Recent decline of roach Rutilus rutilus stock in a large river ecosystem in relation with its population dynamics

Studies dealing with quantitative decline of freshwater fish species with long-time series data are rather scarce and primarily limited to migratory species. We present an original work integrating over two decades investigations on roach (Rutilus rutilus L.) stock, one of the dominant cyprinid fish species in the River Meuse (Belgium). This study aimed to quantify changes in the stock of roach after recent indicators (captures from sport fishermen) suggested a sharp decline. Three methods were used: capture-mark-recapture (CMR) estimation, catch per unit of effort by gillnetting (CPUE) and long-term monitoring of fish-passes. Population dynamics of roach was also investigated. CMR method demonstrated a decrease of 91 % of the roach stock compared to earlier data (1993-2003), with current densities close to 300 roach ha-1. This was also confirmed by a decrease of 95 % in CPUEs and a decrease between 93 and 98 % in two fish-passes during the same period. On the other hand, growth and mortality did not indicate radical changes before and after the decline was found. As roach stock decline is generalizable to the whole Belgian's Meuse following estimations in two other sites, further investigations are needed to explain the reasons underlying this decline in order to take conservation measures. First, the uptake of primary production (i.e. phytoplankton) by invasive clams is supposed. And secondly, an increase in predation pressure by the Great Cormorant is effective as this avian predator was found in high densities from 2000 until 2006.