Systematic analysis of uncertainty is critical for consolidating ecosystem model projections. Sensitivity analysis is an essential step in understanding model uncertainty, but there are many challenges when dealing with complex marine ecosystem models. One of these is related to the quantification of uncertainty in model input parameters, which is especially problematic when limited data leads to the use of arbitrary fixed ranges of variation to define parameter uncertainty. We show the drawbacks of this practice and propose an alternative methodology based on the parameter reliability criterion. This criterion helps to classify the model parameters according to the source of information used to estimate their values and calculate the ranges of variability for each parameter used in a sensitivity analysis. The methodology is illustrated by implementing a sensitivity analysis of the OSMOSE marine ecosystem modelling platform applied to the northern Peru Current ecosystem. We compare the results from the sensitivity analysis based on the parameter reliability criterion with those obtained using fixed ranges of variation. We find that using arbitrary uncertainty ranges can produce different conclusions compared to alternative approaches, such as the one based on the reliability of the parameters. The parameter reliability method can be helpful in situations where direct quantification of uncertainty in model inputs is not available.