Good decisions ideally require good data. Here, we present a straightforward typology for the broad classification of fisheries catch data. At each stage in the reporting chain, from fisher to national/international agencies, fisheries catches can be: known and reported; known and underreported; unknown and overreported; or unknown and underreported. Here, we consider largely the data reporting at the national/international level. Unfortunately, experience has shown that scientists and managers often do not know or are unconcerned with which category their data falls within a country's complete data system, or how to deal with this problem, leading to considerable implications for management. Of these four categories, the underreporting of catches seems the likeliest and most common outcome, which inevitably leads to mismanagement and misallocations of fisheries resources. Attempts to improve catch data should be undertaken, particularly via the development of catch baselines through catch reconstructions and adoption of a transparent and comprehensive country-wide expansion approach. Such an approach not only helps address shifting baselines but identifies aspects of data improvement that can be implemented in future data collection. The taxonomy presented here is a conceptual first-order analytical tool to classify data status, and hence influence management decisions.