What is the "Tragedy of the Commons" and how may it be solved?

The Tragedy of the Commons (also known as the Tragedy of Freedom in a Commons) is an economic situation in which individual economic agents choose to maximise their individual gain when using a shared resource, to the detriment of everyone else. Economic agents make decisions by balancing the positive and negative consequences of a decision, and if the positives outweigh the negatives then they will rationally choose to go forward with that decision. For example, say there is a lake with plenty of fish in it and plenty of fishing boats, fishing at a sustainable rate. A fisherman may ponder if he should buy and use one more fishing boat. Positives: by using one more boat, the fisherman will be able to catch more fish and get more money etc. Negatives: by using one more boat the fish will be depleted quicker, and fishing will start to become unsustainable. In this scenario, by using one extra boat, the fisherman will gain all the positive consequences, andthe whole fishing community will bear the negative consequences, but he will only bear a fraction of the consequence. So rationally the fisherman would choose to buy the boat and catch more fish. But if all the fisherman think in this manner, the fishing will be unsustainable and in the long run there would be no fish left, leaving all the fisherman unemployed.
To combat individuals ruining a shared resource, there are a few solutions:One method is to sell of the public land as private property. If a single company/individual owns the shared resource they will have the incentive to stop people from depleting the resource and will take better care of it. In the above example, the lake can be sold to a single fishing company who now have the incentive to keep fishing sustainable, so as to increase the longevity and success of their company. One real world example of the Tragedy of the Commons is the "Grand Banks fisheries" who experienced the above example, where cod fishing became unsustainable and have never been able to recover since.