Immediate catch reduction or catch increase when the mesh size is changed

Changes in mesh size are often rejected by fisheries management. Fishermen also do not like changes in mesh size. Apparently, a lower, but secure current yield and revenue counts more in the decisions than uncertain benefits in the future. A calculation method is presented that can be used to calculate the immediate changes in yield. It is shown that even large changes in mesh size do not necessarily lead to large changes in yield. Reductions or increases in catches can usually be easily compensated by immediate changes in fishing effort. In addition, it is shown how changes in mesh size affect the biomass of caught undersized fish (discards). As the results depend on the length distribution within the studied area, no general conclusions can be made. The length distribution in the area must therefore be determined in advance each time changes are planned.