This book is the first comprehensive examination of state practice relating to enforcement by non-flag states of the high seas conservation and management measures adopted by Regional Fisheries Organisations. Moving beyond the issue of the implementation of the United Nations Fish Stocks Agreement, it demonstrates an emerging exception, in customary international law, to the rule of primacy of flag state jurisdiction in the high seas fisheries context and delineates the parameters in which this emerging exception allowing for non-flag jurisdiction may be exercised. This book contains extensive factual descriptions of state practice as well as comprehensive legal analyses of that practice.