V. SPAWNING 



A. INTRODUCTION 



Considering the amount of re- 

 search which has been devoted to the 

 Atlantic bluefin tuna, positive informa- 

 tion on its spawning habits is surpris- 

 ingly incomplete. ITie earliest and most 

 extensive information on this subject is 

 from the Mediterranean, but even there 

 much remains to be learned. The situa- 

 tion is far worse in the important east- 

 em North Atlantic region, since, to our 

 knowledge, no larvae or small (less 

 than 12 cm) juveniles have been col- 

 lected in any of its areas which have 

 been assumed to be spawning grounds. 

 Greater advances have been made in 

 the western North Atlantic, but posi- 

 tive information there is limited to rela- 

 tively small areas. Aside from a few 

 larvae collected near the Equator, wc 

 found no information whatsoever on 

 the spawning of the species in the South 

 Atlantic. The paucity of knowledge con- 

 cerning this vitally important phase of 

 the life of the bluefin tuna may be 

 attributed to the serious limitations in- 

 herent in the methods used to obtain 

 this information. 



B. DEFINITIONS 



Larva: Matsumoto et al. (1972) 

 stated that the larval stage of nearly all 

 tuna species ends when the larva has 

 attained 1 to 13 mm standard length 

 (SL) (Figure 63). We have arbitrarily 

 selected 12 mm SL as the size at which 

 the bluefin tuna passes from the larval 

 to the juvenile stage. 



Juvenile: Postlarval fish may be 

 regarded as juveniles until they reach 

 maturity— a considerable size, in the 

 case of the bluefin tuna. In this section 

 on the spawning of this species, how- 

 ever, we have arbitraril> limited the 

 tenn "juvenile" to include individuals 

 from 12 to 120 mm. Bluefin tuna only 

 slightly larger than 1 20 mm are active 

 predators and are taken in some fisher- 

 ies. 



Index of Maturity: The numeri- 

 cal index of maturity most frequently 

 used in this section is the weight of the 

 entire fish divided by the weight of the 



gonads. This index is therefore inversely 

 proportional to the maturity of the fish. 

 Other indices are defined where they 

 first occur in the text. 



C. CRITERIA FOR 

 DETERMINING SEASONS 

 AND AREAS OF SPAWNING, 

 AND THEIR LIMITATIONS 



1. Presence of Ripe Adults 



The evaluation of the condition, 

 or degree of maturity, of the gonads of 

 adult fish is one of Iwo approaches 



used to determine the spawning sea- 

 sons and areas of the bluefin tuna. The 

 presence of fully ripe adults is assumed 

 to indicate spawning. 



Stages of maturity have been 

 evaluated by several methods, some 

 subjective and some objective. The ap- 

 pearance and physical characteristics 

 of the ovaries and the ovarian eggs are 

 among the subjective criteria used for 

 this purpose. Visual diagnosis may be 

 based on the color of the ovaries and 

 the extent and color of their external 

 veining. Their consistency (hardness 

 or softness ) and their size and shape are 



Figure 63. Early life history stages of TImnmis ihynnus of the western central 

 Atlantic (Richards 1089). 



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