Some multi-species fisheries are characterised by production jointness in the sense that several species are caught through a joint production process (literally in the same haul of the net). Other multi-species fisheries (so called purse seine fisheries) are specialized in the sense that species are targeted individually and by-catch is negligible, but over the fishing season the same boat chooses to target several species with varying intensity which also results in a sort of jointness. Both types of fisheries are typically modelled using standard multi-input multi-output profit function forms (e.g. translog, normalized quadratic). In this paper we argue that jointness in the latter, essentially separable fishery is caused by allocation of fishing days input among harvested species. We developed a structural model of a multi-species fishery where the allocation of fishing days input causes production jointness. We estimate the model for the Norwegian purse seine fishery and find that it is characterised by non-jointness, while estimations for this fishery using the standard multi-input multi-output profit function imply jointness.