There is growing international focus on ecosystem based fisheries management. Within the EU this has manifested in the provision of mixed fisheries advice, using the FCube model. The operational implementation and accuracy of this model relies on accurate identification of the technical interactions between fleets, gears and the resulting composition of species in the retained catch. These interactions are defined using units of fishing activity based on gear type and target species assemblage, but with no consideration for spatiotemporal heterogeneity. We assess the relevance of the fishing units used in relation to the spatial and temporal trends in retained catch within the Celtic Sea. To achieve this multivariate analysis (principal component and hierarchical clustering) were conducted to identify homogenous groupings of fishing activity using 5 years of international Celtic Sea retained catch data. The groupings identified demonstrate that a fairly simplistic structure of fishing activity units (country of provenance, fishing location, gear and target species) can effectively describe the complex mixed fisheries scenarios being executed within the Celtic Sea consistently across multiple years. This international and multiannual analysis improves our understanding of the mixed fisheries scenarios within the Celtic Sea and reveals a multifaceted spatial structure in the species assemblages landed, indicating the presence of several distinct mixed fisheries within the region appropriate for mixed fisheries analyses.