River restoration for the replacement of lost spawning grounds due to dam construction

Abstract

The construction of Baixo Sabor Hydroelectric Scheme (BSHS) in the Sabor River (located in northeast Portugal) without fishways has blocked fish migration. However, the Vilariça River, a tributary of the Sabor River, located downstream of BSHS represents an alternative habitat for fish, especially for specie more abundant in this river, the potamodromous cyprinid Iberian barbel (Luciobarbus bocagei). This way, to replace the spawning grounds lost in Sabor River, several compensation measures were implemented in a 2 km long river stretch of the Vilariça River. The measures included restoration activities like bank reinforcement, boulder placement, river water profile increase, and placement of submerged weirs with fish ramps to increase the lotic/lentic sequence. The river water profile increase was done with an adductor system constructed to transfer water from the BSHS to upstream the 2 km long river stretch during spawning. In the river stretch, a River Connectivity Index (RCI) was used and sampling sites of the Iberian barbel were set on to assess the effect of the compensation measures. The RCI and the wetted perimeter method showed that the water velocity of 1.5 m3/s provides good longitudinal connectivity. The results showed that barbel spawns preferentially in a coarse substrate with shallow water and moderate water velocity. The monitoring of the effectiveness of restoration measures during the first 6 years of exploration (2016–2020) showed that the major actions were well achieved, such as the importance of adductor system for attracting the breeding barbels to the river, the stabilization of the riverbank and the increase in the habitat heterogeneity. However, some structures must be intervened, like the replacement of the stone weir destroyed, replantation of native species, and the removal of exotic plants.