TUNAS FROM WESTERN EQUATORIAL PACIFIC 



113 



fin not suitable for freezing in the round because 

 of size or condition were butchered for filleting. 

 Such fish were examined whenever possible for 

 sexual maturity and food habits. 



The big-eyed tuna {Parathunnus sibi) appeai-ed 

 less frequently in the catches than yellowfin tuna 

 but ranked second in percentage composition of 

 tuna landings by weight and number. Approxi- 

 mately S50 tons of this species were received by 

 the mothership. Biological observations were 

 made on big-eyed tuna similar to those for the 

 yellowfin. 



NOTES ON TUNA SPAWNING 



Yellowfin (Neothunniis macropterus) 



Previous studies on the gonads of yellowfin tuna 

 have been largely confined to smaller fish, less than 

 a meter in length, usually captured by surface- 

 trolling gear (Schaefer and Marr 1948a, Wade 

 1950a). Schaefer and Marr's work (1948a) on 

 the spawning of yellowfin tuna in Central Amer- 

 ican waters included large fish, but even so, repre- 

 sentative samples from catches made by bait boats 

 and purse seinere were composed predominately 

 of fish measuring less than a meter. It is ap- 

 parent, therefore, that fishing gear designed to 

 fish at or near the surface catches smaller and 

 younger fish than subsurface gear and for this 

 reason does not supply material that gives infor- 

 mation on the maturation of older fish. 



The longline, on the other hand, captures larger 

 fish. This is evident from figure 1 which shows 

 the plotted length-frequency data for yellowtin 

 tuna taken randomly from longline catches during 

 the season. Length frequencies of fish of less than 

 80 cm. include not only those caught by longlines 

 but also fish taken at the surface by pole-and-line 

 gear. 



Although the proportion of males was usxuiUy 

 greater than that of females among yellowfin tuna 

 examined, the sex ratio sometimes running as high 

 as 80 males to 20 females in samples drawn from 

 landed catches, no effort was made to analyze the 

 condition of male gonads for it is extremely diffi- 

 cult, if not impossible, to make a reliable estimate 

 of the state of maturity by gross examination. 

 Milt was found in the central lumen of practically 

 all testes examined, even in those which, fi'om all 

 appearances, woidd be classified either as spent or 

 as ripening. 



Female yellowfin tuna occurred more frequently 

 among fish below 130 cm. in length than among the 

 larger size groups, but in no case was it observed 

 that the proportion of females exceeded that of 

 males. 



Ovaries of the fish examined were found to be 

 inmiature, ripening, or in spent condition, by 

 Marr's criteria (Schaefer and Man*, 1948a) of 

 gonad classification. Since the yellowfin tuna 

 probably spawns several batches of eggs over an 

 extended period of time, as suggested by Schaefer 

 and Marr (1948a), the ovaries possibly do not im- 

 mediately become much reduced in size after 

 spawning. A long-extended spawning season, 

 with individuals spawning more than once, would 

 result in an ovary ripening over a long period. 

 For these reasons, it is difficult, as others have 

 found, to distinguish between spawning and 

 ripening ovaries. The tabulated results of gonad 

 observations, table 2, probably include both of 

 these categories under the classification of "ripen- 

 ing." 



No ovaries that could be considered ripe or run- 

 ning ripe were found. The absence from catches 

 of individuals ready to spawn has been noted 

 wherever studies have been made on the spawning 

 habits of yellowfin tuna. Sdiaefer and Marr 

 (1948a) observed no running ripe females and hy- 

 pothesized that, as spawning approaches, the fish 

 either migrate beyond the range of the fisliery or 

 stop feeding. In the Philippine Islands, Wade 

 (1950a) found ripe yellowfin in the course of his 

 investigations but no spawning or spent individ- 

 uals. Apparently this phenomenon is not limited 

 to the yellowfin tuna, for it has been observed for 

 big-eyed tuna (Parathunnus sihi), as noted later, 

 and for oceanic skipjack {Katsuwonus pelamis) 

 (Hataietal. 1941), 



During early August, yellowfin females with 

 spent ovaries started to appear among catches 

 made 150 to 200 miles east and northeast of 

 Kapingamarangi Island (1°05' N., 154°45' E.). 

 Some such ovaries, flabby in a^ipearance and dark 

 red in color, might also have been observed in late 

 July if a check had been made then of incoming 

 fish. Spent ovaries were observed up to the time 

 fishing operations ceased in September. 



From these superficial observations of gonads, 

 it was conjectured that the yellowfin tuna to the 

 east of Kapingamarangi Island had spawned in 



