In order to study the migration patterns of the marine sand goby Pomatoschistus minutus between the North Sea and the Schelde estuary using stable C and N isotopes, it is essential that the tissue specific turn over rates are known. Also, it is a prerequisite for this technique that an isotopic gradient between the two migration endpoints is present. The two main aims of this thesis were to assess the turn over rates for muscle, liver and heart tissue of P. minutus and to verify the C and N isotopic gradients in the Schelde estuary.To characterize the tissue specific turnover rates of the juvenile sand goby, we conducted an experiment in laboratory conditions for 90 days. During this experiment sand gobies were fed an isotopically different diet and were sacrificed after 10, 20, 30, 45, 60, 75 and 90 days. This way the change in d 13 C and d 15 N of muscle, liver and the heart tissue could be monitored and plotted against time and increased biomass. Based on the half life times (days) the three tissues could be ranked as follows, for d 13 C: (days): heart (6,2) < liver (10,65) < muscle (24,8). For d 15 N another ranking was found: liver (2,48) < muscle (23,79) < heart (24,2). In conformity with our expectations, a considerable influence of metabolic activity was demonstrated on the rate of isotopic change. Since the models based on increased biomass described more accurately the changes in d 13 C and d 15 N, compared to the time models, we recommend using these models when investigating the migration dynamics of P. minutus . Especially, the models describing the change in d 13 C and d 15 N of muscle, d 13 C of liver and d 15 N of heart were able to make accurate estimates of the experiment time. It was also our intention to investigate the effects of fasting on the d 13 C and d 15 N of the three tissues. When food was deprived for 20 days, a significant enrichment was only found for d 15 N in liver and for d 13 C in heart tissue. The experiment also provided some insight in the fractionation factors ...