This paper examines the allocation problem arising from conflicting demands for marine resource use by (i) commercial fishers, (ii) recreational fishers, and (iii) conservationists. It is shown that decentralised trading of individual transferable quotas (ITQs) is capable of an efficient allocation of resource use between the first two parties. In contrast, it is found that the standard ITQ system is not capable of performing the same ideal co-ordination between the conflicting interests of extractive users, that is, all fishers, and the non-extractive ones, that is, conservationists. The reason is that quota trades between individual fishers and conservationists are inevitably accompanied by (positive) externalities on both other fishers and conservationists. As a result, decentralised quota trades between these parties cannot be efficient. The fundamental economic observation is that quotas for conservation and for extraction constitute two different goods. It follows that a socially optimal market allocation of these two goods requires two prices instead of the single quota price in the standard ITQ system. Thus, to achieve efficiency, the ITQ system has to be extended to incorporate both types of goods. It is shown in the paper that if fishers and conservationists can organise themselves into groups, trades of conservation quotas between the two groups can in principle lead to fully efficient allocation. An interesting implication of this modified ITQ system is that the need for a fisheries authority to set the total allowable catch (TACs) disappears.