The success of regulations of multispecies fisheries may depend critically on understanding output dependencies correctly. An example is purse seine fisheries that target several species over the season but are specialized in the sense that each species are targeted individually. Such fisheries are typically modeled as either independent single species fisheries or using standard multispecies functional forms characterized by jointness in inputs. We argue that production of each species is essentially independent but that jointness may be caused by competition for fixed but allocable input of vessel capacity. We develop a fixed but allocatable input model of purse seine fisheries capturing this particular type of jointness. We estimate the model for the Norwegian purse seine fishery and find that it is characterized by nonjointness, while estimations for this fishery using the standard models imply jointness.