As the ocean isn’t owned by anyone, fisherman will race to the seas and catch as many fish as they can to make as much money as they can.
They have no incentive of “investing” in the ocean/ making sure that there are enough fish left in the end; as some other fisherman will simply catch the fish, which is open to be caught by anyone, instead.

You can compare it with a bank account which is accessible to anyone:
If it were only accessible to you, you’d only take out that, which you gained as interest for that year, and thereby you keep a base capital of money which is always available to you.
If it was accessible to anyone, everyone would take out as much as they could, and therefore depleting the account.

Fish prices going up will only encourage the fishers to fish more of that kind.

So the only way to make sure fish don’t die out is by setting quotas via government regulations.

On the other hand, Greenpeace should be complaining to the governments, asking for these quotas to be set; instead of going after the last people in the supply chain, the restaurants.

1. Why the enemy of environmentalism is capitalism I’ll never understand. Which do you think had a better environment; West Germany or East Germany? North Korea or South Korea? Many of the world’s worst environmental disasters were created by government agencies in Socialist and Communist countries (Chernobyl, China’s Great Sparrow Campaign). The world’s most environmentally conscious and clean countries tend to be more free market.

2. The bluefin tuna that is in Nobu’s kitchen is no longer endangered. It is dead. So eat it up and don’t put in any further orders for more if you’re so concerned about that species.

3. When you’re at Nobu, don’t bother with the bluefin anyway. Get the black cod. You’ll thank me later.

The IUCN Red list lists the species as “Data Deficient”

Surely the obvious solution is for Japan to invest in some research tuna-ing?

As for ‘wild accusations’ – I wasn’t accusing anybody of anything. I was parodying Marco Polo’s apparent claim that business trumps conservation.

Anyway, this Economist article makes interesting reading. However, it’s obviously all codswallop on the very sound scientific grounds that some restaurants haven’t put their prices up.

http://www.economist.com/background/displaystory.cfm?story_id=12502783

Macro Polo makes two arguments.

Premises:
1) The market prices determined by the principle of Supply and Demand are accurate reflections of the actual supply available, and/or market demand.
2) The prices of tuna have not risen.
Conclusion: there is no change in the amount of tuna available, and therefore is not endangered.

Premises:
1) Obstruction of businesses without proper reason is unjust.
2) Environmental activist groups are making demands on restaurants to ban tuna from their menu, which is detrimental to business.
3) The claim that tuna is being endangered is false.
Conclusion: Environmentalists making demands to restaurants regarding removing tuna from the menu lacks proper reason and is unjust.

Whether I agree with those premises and conclusions are a different matter, but if you want to start convincing others of your positions, it would do well to actually argue instead of making wild accusations.