Factors invalidating traditional mesh assessments are discussed with special emphasis on
changes in distribution of fishing effort between areas inhabited by small and large fish. A model
for studying the effects of such changes is constructed.
Both studies of equilibrium situations and simulation studies, taking into account variations in
year class strength, show that traditional mesh assessments may significantly underestimate the
long term effects of mesh size changes. The short term effects will depend on the age composition in
the stock when the mesh change is introduced and on the incoming recruitment.
Finally the more general applicability of the model is discussed
