One of the biggest challenges in fisheries management is the control of illegitimate fishing activities that are not part of formal management structures. These activities are collectively termed IUU fishing - Illegal, Unreported and Unregulated. When catches go unreported they cannot be incorporated into the mathematical models that fisheries scientists use to calculate how much fish can be caught without causing the stock to collapse, thus making sustainability estimates less accurate.
Illegal operators are more likely to catch undersized fish, or disregard other environmental impacts such as bycatch levels and habitat destruction. Finally, because they are illegal, evade paying taxes and levies, and are in effect being subsidised by the environment to absorb the costs of bad fishing practise, they can undercut realistic market prices. This can cause instability and loss of revenue from legal and responsible fishers.
IUU fishing can take place at any scale, from recreational fishers catching more than their quota of rock lobster and linefish, and selling it to the local restaurant, to the wholesale plundering of Patagonian tooth-fish stocks in the distant waters around Antarctica. Once illegal catches have been processed and have entered the formal market it becomes very difficult to distinguish these from legal products, and consumers can unknowingly be contributing to bad environmental impacts.