How can a TAC be divided?

In fishery management, the most effective and practical measure would be the establishment of a “Total Allowable Catch” (TAC) for a given stock and to keep the catch below that level. However, even with the same TAC level, the size of fish being caught makes a big difference in its effect. The catch of a thousand tons of 2kg fish would give much greater impact on the stock than the catch of the same weight of fish of 70kg. As discussed in this column previously, if tuna are allowed to grow up to an adult size, sustainable yield from that stock would be far larger than in the case where many juveniles are caught. Besides, larger tuna can be sold at a substantially higher price than small tuna. Hence there is an even greater economic profit in catching tuna after they have grown up. The problem is that the management of fishery in the real world cannot be adopted based on only biological and economical yields. Many fishers’ livelihoods are dependent on their own fishing. Also an establishment of a TAC to one species of fish can have an effect upon the fisheries on other species.
  Tuna fisheries have multi-species, multi-national and multi-gear aspects. For example, small bigeye tuna are caught by purse seine together with skipjack and yellowfin tuna, while large fish are caught by longline, also with other species of tuna and billfish. Pacific bluefin tuna are caught: at ages 0-1 by trolling along Japanese coasts; at ages 2-4 by purse seine in the Japanese, Korean and Mexican waters, as well as by trap and longline in Japan; matured fish are caught by Taiwanese and Japanese longline. These fishing practices have a direct and real-life relation to fishers’ livelihoods.
  Therefore, we cannot give all the TAC to big fish fisher y in order to maximize biological and/ or economical yields. When adopting a conservation measure, that measure should be based on biological and socio-economic balance. Nonetheless, it is difficult to find a practical method for distributing a TAC among fisheries which take different sized fish. The easiest way out of this dilemma – which has been practiced so far in many cases – has been to cut down the current catches by a certain percentage, regardless of the fisheries catching different size of fish. But there is a significant exception to this r ule: management of Atlantic bluefin tuna. For the fisher y of this species, a minimum size limit has been introduced (30 kg as a minimum allowable size, adopted in 2010, with an exception for the Adriatic Sea). This means that the managers decided to give the priority to increasing sustainable biological catch per given stock. Consequently, all the coastal bluefin tuna fisheries that had been catching a significant amount of small juvenile bluefin tuna in the Mediterranean Sea have been banned. One wonders whether the decision to recover the stock by sacrificing coastal small-fish fisheries was motivated by a desire to prioritise the sur vival of fish farming (fattening)?