This paper analyzes the need to strengthen the law applicable to Flag of Convenience (FoC) States and vessels with the goal of sustaining fish stocks and combatting overfishing on the high seas. FoC States and vessels can escape law enforcement conducted by the other flag states. Due to the lack of political will of FoC States to enforce the law and obligations imposed on FoC vessels to conduct conservation on the high seas, FoC vessels instead contribute significantly to the deterioration of fish stocks on the high seas. FoC vessels overexploit these resources and engage in illegal, unreported, and unregulated (IUU) fishing, which harms the fish stocks on the high seas. The results of the study indicate that there are some legal lacunas in international legal obligations for FoC States and vessels to conserve the fish stocks on the high seas. Existing international legal instruments are ineffective in combating over-exploitation of fish stocks on the high seas by FoC vessels. Hence, strengthen the laws applicable to the FoC States and FoC vessels is urgently needed.