The biological market-sampling programme for marine fish species provides, together with the information from the assessment surveys by research vessels, the information which is essential for relating changes in the fish stocks to changes in the fishery. The market sampling program in the Netherlands comprises the collection of fish samples from commercial catches, the analysis of these samples (estimation of length, weight, sex, sexual maturity and age), and the conversion of the national catches in tonnes to age compositions in numbers together with the additional information on mean weight at age, mean length at age. In a later stage, this information by country is combined to figures representative for the whole fish stock. These combined international data, collected over many years, form the basis for the analysis of population dynamics and for providing biological advice on fishery management.