The aim of the present study was to estimate the length-weight relationship of Ruditapes decussatus and Ruditapes phillippinarum. For this purpose, this study was undertaken using data drawn from two estuaries along the coast of Cantabria (N Spain). The length-weight relationship of both species was estimated for both estuaries, using two models: a classic allometric model and a nonparametric model using local linear kernel smoothers. Additionally, derivatives were used for estimating a minimum size of capture for this species, corresponding to the size where the first derivative reaches the maximum. The confidence intervals, used to draw inferences from these derivatives curves, were constructed using bootstrap methods, and binning techniques were applied to speed up computation in the estimation and testing processes. Within this context, the models application and the individuals’ weight gain showed that (a) the nonparametric model resulted in a better fit of data for both species in both estuaries and (b) a different size of capture for each species. Based on these results some managing specific guidelines are presented.