YELLOWFIN TUNA SPAWNING IN THE CENTRAL EQUATORIAL PACIFIC 



By Heeny S. H. Yuen and Fred C. June, Fishery Research Biologists 



Because of the need for more knowledge about 

 the spawning habits of the yellowfiu tuna, A^eo- 

 thiinnus macropterus (Temminck and Schlegel), a 

 study was initiated as part of the tuna-research 

 program of the Fish and WikUife Service's Pacific 

 Oceanic Fishery Investigations (POFI). Previous 

 studies have been made of the reproduction of 

 this species in various parts of the Pacific — by 

 Schaefer and Marr (1948) and by Mead (1951) in 

 Central American waters, by Bini (1952) off Chile 

 and Peru, by June (1953) in Hawaiian waters, by 

 Marr (1948) in the Marshall Islands, by Shimada 

 (1951) near Kapingamarangi Island (1° N., 155° 

 E.), and by Wade (1950 and 1951) in the Philip- 

 pine Islands. This report is based on data from 

 the central equatorial Pacific. 



In this study we have investigated the time and 

 place of spawning and the size of the fish at sexual 

 maturity. Stages in the resorption of ripe eggs 

 that were not spawned are incidentally described. 

 The appearance of nematodes in the ovaries is 

 noted, and its effect on egg production is discussed. 



The research staff and vessel personnel assisted 

 in tlie collection of ovaries and field data; Richard 

 Shomura helped process the ovaries; Wilvan Van 

 Campen translated the Japanese data; and 

 Tamotsu Nakata prepared the illustrations. 



SOURCES OF DATA 



Ovaries collected on POFI e.xploratorv-fishing 

 trips from February 1950 to June 1954 provided 

 most of the material for this study. The area of 

 collection extended from 8° S. to 10° X. latitude 

 and from 120° W. to 180° longitude. 



The ovaries, preserved in 10-percent formalin. 



were brought back to the laboratory for examina- 

 tion. A record was kept of the date and the place 

 of capture and the fork length of each fish. At 

 the laboratory, the eggs were classified according 

 to physical characteristics, as immature, inter- 

 mediate, maturing, or ripe, as defined by June 

 (1953). Tiie "spawned out" category was omitted 

 because of the difficulty in defining this class, but 

 since only a fraction of the total number of eggs 

 is emitted during spawning the remaining eggs 

 permitted fish that had recently spawned to be 

 classified into one of the four categories. 



Tiie principal features of the four categories are 

 as follows: 



Immature. — The eggs are traii.slucent and range from 

 O.Ul to 0.18 mm. in diameter. 



Intermediate. — The largest eggs are semiopaque owing 

 to the deposition of yo!l< granules; the diameters range 

 from 0.18 to 0.40 mm. 



Maturing, — The largest eggs are fully opaque, with 

 diameters ranging roughly from 0.10 to 1,00 mm. 



Ripe. — The largest eggs are transparent and loo.se, with 

 diameters of about 0.76 to 1.23 mm. A prominent oil 

 globule is present in each egg. 



The ovaries collected after 1951 were subjected 

 to ati added procedure in the laboratory. Tliey 

 were examined for residual eggs — ripe eggs of a 

 previous spawning that were not expelled. These 

 eggs were classified as being in the early stages of 

 resorption if they were still translucent and 

 loose, and as being in tiie late stages if they were 

 massed together and opaque or turning opaque. 



The laboratory classifications of the ovaries are 

 arranged in table 1 by month and locality of 

 capture, bv stage of matm-itv, and by length of 

 fish. 



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