LENARZ ET AL.: YIELD PER RECRUIT OF ATLANTIC YELLOWFIN TUNA 



Table 4. — Landings per recruit (kg) when M = 0.8, initial F = 0.2, growth curve of LeGuen and Sakagawa 

 ( 1973) is used, and yellowfin less than the minimum size are caught and discarded dead. — Continued. 



occur. Therefore, if effoi't is constant the pre- 

 dicted gain with no dumping is greater than the 

 possible loss through dumping if the minimum 

 size were 55 cm. but at 77.5 cm the opposite is 

 true. At both size limits we predict a greater 

 gain with no dumping than possible loss through 

 dumping if effort is doubled. 



Assuming constant recruitment, yield per 

 recruit per unit effort is a measure of fishing 

 success. Table 5 presents the estimated yield 

 per recruit per effort by gear assuming no dump- 

 ing. Increasing the size at recruitment to 77.5 

 cm at the current level of effort would result in 

 a 17% decrease for bait boats, a 9% increase 

 for small purse seiners, a 12% increase for large- 

 purse seiners, and a 25% increase for longliners. 

 Yield per recruit per effort would drop by 

 about 35% for each of the gears if effort doubled 

 and size at recruitment increased to 77.5 cm. 

 If effort doubled and size at recioiitment re- 

 mained 32.5 cm, yield per recruit per effort 

 would decrease by 30% for bait boats, 50% for 

 purse seiners, and 60% for longliners. 



Changes in the average weight of landings 

 should be considered because average weight 

 affects the values of landings particularly in 

 light of size-specific changes in the value of 

 yellowfin tuna. Table 6 presents estimates of 

 the average weight of catches by gear. Figure 

 13 shows average weight isopleths for the en- 

 tire fishery. If effort remained constant and size 

 at recruitment increased to 77.5 cm, the average 

 weight of the catch of the total fishery would 

 increase from 17.7 kg to 30.3 kg. If effort doubled 



and size at recruitment increased to 77.5 cm, 

 the average weight would increase to 24.2 kg. 



Sensitivity of Results to Errors when Ageing 

 Large Yellowfin 



The growth curve used in this study was 

 based on the use of modal progressions to age 

 yellowfin. Unfortunately while this method is 

 probably reasonably accurate for ageing yellow- 

 fin less than about 130 cm long, beyond this 

 size it becomes increasingly difficult to separate 

 modes, and there is a reasonable probability 

 that ages are increasingly underestimated with 

 increases in size. In addition, because tuna 

 apparently spawn over a large portion of the 

 year, the exact meaning of age is not always 

 clear. Alternative methods, such as ageing by 



0,0 



Q5 



LO 1.5 aO 2.5 



MULTIPLIER OF EFFORT 



30 



35 



Figure 13. — Average weight (kg) isopleths for the entire 

 Atlantic vellowfin tuna fishery. 



51 



