The unsustainable extraction of marine resources is caused partly by illegal, unreported and unregulated (IUU) fishing. Stocks of aquatic resources are difficult to measure in inland and marine waters, although certain estimation methodologies may be considered for this purpose. Aquaculture stocks can be estimated more frequently. Aquatic resources may be either cultivated or natural biological resources. Aquatic resources produced within aquaculture facilities (for breeding or for harvest) are considered cultivated biological resources. All other aquatic resources harvested as part of capture production processes are considered natural biological resources.