This study estimates the global extent of illegal and unreported fishing (IUU), finding that annual losses range between $10 billion and $23.5 billion, equivalent to 11-26 million tonnes. The analysis covers 54 countries and high seas, revealing regional variations in IUU levels. Developing countries are most vulnerable, with West Africa's estimated catches 40% higher than reported. The study highlights a significant correlation between governance and IUU levels, with better governance associated with lower IUU. While some areas have seen reductions in IUU, these are recent and follow increased international focus. The paper provides a baseline for assessing future efforts to reduce IUU. The study used a combination of data sources to estimate IUU levels, finding that governance indicators, particularly those from the World Bank, significantly influence IUU levels. The results show that IUU fishing has declined in some regions but remained a major issue globally. The study emphasizes the need for improved governance and international cooperation to combat IUU fishing, which has severe ecological and economic consequences. The findings underscore the importance of addressing IUU fishing to ensure the sustainability of marine ecosystems and global fisheries.This study estimates the global extent of illegal and unreported fishing (IUU), finding that annual losses range between $10 billion and $23.5 billion, equivalent to 11-26 million tonnes. The analysis covers 54 countries and high seas, revealing regional variations in IUU levels. Developing countries are most vulnerable, with West Africa's estimated catches 40% higher than reported. The study highlights a significant correlation between governance and IUU levels, with better governance associated with lower IUU. While some areas have seen reductions in IUU, these are recent and follow increased international focus. The paper provides a baseline for assessing future efforts to reduce IUU. The study used a combination of data sources to estimate IUU levels, finding that governance indicators, particularly those from the World Bank, significantly influence IUU levels. The results show that IUU fishing has declined in some regions but remained a major issue globally. The study emphasizes the need for improved governance and international cooperation to combat IUU fishing, which has severe ecological and economic consequences. The findings underscore the importance of addressing IUU fishing to ensure the sustainability of marine ecosystems and global fisheries.