THE EFFECTS OF MASS REMOVAL ON THE ROACH RUTILUS-RUTILUS (L) STOCK OF LAKE VESIJARVI ESTIMATED WITH VPA WITHIN ONE SEASON

The effects of mass removal on a roach stock was estimated with a virtual population analysis (VPA) within one season. High F/M ratio justifies the use of VPA in short intervals. The high ratio made it possible to obtain robust estimates because the terminal fishing mortality and the estimate of natural mortality had a negligible effect on the stock‐size estimates. Trawling effectively removed almost all the roach from the trawling area. The instantaneous rate of fishing mortality reached values greater than 6.0 during the season in some age‐groups. The estimated stock size was identical with weekly and monthly steps. The biomass of the stock was about 90 kg ha in May before the trawling was started. The results indicate that VPA is a convenient stock assessment method in biomanipulation projects especially if the population cannot avoid fishing by using refuges. Copyright © 1992, Wiley Blackwell. All rights reserved