If those harvesting tuna had to meet the full cost, ie they had to replace the tuna, then tuna would be more expensive. The price doesn't reflect the cost because the planet/ecosystem is being robbed.
If I sell fruit but I get it by stealing it from a neighbour then I'll be able to afford to sell the fruit at a far lower cost than the cost of manufacture and still get a profit.
Fishing, at a low enough volume, it is sustainable, which humans have been doing for dozens of millennia.
The problem comes when the threshold is reached and passed. That is specifically the tragedy of the commons. It's a fractal problem - i.e., even if one country fixes the issue through regulation/taxes/prohibition, another country may still overfish a shared body of water.
The "full cost" wouldn't need to be replacing each fish caught. It would be a fee/tax to ensure a sustainable fish population - this could still result a reasonable market for fish.
>The "full cost" wouldn't need to be replacing each fish caught. It would be a fee/tax to ensure a sustainable fish population - this could still result a reasonable market for fish. //
If I sell fruit but I get it by stealing it from a neighbour then I'll be able to afford to sell the fruit at a far lower cost than the cost of manufacture and still get a profit.