AN EXAMINATION OF THE YIELD PER RECRUIT 



BASIS FOR A MINIMUM SIZE REGULATION FOR 



ATLANTIC YELLOWFIN TUNA, IHUNNUS ALBACARES 



W. H. Lenarz. W. W. Fox, Jr., G. T. Sakagawa, 

 AND B. J. Rothschild' 



ABSTRACT 



Some of the conceptual foundations of yield-per-recruit analysis as a management tool 

 and as applied to the Atlantic yellowfin tuna fishery were critically explored. Problems 

 examined include: (1) estimating the current state of the fishery in terms of a knife-edged 

 recruitment approximation, (2) inferring consequences of management action from the 

 yield-per-recruit isopleth, (3) the difficulty in achieving a maximum yield per recruit when 

 there exist several gear types exploiting different size ranges, (4) the difficulty in obtaining 

 projected increases in yield per recruit when the killing and discarding (dumping) of fish 

 smaller than the optimum size occurs, and (5) the possible interaction between a size limit 

 and the projection of the maximum sustainable yield. 



In employing yield-per-recruit analysis to the Atlantic yellowfin tuna fishery, two ap- 

 proaches were taken — one approach makes use of a wide range of parameter estimates and 

 a number of simplifying assumptions, but little data, and the other approach makes use 

 of considerably more data, but is more confined in the parameter estimates and uses fewer 

 of the simplifying assumptions. The general results of both approaches, assuming no dump- 

 ing occurs, indicate that only minor increases in yield per recruit would occur if the size 

 at recruitment is increased from our estimate of the present size at recruitment and fishing 

 effort remains constant; an increase in fishing effort without changing other aspects of the 

 fishery would not appreciably increase yield per recruit; and an increase in size at recruit- 

 ment and in fishing effort would result in modest gains in yield per recruit. Specifically 

 meeting the request of the International Commission for the Conservation of Atlantic 

 Tunas, we recommended that a minimum size limit regulation in the vicinity of 55 cm 

 (3.2 kg) be enacted. 



The second regular meeting, in Madrid, Spain, 

 on 2-7 December 1971, of the commission of 

 ICCAT (International Commission for the Con- 

 servation of Atlantic Tunas) authorized the 

 "Council to recommend to the Contracting Par- 

 ties that they prohibit landing of yellowfin 

 weighing less than a minimum weight some- 

 where between 3.2 and 10 kg." This recommen- 

 dation was based on studies by members of the 

 Subcommittee on Stock Assessment that showed 

 that theoretically the size at first capture 

 which maximizes the yield per recruit of yellow- 

 fin is between 10 and 25 kg. 



A special ICCAT working group on stock 

 assessment of yellowfin tuna met in Abidjan, 

 Ivory Coast, 12-16 June 1972, to consider fur- 

 ther scientific aspects of size regulation and 



other matters pertaining to the Atlantic yellow- 

 fin fishery (ICCAT, 1972).- Studies on yield 

 per recruit were presented by Hayasi, Honma, 

 and Suzuki (1972) ;■' Joseph and Tomlinson 

 (1972);^ and Lenarz and Sakagawa (1972)." 

 A similar study was published by Wise (1972) 



' Southwest Fisheries Center, National Marine Fisheries 

 Service, NOAA, LaJolla, CA 92037. 



- ICCAT. 1972. Report of the meeting of the special 

 working group on stock assessment of yellowfin tuna 

 (Abidjan, June 12-16, 1972). Manuscript on file at ICCAT 

 General Mola 17, Madrid, 1 Spain. 



•^ Hayasi, S., M. Honma, and Z! Suzuki. 1972. A com- 

 ment to rational utilization of yellowfin tuna and albacore 

 stocks in the Atlantic Ocean. Far Seas Fisheries Research 

 Laboratory, Orido 1000, Shimizu, Japan. Unpublished 

 manuscript. 



-* Joseph, J., and P. K. Tomlinson. 1972. An evaluation 

 of minimum size limits for Atlantic yellowfin. Inter- 

 American Tropical Tuna Commission, La JoUa, Calif. 

 Unpublished manuscript. 



5 Lenarz, W., and G. Sakagawa. 1972. A review of the 

 yellowfin fishery of the Atlantic Ocean. Southwest Fish- 

 eries Center, National Marine Fisheries Service, La Jolla, 

 Calif. Unpublished manuscript. 



Manuscript accepted June 1973. 



FISHERY BULLETIN: VOL. 72. NO. 1. 1974. 



37 



