gravel bars as spawning grounds and nurseries for European running water cyprinids

Gravel bars are common riverine habitat features most frequently regarded as useless obstacles to water flow thus as potential hazards for flooding in urbanised areas. Dredging policies are thus most common and frequently applied since dredged bars are progressively reconstituted during spates. However there is a dearth of knowledge on the actual impact of dredging and removal of gravel bars on fish communities, essentially because few investigations have been dedicated to the ecological importance of these structures as fish habitats. These considerations mainly apply to running water Cyprinids which are essential components of the ichthyofauna of European rivers. This study was conducted in 1993-1994 in a 10 km stretch of the 'barbel zone' in the River Ourthe (max width : 50 m ; mean annual discharge : 43 m3/s; River Meuse basin, Belgium). It aimed to assess the importance of gravel bars as potential spawning grounds and nurseries for running water Cyprinids and to model the impact of their removal in terms of loss of carrying capacity for 0+ juvenile fish. The common barbel Barbus barbus was selected as a target species for the study of spawning grounds, based on detailed knowledge of spawning behavior and on the representativity of their spawning grounds for other species of the community. Only six sites, covering an overall 886 m2 (1,75 % of the stretch surface) were identified as spawning grounds of which the two largest (450 and 350 m2) corresponded to large gravel bars targeted by hydraulic works. Fish sampling with DC powered electric fishing frames revealed that few 0+ juveniles (< 20 g/m2) were encountered in lotic or lentic habitats independent of the gravel bars during summer and autumn. In contrast large mixed 0+ juvenile cyprinid shoals were captured along the edges of the bars (up to 240 g/m2, mean : 84 g/m2), substantiating their functionality as nursery habitats. Based on models of habitat utilisation by 0+ fish and mapping of habitat availability the overall carrying capacity of the two large gravel bars for 0+ rheophilous cyprinids was estimated at 35.000 and 18.500 fish in late summer. Dredging these gravel bars as a preventive measure against the flooding of urbanised countryside would reduce the carrying capacity of these sites for 0+ juveniles by an 80 % margin. Additional and more severe hydraulic works including river bed profile and bank rectification would imply a further decrease of the capacity down to 0.7 - 1.6 % of its present value. These results highlight the ecological importance of gravel bars and the potential impact of regular dredging practices on the fish community of the River Ourthe, especially in stretches where man-made obstacles may restrict the free circulation of spawners. Perspectives for a more ecological management of rivers and gravel bars are discussed, focusing on the possibility of integrating the current knowledge on fish biology and local population dynamics into the planning of hydaulic works.