POSTLARVAL NEOTHUNNUS MACROPTERUS, AUXIS THAZARD, AND 

 EUTHYNNUS LINEATUS FROM THE PACIFIC COAST OF CENTRAL 

 AMERICA 



By Giles W. Mead, Fishery Research Biologist 



UntU 1942, none of the spawning areas of the 

 several species of eastern Pacific tunas was laiown. 

 Since that year several such regions have been 

 identified and in each case the discovery has been 

 made by indirect means, through the collection 

 and identification of the pelagic postlarvae, for 

 the ripe eggs of the tuna have rarely been found. 

 Knowledge of the location and extent of the 

 spawning grounds of the tunas depends, therefore, 

 on being able to identify the young taken in 

 plankton collections. This paper provides a de- 

 scription of the identifying characters of the 

 juveniles of several tunas. 



In the late spring of 1949 I had the opportunity 

 to make collections of pelagic postlarvae in waters 

 off the Pacific coast of Central America. Supple- 

 menting this material, a series of uncatalogued 

 specimens ' from the California Academy of 

 Sciences, which was collected off Central America 

 during the 1932 cruise of the Zaca, was examined. 



OBSERVATIONS ON ADULTS 



The fishes collected in the spring of 1949 were 

 taken from the motor vessel Alphecca, a tima 

 clipper fishing for the Westgate-Sun Harbor Co. 

 of San Diego, Calif. Actual fishing was confined 

 to the month of May in waters from 50 to 150 

 miles off the west coast of Nicaragua and El 

 Salvador. Diu-ing this period the 240-ton catch 

 consisted of j'ellowfin tuna, Neothunnus maeropter- 

 11^ (Temminck and Schlegel), and oceanic skip- 

 jack, Katsuwonus pelamis (Linnaeus), the former 

 comprising the bulk of the catch by weight and 

 number. Gonads of 25 of each species were 

 examined for degree of maturity. It was apparent 

 from this examination that the yello\vfin tuna 

 more than 75 centimeters long and all the oceanic 

 skipjack were in advanced stages of sexual ma- 

 turity. (Total lengths are taken from tip of snout 



I Made available by Lillian Dempster of the California Academy of 

 Sciences. 



963182-61 



to distal end of the shortest caudal fin ray.) 

 Ovaries were swollen and turgid, although no ova 

 were visible to the imaided eye. Testes of both 

 yellowfin tuna and oceanic skipjack had milt in the 

 central duct. Several large male yellowfin were 

 running ripe, but no females in a similar condition 

 were observed in the catch. Two female black 

 skipjack, Evthynvus lineatus Kishinouye, 54.4 and 

 55.0 cm. m length were taken. Their ovaries were 

 similar in degree of maturity to those of the oceanic 

 skipjack. Two ripe female sierra mackerel, Scom- 

 heromorus sierra Jordan and Starks, were taken in 

 a bait haul at Alacapule, Mexico, in the Gulf of 

 Cahfornia. Eckles (1949) has described the post- 

 larvae of this species. Althougti numerous at- 

 tempts were made with a high-speed plankton net 

 to recover the eggs from the surface layers of 

 waters where mature fish were found, none proved 

 successful. 



Apparently the spa\vning season for the tunas 

 is a long one and the spawning area large. Ehren- 

 baum (1924) outlines the probable spawning 

 grounds in the Mediterranean region and in the 

 Atlantic for the species represented in his collec- 

 tions by larvae and postlarvae. He also describes 

 the degree of maturity and possible migrational 

 routes of the adults. Similarly, various Japanese 

 workers have attempted to delimit spawning areas 

 in the western Pacific, and at present extensive 

 work is being done near the Hawaiian Islands and 

 the Phihppines. The spawaiing areas of the tunas 

 in Central America are now knowm to extend from 

 Panama north to Nicaragua and El Salvador and 

 off shore to a distance of more than 100 miles. It is 

 also probable that spawnmg of yellowfin tuna and 

 oceanic skipjack occiu-s off Mexico, since the Zaca 

 collections made there include frigate mackerel 

 and one larval black skipjack. It is not unlikely 

 that futiu-e work will show that this spawning 

 area extends throughout the tropical waters of 

 Central America. 



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