Small bigeye tuna associated with FAD

In previous columns, I have taken up the subject of purse seine catch of very small tuna, particularly of bigeye associated with FAD. Some people claim that catching tuna spawners would have the largest negative impact on a tuna stock. Others claim that catching juvenile tuna would be worse as fish caught have no chance of spawning in their life time. The fact is that catching tuna has some negative impact on the stock, regardless of at what life stage it is taken and it matters little whether they are allowed to spawn or not, because in the case of tuna, there is little correlation between the abundance of spawners and that of the resulting recruitments.
On the one hand, limiting the catch of large fish (spawners) would reveal more of an instant effect on the apparent stock size, whilst on the other hand, limiting juvenile catches would show its effects in the longer term. The effect of catch limits should be evaluated in a balance with fishing mortality, growth and natural mortality of fish at each life stage. Generally, catch quota are not set in terms of number of fish but in weight. Therefore, 1 ton of large fish represents only 10 to 20 fish, while 1 ton of small fish can represent 500 to 1000 fish. Even if the natural mortality occurring during the course of growing from small to large fish is taken into consideration, catching 500 small fish has more of a negative impact on the stock.
This is related to the analysis of yield per recruit. In other words, the question is: given a certain amount of recruitment, at what size (or age) should fish be caught in order to obtain the largest sustainable yield? For bigeye tuna, the curve showing yield that can be obtained at a certain size of fish increases rapidly from small fish until it reaches maximum level at the size of 60-70 kg (optimal size). Therefore, a catch of 1 ton at the optimal size would have the least impact on the stock, while 1 ton of catch of smaller fish would have a greater negative impact on the stock; and the smaller the fish are, the resulting impact is worse. After reaching the optimal size, the yield curve becomes almost flat but with a decreasing trend. Therefore, 1 ton of catch of fish larger than this optimal size would have somewhat more of an impact to the stock but the difference is much less than in the case of smaller fish.
People often combine all the immature fish in one category “immature/juvenile”. However, this is totally wrong. As explained above, 1 ton of catch of fish of 2 kilos has an absolutely greater impact on the stock compared with 1 ton of fish of 40 kg, even though both are immature/ juveniles. Purse seiners fishing on schooling fish catch relatively large immature/juveniles and sometimes even matured fish. However, purse seine set on fish associated with FAD catches almost exclusively very small fish. Therefore, those two types of purse seine operation should not be mixed up. In addition to this, species are generally not mixed in a free swimming school and the purse seine fishers can generally identify species in a school. Therefore, they can select species in their catches, avoiding bigeye if necessary. However, fish aggregated under FADs are mixed in species and size. Therefore, with current technology, fishers can not select species and/or size of fish in FAD fishing.
So far, the discussion has only considered the biological aspects of fish and fisheries. Even though we know that catching bigeye at 60-70 kg would provide us the maximum biological yield for a given recruitment, for socioeconomic reasons, it is difficult to catch only fish at the optimal size. For example, if we prohibit catch of small fish, some fisheries of coastal developing states would have to be banned because only small-sized fish are available in their waters. In addition, the longline effort has to be increased by several folds to achieve that objective and thus the fish-harvesting cost would soar. Furthermore, tropical tuna fisheries are generally multi-specific, i.e. they target skipjack and yellowfin more than bigeye.
Therefore, if regulation is based on bigeye, that regulation might curtail the catches of other species which are adequately abundant.
Considering all these elements, stock management policy needs to be approached with a sense of balance and careful reflection of all factors. Scientists believe that any management measure for the bigeye stocks would not be successful unless the catch of very small fish associated with FAD is well regulated. Since this is the common concern in all the world oceans, can we find any appropriate measures to limit taking very small bigeye in FAD fishing?