A new system proposes to track each individual fish from when it is caught to when it is served in a restaurant, using blockchain, to stop illegal fishing of Patagonian toothfish.

It is easier to preserve a database record across time than to make sure it always corresponds to the same actual fish.

In this particular case, that problem may not matter. It may be sufficient to make sure that the number of toothfish that are sold is no more than the number that were caught lawfully. If customers keep restaurants honest by checking the record of an individual fish, they might find it impossible to make use of the fish caught by poachers.

If the poachers of this fish, a luxury product, can't sell to expensive restaurants, they won't find it profitable.

However, if a restaurant could sell the "same" fish multiple times, the system would not achieve its goal.