Summary Several aquatic living resources are overexploited. An example of this is a barnacle fishery, which is a commercial species that commands high prices in the market. We feel that not enough is known about the management of this species. In particular, not enough is known about its growth pattern. Thus, we model the length–weight relationship of this resource for each harvest site. We use regression models with factor-by-curve interactions. Specifically, a global test to compare regression curves and their derivatives between levels and a local test to draw inferences about critical points are provided.