... ICCAT has also recommended certain specific rules on minimum landing sizes of bluefin tuna;
Minimum fish and shellfish landing sizes
Ensuring that fish are not caught too young has biological and economic benefits. It allows for better catches once fish are bigger and it also ensures that the number of adults able to reproduce is sufficient to maintain healthy stocks. To discourage the targeting of small fish, minimum sizes below which it is illegal to market fish and shellfish have been set for a range of species. Undeniably this leads to some discarding - dumping fish and shellfish overboard because it would be illegal to land them. But, on balance, the risk of discarding is preferable to the absence of any rules which would see the targeting and destruction of small fish on an even larger scale.