■52- 



III. C.l. Effects of Regulations 



The current size regulation for bigeye tuna has not been 

 in force long enough to assess its specific effects. Theoretically, the 

 size limit should increase yield-per-recruit to the fishery. 



At its 1980 meeting, ICCAT's Working Group on Juvenile 

 Tropical Tunas assessed the effects of area- time closures designed to 

 protect small yellowfin and bigeye tunas, and thus increase yield-per- 

 recruit. Potential gains to yellowfin-bigeye catches were contrasted to 

 possible losses to catches of skipjack tuna in multi-species surface 

 fisheries. Six fishing areas having combined monthly catches of yellowfin 

 and bigeye tuna in excess of 50 mt were identified (Figure 13). Twenty- 

 three area-month combinations were chosen for detailed examination. The 

 following conclusions were made: 



1) Ignoring losses to catches of skipjack tuna, closing any or a 

 combination of these strata would result in a maximum gain of 10% to 

 yellowfin-bigeye catches. 



2) Assuming that skipjack tuna not taken in closed strata would be 

 taken in other areas or at later times, an overall gain in at least yield- 

 per-recruit of bigeye and yellowfin would result from closures. Yield-per- 

 recruit to baitboats would decrease while yield-per-recruit to purse 

 seiners, which catch a broader size range of fish, would increase. 



3) It was assumed that any skipjack tuna not taken would be lost, and 

 that these losses would particularly affect baitboats. Although available 

 data were insufficient to predict skipjack losses, the consensus of the 

 Working Group participants was that losses to skipjack catches would exceed 

 gains to yellowfin/bigeye catches. The Group concluded that imposing the 

 area-time closures investigated would probably result in a decrease in the 

 combined catch of yellowfin, bigeye, and skipjack tuna. 



IV. STATUS OF CURRENT RESEARCH NEEDS AND EFFORTS 

 IV. A. Major Research Problems 



The uncertainty of the Composition of the Atlantic bigeye stock 

 structure makes it difficult to evaluate and assess the effect of the 

 fishery upon this tuna. Analyses of models and determination of MSY, 

 therefore, can only be regarded as "best guesses." In addition, the ICCAT 

 minimum size limit regulation has not been in effect long enough to 

 determine its effect upon the fishery. 



