How can fishing be referred to as a "tragedy of the commons"?
Individuals reap an economic benefit from maximizing the number of fish they extract from the seas, but the general fishing population suffers because of the reduced (or extinction) of fish stock.
"Commons" refers to fields available to all (notably in England) where any one could graze their livestock. Farmers who placed their sheep or cattle in the "commons" benefited from this action; basically the more sheep or cattle they feed from the commons, the greater the economic benefit to themselves. However, with each farmer attempting to achieve maximum benefit by using the commons in this way, the commons was overgrazed resulting in a loss to all.
The same principle can be applied to fishing. Without international regulation, the fishing boats of any nation seek to maximize their income from fishing with the result that fish stock become depleted, damaging the fish environment for everyone.