Longline or Purse seine?

The tuna industry can be grouped into two major sub-industries. One is the high-priced fresh fish (particularly of sashimi) market mostly supported by longliners. The other is low-priced canned tuna market where tuna have been mainly supplied by purse seiners in recent years. The demand for canned tuna has been expanding very rapidly and accordingly, the purse seine fleet has also expanded rapidly.
Some people argue that purse seine fishing, particularly with Fish Aggregating Device (FAD), is the most economic fishing method with less impact to earth ecology. It is true that among industrialized fisheries, the quantities of fish captured per gallon of fuel is highest with purse seine fishery on FADs, if we disregard the economic yield or fish prices.
If the supreme objective of the tuna industry were the maximum reduction of carbon dioxide produced by the combustion of fuels by fishing vessels, the best would be to fish near the coast using only man power (like artisanal vessels without any engines). We all know that is not really a pragmatic option. Then, if we were to catch all tuna only using purse seiners on the basis of their fuel consumption levels, what would happen? Total catch which can be sustainable (MSY) would be much lower, as shall be explained later. In addition, without longline fishery, there would be no tuna available for sashimi consumption and the reduction in total economic yield would be very substantial.
Purse seiners catch a great number of relatively inexpensive young small-sized tuna, while longliners catch much less number of very high-quality expensive large-sized fish. It is known that a cohort of tuna gains mass until a certain size/age (i.e. gain by growth exceeds loss by natural mortality) and thereafter the cohort mass will decline (i.e. natural mortality loss is greater than growth gain). This critical point is about 40 kg in yellowfin tuna and 70 kg in bigeye tuna. These critical sizes correspond to the fish captured by longliners. Therefore the total weight of fish which can be sustainably taken by purse seiners alone would be much less than those taken only by longliners. Indeed, the current maximum sustainable yield (MSY) of bigeye tuna taken by longline and an expanded level of purse seine fisheries in the Pacific Ocean has been reduced to almost a half of the level of 20 years ago when bigeye tuna were taken by longline alone.
If we were to use only longline for catching tuna in order to increase MSY of bigeye, what would happen? It will ease the pressure on the bigeye stock. However, to achieve the current level of tuna catches (in weight) by longline alone, a substantial increase in the number of longline vessels would be required, and even then, we may still not catch as much fish as currently being caught. Further problem is that skipjack cannot be harvested at all and yellowfin catches will also drop far below the MSY level. The canning industry will be collapsed due to severe shortage in tuna supply and higher price of fish. Therefore, it is not realistic either.
In conclusion, we have to seek a point of compromise through a fair balance of social, economic, environmental and biological factors. The duty of scientists is to find an unbiased and transparent means of achieving equilibrium among these various factors, without being affected by prejudiced propaganda, money or political pressures.