-30- 



Total yield to the fishery partially depends on the sizes of 

 fish caught. Any increase in the effective size-at- first-capture should 

 increase yield up to a point. Conversely, increased fishing on small 

 yellowfin would tend to decrease the total long-term yield. 



III.C.l. Effects of Regulations 



On the basis of several studies, ICCAT, in 1973, 

 adopted a minimum size limit on Atlantic yellowfin of 3.2 kg or 55 cm. It 

 was estimated that under the then-current fishing pattern, this would 

 either give a small increase in yield-per-recruit (YPR) or, if undersized 

 fish were discarded dead, would not affect the YPR. Since the regulation 

 was adopted, the fisheries have changed dramatically: purse seine catches 

 have increased greatly and have shifted to larger fish, catches of both 

 baitboats and longliners have decreased, and there has been a shift to 

 smaller fish by the baitboats. 



Analyses were conducted to assess the amount of change 

 in YPR since the adoption of the regulation. Results indicated that YPR 

 for the entire fishery increased anywhere from 3 to 18% since the adoption 

 of the regulation. The longline fishery experienced a 57% reduction in 

 equilibrium yield-per-recruit, the purse seine fishery a 55% increase, and 

 the baitboat fishery a 45% decrease. The longline fishery's decrease in 

 yield-per-recruit was probably due to increased competition from purse 

 seine fisheries. The purse seine fishery's increase in yield-per-recruit 

 was due to increased effort on larger fish. The baitboat's decrease in 

 yield-per-recruit was due to reduced effort on large fish and increased 

 effort on very small fish. 



The regulation may also have resulted in increased 

 occurrences of undersized yellowfin tuna being landed as bigeye tuna, and 

 increased dumping of dead undersized fish at sea. Several ICCAT member 

 countries have initiated research to quantify the extent of discards at 

 sea, to describe in detail the areas where juvenile yellowfin tuna are 

 available, and to describe the interaction between juvenile yellowfin and 

 juveniles of other tuna species. 



A size regulation on bigeye tuna, similar to that on 

 yellowfin tuna has also been adopted by ICCAT members to increase the 

 yield-per-recruit of bigeye tuna and to reduce the landing of undersize 

 yellowfin tuna as bigeye. Additional measures to reduce the taking of 

 small yellowfin tuna are being evaluated by ICCAT's Juvenile Tuna Working 

 Group. The Group has identified areas of high juvenile tuna catches and is 

 looking at different alternatives to the current size limit. 



